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Land use implications of onsite water and sewer systems

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Title:
Land use implications of onsite water and sewer systems
Creator:
Ryan, Doug
Language:
English
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vi, 102 : illustrations, maps (some color), plans ; 28 cm

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Subjects / Keywords:
Sewage disposal ( lcsh )
Sewage disposal in the ground ( lcsh )
Sewage disposal ( fast )
Sewage disposal in the ground ( fast )
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Academic theses. ( lcgft )
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )
Academic theses ( lcgft )

Notes

Bibliography:
Includes bibliographical references (leaves 98-102).
General Note:
Submitted in partial fulfillment of the requirements for a Master's degree in Planning and Community Development, College of Design and Planning.
Statement of Responsibility:
by Doug Ryan.

Record Information

Source Institution:
University of Colorado Denver
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Auraria Library
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
09003600 ( OCLC )
ocm09003600
Classification:
LD1190.A78 1981 .R92 ( lcc )

Full Text
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LAND USE IMPLICATIONS OF ONSITE WATER AND SEWER SYSTEMS
By
Doug Ryan
i
Studio III Project Planning and Community Development University Of Colorado at Denver Spring 1981


TABLE OF CONTENTS
INTRODUCTION
Chapter 1 - ONSITE SEWAGE DISPOSAL SYSTEMS Background
The Septic Tank - Soil Absorption System Alternative Systems:
Aerobic Treatment Units. Seepage Pits.
Mound Systems. Evaporation Systems. Systems in Fill Material. Surface Discharge Vaults. Greywater Systems. Composting Toilets. Systems Recycling for Potable Reuse. Wastewater Characteristics Site Characteristics:
Hydraulic Conductivity. Soil Type.
Depth to Bedrock. Depth to Groundwater. Slope. Climate.
Sewer System Design
Performance of Onsite Wastewater Systems:
Septic Tanks. Aerobic Units. Soils as a Treatment Unit. Microbiological Aspects. Chemical Aspects.
Disposal of Residuals
Page i - vi
1 - 2 3 - 7 8-18
19 - 21 22 - 24
25
26 - 35
36
a


Management of Onsite Systems:
37
38
Wastewater Management Planning. Site Evaluation.
System Design. Supervision of Construction.
Owner Education. Septage Treatment and Disposal. Monitoring and Enforcement.
Administration.
Chapter 2 - HYDROLOGY OF ONSITE WATER WELLS
Groundwater and the Hydrologic Cycle 39 - 40
Aquifers as Areas of Storage and Transport 41 - 44
The Effect of Wells 45 - 46
Water Budgets 47
Well Construction 47
Water from Crystalline Rock Formations 48 - 51
Chapter 3 - INTERACTIONS BETWEEN WATER AND SEWER SYSTEMS 52 - 53
Flow of Effluent Within the Groundwater System 54 - 55
Flow of Effluent Above the Groundwater Table 56 - 57
Minimum Safe Distances Between Wells and Absorption
Systems 58 - 61
Other Sources of Nitrate Contamination 62
Aerial Extent of Groundwater Contamination 63
Accumulation of Contaminations Over Time 64
Chapter 4 - PRINCIPLES FOR WATER QUALITY PROTECTION
Site Characteristics: 65 - 66
Topography. Soil Characteristics. Hydraulic Conductivity. Depth to Water Table. Evaporation Potential.
b


Selection of Alternative Treatment Systems 66
System Design 66
Installation 67
Relationship to Wells 67
Operation of Systems 67 - 68
Maintenance 68
Well Construction 68
Chapter 5 - LAND USE IMPLICATIONS
Background 69 - 70
Regional Issues 71 - 73
Designation of Areas Suitable for Onsite Systems 74 - 80
Community Issues 81 - 87
Site Specific Issues 88 - 94
Chapter 6 - CONCLUSIONS 95 - 97
References 98 - 102
c


INTRODUCTION
The use of onsite wastewater treatment and disposal systems as a method of sewage disposal is common in the United States, It has been estimated that 18 million housing units (25% of the total) utilize onsite systems (1). The vast majority of these consist of a septic tank followed by some type of soil absorption system.
Properly designed and operated septic tank-soil absorption systems provide a high level of wastewater treatment. In rural areas they have been in use for decades with no adverse environmental effects. In other areas with poor soils, or high population densities, disease outbreaks and other water pollution problems have resulted.
Wells are the most common source of water in the non-urban areas where these systems are utilized. This poses both good and bad consequences. Ecologically, the use of wells and soil absorption systems in conjunction with each other is a sound one. Water drawn from the well is returned to the ground, replenishing the supply. The advantages of this arrangement break down if, for whatever reason, the groundwater is contaminated by the sewage effluent resulting in health hazards to the users.
The most serious of these health hazards are nitrate contamination or bacteriological contamination. In a recent study it was found that out of 14 groundwater quality variables sampled in an area which utilized onsite soil absorption systems, that nitrate was the only variable which showed significant variation and exceeded standards considered safe for human
l


consumption (2). Serious poisonings in infants have occurred following the ingestion of well water containing high nitrate levels (3)„ High nitrate concentrations in water interfere with the blood’s ability to transport oxygen (4).
In certain instances relatively high survival rates of bacteria in water can allow pathogenic bacteria to remain in high enough numbers to cause disease outbreaks (5)„ In Florida, 1,200 cases of gastrointestinal illness was traced to the bacterium Shigella sonnei when a municipal well was contaminated with the effluent of the onsite disposal system from a church/day care center (6). Many smaller, less dramatic cases of water borne disease outbreaks have also been recorded (7)„
In order to deal with these problems, tremendous amounts of money have been spent for the construction of central wastewater collection and treatment systems. A large part of this cost was paid by the Federal government, but small communities were burdened beyond the ability of its users to repay. This is especially true in mountain areas, due to the high cost of providing a collection system in rough terrain. The Environmental Protection Agency now requires a cost-effectiveness analysis of sewer projects which request Federal funds. The agency's policy is that if site conditions are suitable, the conventional septic tank-soil absorption system is the best type of disposal system (8) .
In areas where community sewer systems have not been provided, health departments have attempted to regulate the design and installation of onsite
li


systemso In departments with more advanced programs, polluted groundwater and nuisance problems associated with failure are no longer seen as inherent shortcomings of the systems, but rather as problems with their misapplication and misuse (9). Much has been learned in the last several years about how onsite systems work, why they fail, and what conditions are necessary for their successful operation. The emphasis has been on how to design alternative systems for poor or marginal soil conditions, or how to increase the longetivity of onsite systems (10).
Individual sewage disposal systems do not provide 100% treatment of domestic wastes. The effluent leaving the soil absorption system will always contain some contaminants in excess of safe drinking water limits. In order to protect groundwater from contamination by this effluent, minimum distance separation requirements between wells and soil absorption systems have been developed. The most widely accepted distance is 100 feet (11).
In areas with soils of high filtering capability this distance may be reduced. Mountain areas offer special problems because effluent can travel through fracture systems with little treatment before entering the groundwater. Separation distances as high as 200 feet are called for in mountain areas (12, 13) .
In order to insure that safe separation distances can be met, and to allow for some safety factor, some health departments have established minimum lot size requirements where wells and onsite sewer systems are used. Jefferson County, Colorado presently reauires 3.5 acres lots for all new subdivisions in this category (13).
iii


This approach has produced satisfactory results in the past, but rural areas are experiencing pressures to develop at higher densities. Rising energy costs have changed peoples ideas about land use patterns. Rather than getting as far away from work as possible, people want to live close to their job. Large lots of several acres are seen as wasteful. The cost of developing roads and other utilities and the cost of local governments to maintain these services is very high. A large portion of the non-urban population either cannot afford, or does not wish to live in very low density developments. The result has been a trend toward cluster communities or smaller lots. Depending on one's point of view, several advantages can be seen from this trend. More people can be accomodated in a given area while at the same time preserving open space. Ecologically sensitive areas, or areas in which natural hazards occur, can be identified and preserved.
This move to higher densities has caused problems for health officials and planners alike. Today's realities about land use planning have clashed with the goal of groundwater protection. It is known that high density residential developments where wells and onsite sewer systems are utilized can develop significant groundwater contamination problems. It is also known that poor soil conditions or other site problems are a major cause of sewer system failures. Even with these problems, economics of rural development dictate that it is equally clear that due to a variety of reasons, onsite sewage disposal systems will continue to play an important role in non-urban communities.
IV


The problem for both planners and health officials is that not enough is known about the relationship between land use and ground-water quality in cases where individual sewer systems are utilized.
A review by the Environmental Protection Agency revealed that little is known about the dynamics of the interaction between groundwater and soi1-absorption of sewage (14).
This paper will examine these relationships. The premise of my work is that if the interactions between groundwater, wells, and onsite sewer systems are understood then land use patterns can be designed which will preserve groundwater quality and allow for the attainment of other land use goals as well.
The order of presentation will be as follows: first the operation of onsite wastewater systems will be described. Particular attention will be paid to the effect of natural conditions on the performance of these systems, and the quality of effluent produced. This will be followed by a discussion of the hydraulogic aspect of groundwater availability. Geologic conditions, weather, land use, and method of sewage disposal all play important roles in this regard. From an understanding of how both sewer and water systems operate in the environment, standards or principals will be formulated regarding the protection of groundwater supplies from contamination by sewer systems. The implications for land use in these standards will then be examined.
v


It has been suggested that land use planning controls, not sewer sizing and locations, or regulations of onsite treatment and disposal systems, should be the critical point of leverage in controlling surburban development (15). A better approach is to consider the provision of safe sewage disposal as part of the planning process. Of course, the importance of high quality groundwater is so critical that the provision for safe sewage disposal will always be one of the important factors in making land use decisions.
A successful combination of onsite sewage disposal system regulation and land use planning will require a greater degree of coordination between health and planning agencies than takes place at present. In the long run, however, it can mean cleaner water supplies and land use patterns which reflect other equally important community goals.
vi


Chapter 1
ONSITE SEWAGE DISPOSAL SYSTEMS
Background
Sewage disposal, like many other facts of life, used to be much simpler than it is today. Before rural electrification farm families relied on privies as a method of waste disposal. But electric power was soon followed by piped in water systems and the need for a water carried sewer system was borne.
Homeowners installed the earliest systems themselves. Cesspools were the most common method of disposal. These were sometimes followed by subsurface irrigation systems in an attempt to increase soil absorption in cases where backups or sewage surfacing occurred. Later, departments of health began publishing plans for home treatment systems.
A septic tank was installed to act as a settling basin prior to soil absorption through buried trenches lined with open tile pipe. A drain tile length of 40 feet per person was commonly used despite the differences in soil conditions encountered (16).
Improvements were made on these designs, such as the addition of course aggregate around the tile to provide better liquid spreading. Failures with these systems occurred frequently. Research by Henry Ryon of the New York Health Department into the cause of failures led to the development of site testing criteria which could be used to design absorption systems (17). The procedure developed by Ryon was the percolation test. It measures the soil’s
1


ability to absorb water. A formula for determining the square footage of absorption area required was developed based on observations of properly functioning systems in soils with known percolation.
Failures of isolated systems constituted more of a nuisance than a health hazard until the housing boom following World War II led to the development of higher density rural homesites. It soon became evident that much was still to be learned about onsite sewage disposal. The U.S. Public Health Service conducted research beginning in the late 1940's and published the first Manual of Septic Tank Practice in 1957„ This included a consideration of a variety of site characteristics besides percolation.
Much has been learned about sewage disposal since the manual was first published. Technical improvements have allowed for the development of areas which were unsuitable for onsite disposal with the technology of 1957. Areas with high groundwater tables and poor soil conditions can be fitted with alternative disposal systems which provide high levels of environmental protection.
The primary goals of wastewater treatment are the protection of public health and the preservation of water quality. The following sections will examine the present state of onsite wastewater disposal systems in the light of these goals.
2


The Septic Tank - Soil Absorption System
The septic tank - soil absorption system is the most widely used method of onsite wastewater disposal in the United States. Its purpose, as with any other system, is to treat sewage in such a way as to remove harmful bacteria and viruses, and to break down chemicals so as to eliminate a nuisance or health hazard. As we shall see, onsite systems do not remove 100% of the contaminants from sewage. But given a properly designed system in good soil conditions, very high levels of treatment can be achieved.
Sewage enters the septic tank from the home's drain line. The septic tank provides a minimum level of treatment. Its main purpose is to remove solid material in order to protect the soil absorption system from being sealed off by the high solids content of raw wastewater (18). This is accomplished through a combination of physical settling and anaerobic biological decomposition. Under anaerobic conditions (conditions without oxygen), bacteria utilize the organic matter in sewage as an energy source. Carbon dioxide, organic acids, methane, ammonia and other nitrogen compounds are byproducts of this process.
Not all solids entering the tank are broken down. These either settle out in the bottom of the tank to form a sludge layer, or remain suspended at the top of the liquid. A series of baffles are provided to prevent their release and to allow for more even flow through the tank (19). Figure 1 shows a typical septic tank design.
The size of the tank is very important because the longer the sewage is retained, the greater the breakdown and settling of solids. Septic tanks
3


TYPICAL TWO-COMPARTMENT SEPTIC TANK
Plan
Sanitary
Outlet
Longitudinal Section
Figure 1. From EPA(l)
4


should be sized to provide at least a 24-hour retention time.
Effluent leaves the septic tank and flows, usually by gravity, to the soil absorption system. The most common design for the absorption system is the use of a trench or bed with perforated plastic pipe surrounded by gravel as a distribution system. Figures 2 and 3 illustrate typical layouts. The effluent then percolates through the soil layer. "The soil through which these fluids pass acts as a physical filter, chemical reactor and biological transfer in further processing the septic tank effluents and modifying its quality attributes" (20).
Following an initial break-in period, a clogging mat forms at the soil surface below the gravel layer. This mat slows the movement of water into the soil. This has a beneficial effect because it helps to maintain unsaturated, aerobic conditions beneath the field. As will be discussed later, unsaturated soil is much more efficient at providing treatment of septic tank effluents than is saturated soil.
Absorption systems are sized according to the infiltrative capacity of the soil. The most common method of determining this capacity is the percolation test. It involves observing the drop in water level in a test hole excavated into the onsite soils. Many investigators believe that absorption systems should be sized according to the infiltrative rate of the clogging mat which ultimately forms (1) .
A properly functioning soil absorption system is able to accept all the effluent generated from a household without backing up into the house or
5


making its way to the surface of the ground. The soil under the system must be able to provide a high degree of treatment before the effluent reaches the groundwater table.
The importance of adequate site evaluation prior to installing these systems cannot be overestimated. Further sections will discuss this in some detail. The amount and character of soil cover on the building site, the depth to bedrock or impermeable layers and the depth to groundwater all have major effects on the amount of treatment a soil absorption system can provide.
In order to overcome adverse site characteristics, several alternative systems are available which may be suitable in areas where the standard septic tank - soil absorption system is not feasable. These are reviewed in the following section.
Figure 2. Absorption Field. From EPA(l)
6


Figure 3. Section through an
absorption trench. From EPA(1)


Alternative Systems
The septic tank - soil absorption system is the preferred method of onsite wastewater disposal when conditions are favorable for its use. This is because of their relatively low installation and maintenance cost. But as much as 68% of the land in the United States has been estimated to pose serious constraints for the operation of conventional systems (19)* Figure 4 shows areas of limitations. The following treatment and disposal methods are summarized from a more comprehensive report by the Environmental Protection Agency (1),
Aerobic Treatment Units, Aerobic units incorporate a wide variety of designs but all work on the same principal of providing a high rate of oxygen transfer to the wastewater with an intimate contact between microbes and the waste. In an aerobic environment, pollutants are used as an energy source by bacteria and other organisms. Byproducts of this process include carbon dioxide, water, inorganic nitrogen, and a mixture of cell mass and nondegrad-able material known as sludge. Smaller amounts of organic and inorganic compounds are also formed.
Aerobic units normally require a power source for operation and must be maintained on a regular basis in order to produce satisfactory results.
Effluent quality is usually much better than from a septic tank and some states allow surface discharge after disinfection. The total nitrogen content of effluent from these systems is comparable to that of a septic tank, although it is in an oxidized rather than reduced state. Figure 5 illustrates a typical aerobic treatment system.
8


Figure 4. Limitations for standard septic systems. From Mitre Corp.(19).


Batch - Extended Aeration
Flow-Through Extended Aeration
Figure 5. Aeration treatment units. From EPA(l).


Seepage Pits. Seepage pits or dry wells are deep excavations in which wastewater is allowed to seep into the ground much like a standard field or trench. The raw sewage is treated in a septic tank or aerobic unit prior to discharge into the pit. Seepage pits are most useful in tight areas where a bed or trench system can not be designed. Groundwater levels must be at least four feet below the bottom of the excavation in order to provide acceptable treatment through the soil. Figure 6 shows a typical seepage pit design.
Mound Systems. A mound is a soil absorption system constructed above the ground surface. A suitable fill material is imported and placed below the distribution system in order to compensate for a high groundwater table or a lack of acceptable quality soils. A study of land use in Wisconsin indicated that the use of mounds could substantially increase the amount of land suitable for onsite sewer systems (10). Loading of the mound can be by gravity flow or through a low pressure distribution system. Figure 7 illustrates a typical mound design.
Systems in Fill Material. Fill systems are similar to mounds in that they use imported material as a filtering agent in cases where onsite soils are not suitable. The difference is that fills are constructed at grade.
Sand or imported soil are the most common materials used in constructing these systems. They may be considered in areas where the uppermost soil percolates very slowly and is over faster soils or where fractured bedrock exists within four feet of the ground surface. Fills are not suitable in areas with high seasonal groundwater tables (higher than 6 feet from the surface). Figure 8


Figure 6. Typical seepage pit.
{ I I \ I soil-fill
topsoil
t
2 feet
Original soil
Creviced bedrock
Figure 7. Typical mound system.
l ?


illustrates a fill system common in mountain areas where shallow soils and fractured bedrock exist.
Evaporation Systems. Evapotranspiration systems are used to dispose of wastewater through evaporation so that no discharge into the groundwater is required. A modification of this concept is the combined evaporative/ absorption system. As with the other disposal alternatives, these systems are always proceeded by a septic tank or aerobic treatment unit.
A typical evapotranspiration system consists of a sand bed placed over a distribution system similar to the standard soil absorption bed. The surface is covered with soil and planted with vegitation. The system functions by raising the wastewater to the upper portion of the sand layer by capillary action and then evaporating it into the atmosphere either directly or through plant tissue. Figure 9 illustrates a cross section of a typical evaporative system.
These systems are utilized where soil permeability is very low or ground-water levels are too high to permit a subsurface discharge. Climate has a significant effect on the application and performance of evaporative systems.
In some areas, natural precipitation is greater than the yearly evaporation rate and the net loss through evaporation is zero. In other areas where groundwater is scarce, evaporative systems are not appropriate because the effluent is not recycled to the groundwater supply.
Wastewater lagoons are also evaporative systems. They work well in arid regions which receive little natural precipitation and large amounts of sunlight.
13


Figure 8. Absorption system in fill material,
Impermeable plastic liner
Figure 9. Evapotranspiration system.
14


Lagoons are more common for small communities or cluster applications than for individual onsite disposal. Lagoons must be lined to prevent seepage into groundwater in areas with high water tables.
Surface Discharge, Surface discharge systems utilize a primary treatment method, such as a septic tank or aerobic unit, followed by additional treatment (in the case of septic tanks) or disinfection prior to discharge on the ground surface or into a stream. Many states discourage surface discharge of onsite treated wastewater. This is due to poor performance of these systems and to the burden of monitoring and maintenance placed on homeowners and local officials (21),
Vaults. In some instances, sealed vaults similar to a septic tank but with no outlet device, are utilized to collect sewage prior to its transportation to a remote site for treatment and disposal. A municipal sewage treatment plant is the most common treatment alternative. Vaults pose the disadvantage of a very costly operation for the routine pumping required. In mountain areas with high snowfall, they may not always be accessible for service. For these reasons, vaults should be considered only as temporary solutions to waste-water disposal.
Greywater Systems. In cases where toilet wastes are treated separately, such as a composting toilet, consideration must be given to treatment of greywater. Most states do not distinguish between combined sewage and greywater as far as requirements for disposal are concerned. Septic tank - soil absorption systems, or any of the other alternative systems, are suitable for greywater
15


treatment and disposal. As would be expected, greywater does not contain the same concentration of contaminants as combined sewage. It has been shown that 91% of the total nitrogen in sewage is derived from toilet wastes (22). Since very little nitrogen is removed from the household wastewater system, it is clear that the environmental effects of segregating toilet wastes from greywater are significant. Brandes has demonstrated that the average nitrogen concentration from a greywater septic tank was 14 times lower than a septic tank treating only toilet wastes and that sludge buildup is 8 to 10 times shorter than for a septic tank receiving combined sewage (23). Greywater irrigation of gardens and greenhouses appears to be an acceptable method of treatment if properly controlled (24).
. Composting Toilets. Composting toilets receive toilet and kitchen waste, and convert them through aerobic decomposition to humas. About 80 pounds of compost are produced per person per year. The composting bin is ventilated to the roof. Homeowners have reported satisfactory performance with no objectional odors. A greywater system must be provided in conjunction with a compost unit. Figure 10 illustrates a typical compost toilet installation.
Figure 10. Compost system.
16


Systems Recycling for Potable Reuse
A technology which is on the horizon now is that of a household system for resue of all household water. The Pure Cycle firm in Boulder, Colorado manufactures such a system (25). It consists of a storage module, a sophisticated treatment process and a computerized monitoring system which can shut down the system and inform the service center of its status. The system is charged with water initially and then only small additions are made to make up for consumptive losses.
Treatment begins with an aerobic, anaerobic biological process consisting of a rotating biodisk within a digestion tank. Next, an ultrafiltration stage removes particles down to large molecular size.
Organic absorption then takes place on a carbon filter. This is followed by demineralization in an ion exchange unit. The last treatment step is ultraviolet sterilization.
The application of this technology depends on the availability of an authorized service entity. The Pure Cycle Corporation requires that a service unit be located within 50 miles of any installation.
Recycling systems offer the advantage of being independent of onsite soil or water table conditions and are therefore suitable for any site which is within service range. The Colorado Department of Health has tested the effluent quality of the Pure Cycle system and accepts the unit for installation within the state (26).
17


Popular acceptance of recycling systems depends on the public's attitude toward drinking directly treated wastewater and on the cost comparison with available methods of obtaining water and sewer„
18


Wastewater Characteristics
A knowledge of wastewater characteristics is necessary in order to properly manage sewage disposal systems. Domestic sewage is a combination of kitchen, toilet and bathing wastes. When these are diluted with water, the solids content becomes relatively small. Domestic sewage contains 99.8% water and .2 mineral and. organic solids (4).
The concentration of these substances in sewage is expressed in terms of milligrams per liter. One milligram per liter is one thousandth of a gram in one liter of water, or one part per million by weight. Table 1 lists the approximate composition of untreated domestic sewage.
The average wastewater flow from a typical residence is 45 gallons per person per day. A considerable variation in flows can exist, but the maximum seldom exceeds 75 gallons. Table 2 shows a breakdown in flow from various household fixtures. This information is important in determining the effect of water conservation measures. Estimates of non-residential waste flow volumes are made in Table 3.
Table 2. Sweage flow from household fixtures.
Separate Flow - Residential Use
Bath/Shower 14.7 .014
Dishwasher 1.8 .002
Kitchen Sink 4.4 .045
Additional for Garbage Grinder 1.4 .052
Laundry Washer 19.5 .037
Lavatory 8.4 .021
Water Closet 24.8 .029
19


TABLE 1
APPROXIMATE COMPOSITION OF MEDIUM STRENGTH DOMESTIC WASTEWATER
From Anderson-Nichols(59).
Value
Parameter (mg/1, except as noted)
pH/ units ................. 6.5 to 7.5
Dissolved Oxygen.............. 0 to 3
Biochemical Oxygen Demand .... 220
Chemical Oxygen Demand ................. 610
Total Organic Carbon.................... 240
Total Phosphorus............*. 30
Phosphates ............................ 10
MBAS..................................... 23
Total Solids ........................ 700
Total Suspended Solids ................. 300
Total Dissolved Solids ................. 400
Total Nitrogen ......................... 35
Kj.eldahl Nitrogen....................... 35
Ammonia.................................. 25
Organic ................................. 10
Nitrate .................................. 0
Nitrite ................................. 0
Boron..................................... 0.25
Sodium................................... 55
Potassium.............................. 11
Magnesium ............................... 5
Calcium.................................. 11
Zinc...................................... 0.20
Copper ................................... 0.04
Lead ..................................... 0.03
Nickel ................................... 0.01
Mercury .................................. 0.07
Chromium.................................. 0.04
Sulfate ................................. 20
Chlorides................................ 45
Grease ................................. 100
Alkalinity as CaCC>3.ft................. 120
Coliforms - Total 105/100 ml . 150
- Fecal......................... 3
Temperature, *C ......................... 37
20


Table 3.
QUANTITIES AND BOD STRENGTH OF SEWAGE
TYPE OF ESTABLISHMENT
Residential
Hotels & Motels without private baths Hotels & Motels with private baths Multiple family dwellings or apartments Rooming houses Single family dwellings
Commercial & Miscellaneous
Airline Catering Airports (not incl. food)
Airports
Bus service areas (not incl. food)
Country clubs (not incl. food)
Day workers at offices Drive-in theaters (not incl. food) Factories and plants (excl. of industrial wastes)
Laundries, self-service Food service establishments (toilet and kitchen wastes)
Food service establishments (kitchen wastes)
Food service establishments (with paper service)
Additional for bars & cocktail lounges Movie theaters,churches (not incl. food) Stores
Work or construction camps (semipermanent) with flush toilets Work or construction camps (semipermanent) without flush toilets Travel trailer parks with individual water & sewage hook-up
Travel trailer parks without individual water & sewage hook-up
Institutional
Hospitals
Institutions other than hospitals Mobile home parks Schools, boarding
Schools, day (without cafeterias, gym or showers)
Schools, day (with cafeterias, but not gym or showers)
Schools, day (with cafeterias, gym and showers)
GALLONS/PERSON/DAY (Average) (Unless Otherwise Stated)
50
60
75
50
75
3 gal/meal served 5 gal/passenger 10 gal/employee/day 5
30
15
10 gal/space/day 35
400 gal/washer/day
10 gal/patron/day
3 gal/meal served
1.5 gal/meal served 2
5 aal/seat/dav 400 gal/public toilet/ day
50
35
100 gal/unit/day 50 gal/unit/day
250 gal/bed space/day 125 gal/bed space/day 75 100
15
20
25
LBS. B0D5 PER
PERS0N/DAY (Unless Otherwise Stated)
.15
.15
.20
.15
.20
.03 Ibs/meal served .02 lbs/passenger .06 lbs/employee/day .02 .02 .06
.06 lbs/space/day .08
2.00 lbs/washer/day
.06 lbs/meal served
.03 lbs/meal served
.01 lbs/meal served
.02
.03 lbs/seat/day
2.00 lbs/public toilet/ day
.17
.02
.50 lbs/unit/day .17 lbs/unit/day
.20 lbs/bed space/day .17 lbs/bed space/day .20 .17
.04
.08
.10
21


Site Characteristics
Most onsite disposal systems discharge below the ground surface, allowing the effluent to percolate into the groundwater table,, How fast that effluent reaches the water table, and the amount of treatment provided by the soil depend on specific site characteristics.
Hydraulic Conductivity. The hydraulic conductivity refers to the ability of a liquid to move through soil. Absorption systems are sized according to this ability. The percolation test is the most common way of measuring hydraulic conductivity. More sophisticated techniques have been devised, but correlation between them is poor (27). Actual conductivity depends on soil permeability, depth to the water table and depth to any impervious strata (28).
Percolation test results should be correlated with data on soil borings. If the test results vary considerably from expected valves for the soil type involved, then more investigation is called for (1). For standard soil absorption systems, the percolation rate should be between 5 and 60 minutes per inch (11).
Soil Type. Soils vary in composition from sands to clays. Course textured soil have rapid percolation rates whereas fine soils percolate slowly under saturated conditions.
Soils with a variety of partical sizes and moderate percolation rates are most suitable for absorption systems. The Soil Conservation Service ranks soils according to their limits for onsite disposal systems. Table 4 shows the criteria used.
22


Depth to Bedrock. The depth to bedrock is important in two regards. First is that fractured bedrock provides for rapid movement of effluent to the groundwater table with little treatment. In many mountain areas with little or no soil cover over bedrock, conventional absorption systems are not suitable. Secondly, if the bedrock layer is impermeable then effluent may pond and restrict the hydraulic conductivity of the soil.
Depth to Groundwater. It is important to maintain as much distance between the point of effluent discharge and the water table as possible.
The soil zone above the water table is generally oxygenated and provides adequate treatment. Once the effluent has reached the oxygen deficient conditions within the water table treatment efficiency declines. A minimum of 6 feet between the ground surface and water table is necessary for the installation of a standard soil absorption system.
Evidence of groundwater may be obtained by examining the soil profile boring. If no water is evident, the soil should be checked for mottling which may indicate a seasonally high water table (1).
Slope. The type and degree of slope determine surface drainage patterns. Absorption systems should be located away from runoff collection or erosion areas. The degree of slope has a practical consideration as to the type of system that may be suitable. Slopes over 30% may cause effluent to short circuit out the toe of the absorption system (29)„
Climate. Precipitation falling on the disposal fields adds to the effluent volume and should be considered in choosing between disposal alternatives.
23


In some areas, evaporative systems are not suitable because precipitation rates are too high. Snow cover halts evaporation, but does not normally harm a properly designed absorption system. Biological and effluent heat are usually enough to keep a system from freezing.
Table 4.
SOIL LIMITATIONS RATINGS USED BY SCS FOR SEPTIC TANK/SOIL ABSORPTION FIELDS
Property SI i ght
USDA Texture —
FIoodi ng None, Protected
Depth to Bedrock, in. >72
Depth to Cemented Pan, in. >72
Depth to High Water Table, ft below ground >6
Permeability, in./hr 24-60 1n. layer layers <24 in. 2.0-6.0
Slope, percent 0-8
Fraction >3 in., percent by wt <25
Li mi ts Moderate Severe Restri cti ve Feature
— Ice Permafrost
Rare Common FIoods
40-72 <40 Depth to Rock
40-72 <40 Depth to Cemented Pan
4-6 <4 Pondi ng, Wetness
0.6-2.0 <0.6 Slow Perc. Rate
— >6.0 Poor Filter
8-15 >15 SI ope
25-50 >50 Large Stones
24


Sewer System Design
After the site evaluation has been performed, the most suitable alternative should be chosen among the various system designs. Table 5 gives a method for determining appropriate designs.
It is beyond the scope of this paper to present design details of the various sewer systems described. The Environmental Protection Agency's Design Manual for Onsite Wastewater Treatment and Disposal Systems gives detailed data in this regard (1). Design requirements and construction procedures are critical to the proper functioning of onsite systems. These items should be an integral part of the management of these systems.
Table 5. Disposal methods under various site constraints. From EPA(l).
Site Constraints
Soil Permeability Depth to Bedrock Depth to i Slope Small Lot Size
Very Rapid Shallow
Method Rapid- Moderate Slow-Very Slow and Porous and Nonporous Deep Shallow Deep 0-5% 5-15% 15%
Trenches X X . X X X X X X
Beds X X X X X
Pits X X X X X X X
Mounds X X X X X X X X X X
Fill System X X X X X X X X X X X X
Sand-Lined Trenches o Beds X X X X X X X X X
Artificially Drained Systems X X X X X X
Evaporation Infiltration Lagoons X X X X .
Evaporation Lagoons (lined) X X X X X X X X X
ET Beds or Trenches (lined) X X X X X X X X X X
ETA Beds or Trenches X * X X X X X X
25


Performance of Onsite Wastewater Systems
This section will examine the performance of onsite sewer systems in the environmento The purpose of it is to determine what effects the discharge of effluent has on groundwater quality. It will be shown that while these systems provide a high level of treatment, some contaminants do reach the groundwater in concentrations greater than considered safe by health authorities. Weather these will be present in significant concentrations in the water being drawn by nearby wells depends on several factors, some of which are land use issues.
Septic Tanks. The septic tank is normally the first component of the onsite system. Table 6 gives effluent data from various septic tank studies.
The total amount of nitrogen changes very little from influent concentrations, although it may change form.
Several factors effect septic tank performance. These include internal geometry, loading rate, inlet and outlet arrangements, and operation and maintenance procedures. Multiple compartment tanks provide a higher retention of solids than do single tanks.
Aerobic Units. Aerobic treatment units rely on the addition of oxygen to aid in the biological digestion of sewage. Table 7 contains effluent concentrations from small aerobic units. Much higher reductions of chemical and biological oxygen demand are achieved than from a septic tank.
Soil as a Treatment Unit. The properties of soil allow for several processes which act on the wastewater to effect its treatment. These processes are filtration, sorption and oxidation (30).


Table 6.
SEPTIC TANK EFFLUENT CONCENTRATIONS AND PERCENT REMOVED
From Anderson-Nichols(59).
Value
Parameter (mg/1, except as noted)
pH, units...................... 7.it
Dissolved Oxygen ......................... 0
Biochemical Oxygen Demand .... 160
Chemical Oxygen Demand ................. 323
Total Organic Carbon.................... 129
Total Phosphorus......................... 18
Phosphates ............................. 34
. MBAS.......................... 7.6
Total Solids ........................... 378
Total Suspended Solids .................. 90
Total Nitrogen asN....................... 32 •
Ammonia Nitrogen............... 27'
Organic Nitrogen......................... 8
Nitrate .................................. 0.14
Nitrite........................ 0.061
Chlorides...................... . 95
Alkalinity............................ 390
Coliforms - Total 105/100 mi . 11-110+
- Fecal........................ 0.17
Percent
Removed %
27
47
46
40
240 increase 67 46 70 8
8 increase 20
increase increase 111 increase 225 increase
Table 7.
AEROBIC TANK EFFLUENT CONCENTRATIONS AND PERCENT REMOVED
From Anderson-Nichols(59).
Parameter
Value
(mg/l, except as noted)
Percent removed %
pH, units ....................
Dissolved Oxygen .............
Biochemical Oxygen Demand ....
Chemical Oxygen Demand .......
Total Organic Carbon .........
Phosphates as P ..............
Total Suspended Solids .......
Nitrate as N .................
Nitrite as N .1...............
Coliforms lOVlOO ml...........
7.7 —
2.76 —
41 82
158 74
40 83
37 270 increase
57 81
8 increase
2 increase
72 52


Filtration is the process by which soil blocks the passage of suspended solids and also retains microorganisms to facilitate biological treatment of disolved and suspended organic matter. The soil surface immediately below the distribution system traps larger particles first, followed by an increased efficiency as these particles contribute to the filtering process. As a result, a crust or clogging zone builds up which has greater filtering capacity than the original soil.
This clogging slows down the movement of effluent through the soil. In extreme cases, the system will fail by backing up into the house or surfacing. An equilibrium exists between the formation of the clogging mat and its decomposition (28). Failures can be reduced by increasing the size of the absorption system (8). Research has shown that this mat forms for both septic tank and aerobic unit effluent (18), In reducing flow, the mat helps to maintain unsaturated conditions beneath the field.
Sorption is the binding of one substance to another. Soil has a very high amount of surface area which reacts chemically and electrostatically to trap disolved and suspended materials present in the effluent.
Oxidation, through either chemical or biological means is the conversion of compounds to oxidized states. Carbon dioxide, water, nitrates and sulfates are some products of this process. Oxidation can only take place in aerobic environments. A well drained soil will maintain this condition. Oxidation is preferable to reduction, which occurs in oxygen deficient environments because reduction takes place at a much slower rate and produces unwanted byproducts such as acids, alcohols and amins.
28


Each of these processes function more efficiently during conditions of unsaturated flow through the soil. This is because liquids flow at a slower rate in unsaturated soil. Flow occurs only in the smaller pores, where particles are closer together resulting in increased contact with the soil.
Void spaces in the soil serve to keep the effluent aerated (8).
Good quality soils, loaded at rates which allow unsaturated flow can reduce 75 to 90 percent of the suspended solids, oxygen demand and soluble organic carbon present in septic tank effluent (31).
In mountain areas where soil development is poor, standard absorption systems do not function well. Fractured crystiline rock does not contain the smaller size particles necessary for treatment. Flow through this material is very rapid, resulting in quick recharge of groundwater with poor quality sewage effluent (32) „
Microbiological Aspects. Many field and laboratory studies have examined the efficiency of soil in removing pathogenic, or disease causing organisms.
The results of these studies indicate that soil has a remarkably high potential for filtering out bacteria and viruses. Figure 11 illustrates the results of one such study conducted in unsaturated sandy soil. The type and numbers of bacteria found in the liquid one foot below the trench are similar to those found in natural soil (8). Most of this removal occurs at the clogging mat.
If the absorption system is overloaded and saturated conditions occur, then chances are good that more bacteria will penetrate deeply (33). Ziebell noted bacterial breakthrough in silt loam cores loaded at high rates, but when
29


FEET
ABSORPTION FIELD CROSS SECTION
o- FS Bacteria, 100 ml or 100 g of soil Total FC conforms Total bacteria X 107
Trench — i ft —H
1 - o • -— <200 <200 <600 0.6

;$ Liquid * 160,000 1,900,000 5,700,000 3.0
2 “ •.v.v.v ■ *.*•*.*.’ ■'■'ik Clogged zone ; : T • 54,000 4,000,000 23,000,000 4,400
— \_ <200 17,000 23,000 6.7
3- • —— ■ <200 <200 <600 3.7
•
— Natural '— <200 700 1,800 2.8
soil
Figure 11. Removal of bacteria by soil. From EPA(8).
30


the loading rates were reduced to allow for unsaturated flows very little bacteria escaped (16).
The bacteria that do reach the groundwater table travel much farther than through unsaturated soil. Movements of several hundred feet have been noted (19). Bacteria can also move considerable distances through fractured rock, as is often found in mountain areas. If adequate precautions are not taken the combination of thin soils, greater topographic relief and increased rock jointing greatly increase the possibility of contamination of mountain ground-water supplies (34). Table 8 is a summary of travel distances of bacteria.
Virus removal in soils is a result of sorption, inactivation and retnetion (8). Removal is dependent on the degree of saturation of the pores through which the effluent flows. Pores which are not saturated provide greater opportunity for viruses to come into contact with surfaces which can absorb them. Because of the difficulty of detecting viruses in water, not as much work has been done as with bacterial studies but it is felt that viruses are as effectively removed as bacteria (19).
It can be seen that while soil is very efficient in removing bacteria and viruses, under adverse conditions contamination of groundwater can occur. The geologic conditions least suitable for the removal of pathogens are: shallow soils over creviced bedrock, shallow soils over high groundwater and impermeable soils (5). In extreme cases, soil absorption systems will not be appropriate although alternative systems can usually compensate for these conditions.
31


Table 8
SUMMARY OF DISTANCES OF TRAVEL OF BACTERIA REPORTED IN LITERATURE CITED
From EPA(l).
NATURE OF POLLUTION POLLUTANT OBSERVED DISTANCE OF TRAVEL TIME OF TRAVEL
Sewage polluted trenches intersecting groundwater Coliform bacteria 65 feet 27 weeks i
Polluted trenches intersecting groundwater Coliform bacteria Uranin 232 feet 450 feet —
River water in abandoned wells Intest, pathogens Tracer salts 800 feet 800 feet 17 hours 17 hours
Sewage in bored latrines intersecting groundwater Coliform bacteria Anaerobic bacteria 10 feet 50 feet
Sewage in bored latrines lines with fine soil Coliform bacteria 10 feet
Sewage in bored latrines intersecting groundwater Coliform bacteria 35 feet
Sewage in bored latrines intersecting groundwater Coliform bacteria 80 ft; regressed 59 20 feet
Coliform organisms introduced into soil Coliform bacteria 50 meters 37, days
Sewage effluents on percolation beds Coliform bacteria 400 feet
Sewage effluent on percolation beds Bacteria 150 feet
Sewage polluted ground-water Bacteria A few meters
Introduced bacteria Bacillus prodigio-sus 69 feet 9 days
3 2


Chemical Aspects. As was noted previously, soil absorption systems remove
a large portion of the organic compounds present in wastewater. The most important point with regard to their removal is to maintain unsaturated conditions in the soil. This will prevent the formation of undesirable byproducts of anaerobic decomposition. Some organics, such as gasoline and phenols are not removed by the soil and can travel distances of several miles in soils (30) , This factor has practical implications for homeowners who should be aware of the limitations imposed by onsite systems. It also limits the kinds of industrial users for which soil absorption wastewater systems may be appropriate.
Most pesticides, because of their complex chemical structure and large molecular size move slowly through soil. The effectiveness of soil in degrading these is a function of both soil and pesticide properties (30)„
For safety reasons, no disposal of pesticide residuals should be allowed in subsurface systems.
Numerous inorganic compounds are found in wastewater, A considerable portion of these are normally found in groundwater. Thus, an increase in the mineralization of groundwater can be expected in areas where absorption systems are used (19). The substances which are of the most concern from an environmental or health standpoint are nitrogen and phosphorous (16).
Phosphorous does not constitute a health hazard in drinking water, but it is one of the major nutrients that contribute to algal production and eutrophication of surface waters. Because groundwater is often connected with
33


surface waters through the hydrologic cycle, phosphorous contamination is a significant issue. Phosphates are removed by soil, although travel distances of 2,000 feet have been observed (35). Other observers have noted that phosphorous is absorbed within a few inches of the loading surface (30).
Nitrogen presents a potential threat to public health. As noted in the introduction, nitrates in water interfere with the oxygen carrying capacity of the blood. Because of this health effect, nitrates are considered to be the most important ion associated with soil absorption of wastewater (35).
In the septic tank, most nitrogen from the incoming sewage is converted to ammonia and released. In the absorption system this is converted to nitrates in the unsaturated zone below the clogging mat. Almost complete conversion (nitrification) occurs in the first 2 cm of soil (37). Nitrification will not take place under anaerobic conditions, such as occurs if the absorption system is constructed in the groundwater table. In this case, ammonia will be the predominate form of nitrogen. Ammonia is absorbed by the soil and will not migrate long distances. This may seem as an advantage, but it should be remembered that the treatment of most other sewage contaminates will be severely reduced. If the water table drops, such as during seasonal fluctuations, then the ammonia will be converted and begin to migrate. Nitrates are not absorbed by the soil and will migrate freely with the effluent.
Under most conditions, the nitrate concentration of the effluent beneath the absorption system approximates the total nitrogen content of the septic
34


tank effluent. In some conditions with subsoils containing high organic levels, nitrogen fixation (physical binding) occurs. At high flow rates, however, such a system will become saturated after a few years of operation and nitrate release increases to normal levels (38).
Nitrates can be converted to nitrogen gas through a process called denitrification if an organic energy source is added under anaerobic conditions. A method of producing these conditions has been demonstrated.
It consists of adding a basin or tank below the absorption system to which methanol is added as a carbon source (39) „ This method provides 85-90% removal of nitrogen, but is labor intensive and requires a high capitol cost.
For onsite applications, this technology is tentative (1).
Studies have shown that a family of four discharges an average of 70-75 pounds of nitrogen annually into a septic system (40). Concentrations in the unsaturated soil immediately beneath the absorption system range from 20 to 130 parts per million (40) . This is compared to the U.S. Public Health Service Standard of 10 parts per million in drinking water. The only practical way of reducing nitrate concentrations under these circumstances is through dilution with uncontaminated groundwater.
35


Disposal of Residuals
All wastewater treatment systems accumulate some type of residuals which must be removed and disposed of. An understanding of proper techniques is important if environmental degradation is to be avoided.
Table 9 lists residual from various process and approved methods of disposal.
Table 9. From EPA(l).
RESIDUALS GENERATED FROM ONSITE WASTEWATER SYSTEMS (1)
Source Res1 dual Frequency of Removal
Septic tank Septage 2 to 5 yr
Aerobic unit Sludge 1 yr
Holding tank Sewage ( week to months
Holding tank Blackwater 6 months-1 yr
Recycle systems Recycle 6 months-1 yr
Residuals Compost toilet; Compost large small Incinerator toilet Ash 6 months-1 yr 3 months weekly
Sand filters Scum 6 months
Characterise cs D1sposal
High BOD and SS; odor, grease, grit, hair, pathogens Pump out by professional hauler for off-site d1sposal.
High BOD and SS; grease, hair, grit, pathogens Pump out by professional hauler for off-site d1sposal.
Strong septic sewage; odor, pathogens Pump out by professional hauler for off-s1te d1sposal.
High BOD and SS; odor, pathogens Pump out by professional hauler for off-s1te disposal.
Variable depending on unit processes employed Pump out by profeslsonal hauler for off-s1te disposal.
Relatively stable, high organics, low pathogens Homeowner performs ons1te di sposal; garden burial.
Dry, sterile, low volume Onsite burial by homeowner or disposal with rubbish to landfill
Odor, pathogens, low volume Onsite burial by homeowner or off-s1te d1sposal
36


Management of Onsite Systems
It is clear that many variables effect the performance of onsite sewage disposal systemso All stages of their design, installation and use are important if satisfactory results are to be obtained,, For this reason, management programs should be designed to control these factors. The following functions should be part of an effective program of management.
Wastewater Management Planning. A management plan should identify those areas which will be served by central collection facilities. Those areas which will not be served by central sewer should be ranked according to their relative suitability for onsite systems.
Site Evaluation. The management plan should provide for evaluation of specific onsite conditions to determine the feasibility of various treatment and disposal options. Personnel trained in the examination of soils and geologic conditions can determine the perameters that will be used in designing the system (41).
System Design. After choosing which treatment alternative best fits a given site, the actual design is made. The design must take into account all relevant factors, including waste flow characteristics, soil conditions, depth to groundwater, topography, hydrology and local codes and ordinances.
Supervision of Construction. The installation of an onsite system must be in accordance with its design if proper operation is expected. This is most often accomplished by a combination of construction inspection by health departments and licensing of systems contractors.
37


Owner Education. Occupants of dwellings utilizing onsite systems should
be informed of the type and location of the disposal system utilized and of any relevant factors which will effect its operation. Examples of the later include wastewater flow limitations and a list of substances which should not be disposed of in the system. The importance of periodic maintenance should also be stressed.
Septage Treatment and Disposal, Provisions must be made for adequate treatment and disposal of septage. This may include a contractual arrangement with a municipal treatment facility to accept the sludge. Systems cleaners should be licensed to insure an understanding of how these residues must be handled.
Monitoring and Enforcement. Inspections should be performed to insure that sewer systems are operating properly. A complete program would include water quality testing, enforcement of maintenance requirements, and inspection of systems to determine if back-ups or surfacing are occurring.
Administration. Management agencies may take many forms. State agencies, local governments, special purpose districts and private institutions have all been designated as management agencies. In some instances, a homeowners association or a special district may actually own the sewage disposal systems. The cost of installation, maintenance and repair are borne by the management agency, and a tax or fee is collected to cover expenses. In other areas the concept of management agencies have been hard to sell. Dires has presented a useful paper on promoting the idea of management (42). The relationships between planning and implementing such an idea are not as direct as might be hoped.
38


Chapter 2
HYDROLOGY OF ONSITE WATER WELLS Groundwater and the Hydrologic Cycle
The hydrologic cycle consists of an endless circulation of water between the oceans, atmosphere and land. Figure 12 illustrates the major components. While it is groundwater that is of primary interest here, it cannot be considered alone. Natural conditions, such as drought lower the water table and decrease the amount of water available for man's use. Man effects the cycle in several ways. The building of communities, with roads, parking lots and roofs decreases the land area available for groundwater recharge. Drawing water through wells lowers the water table. Conversely effluent from onsite wastewater systems recharges the water table.
Subsurface water within the zone of saturation is properly referred to as groundwater. This saturated zone is essentially a natural reservoir whose capacity is the total volume of pores or openings in the rocks (43)„
39


Impermeable rocks
Figure 12. The Hydrologic Cycle. From Hofstra, Hall (49).
40


Aquifers as Areas of Storage and Transport
Groundwater occurs in two major types of formations. When the upper limit of the aquifer is defined by the water table itself, it is known as a water table aquifer. If water is found between impermeable layers, it is termed an artesian aquifer. Figure 13 shows both types of aquifers.
Aquifers perform two functions, that of transport and storage. Ground-water is constantly moving between areas of recharge and discharge. This movement is usually slow, on the order of a few feet per day in porous materials to a few inches per day in tighter aquifers (44) .
Groundwater storage relates to the porosity of the water bearing formation. Porosity is a measurement of the volume which consists of openings or voids within the rock. It is a measure of how much water an aquifer can hold, but not the quantity which can be extracted. The yield is a function of pore size and the degree to which they are interconnected (45). In general, fine grain materials have low specific yields while corse grain material has a high yield. Only those formations which have sufficient porosity and water yielding ability to permit the removal of water at usable rates are called aquifers.
Several different types of aquifers exist. Their properties determine the availability of water. Bedrock aquifers consist of natural bedrock formations. These may be sedimentary, igneous or metamorphic. Alluvial aquifers occur in deposits made by the action of modern rivers. Glacial aquifers are formed from glacial deposits (46).
41


The most productive aquifers are deposits of clean, corse sand and gravel, porous sandstones, cavernous limestone and broken lava rock. Silts and clays are among the least productive. Igneous and metamorphic rocks, such as found in many mountain areas are very hard and dense and possess little water storage capacity.
Groundwater flows in the direction of the hydraulic gradient, which is determined by the difference in pressure, or head, between two points. The hydraulic gradient typically follows the contours of the land. Figure 14 illustrates a typical configuration for an unconfined aquifer. The extent to which topography effects the regional flow system can be seen in Figure 15. The upper portion shows a uniform gentle incline, and the flow follows the contour. The lower figure shows the effect that hilly terrain produces.
Numerous subsystems are created within the major flow system (45) . Those areas which separate groundwater basins are known as groundwater divides.
It is generally believed that very little interbasin transfer takes place.
The rate at which water flows through an underground formation is directly related to the permeability of the material and the hydraulic gradient. According to this relationship, which is known as Darcy's Law, flow through an aquifer will be more rapid in areas of greater topographic relief given the same water bearing materials (45). It also means that it takes less energy to draw water from a highly porous aquifer than from a nonporous. aquifer.
42


\ Wittf
feftATlbl*
wsajsw
GrwiTy "*“* Js*«P*9«liPfin9*
WATER TA»U IUNCONFINEDI AQUIFER
^ Fk»Hn9T
ArtwiwWtii!
nmi*N wtu
'CONFINED)
AQUIFER
R*di»9« ' Atm
Figure 13. Artesian and water table aquifers. From EPA(44).
H
II
II
Figure 14. Shape on an unconfined aquifer. From Snow(50).
43


Figure 15. The effect of topography on groundwater flow. The upper portion depicts flow through a gentle incline. The lower portion illustrates a hilly terrain. From Freeze, Cherry(45).
44


The Effect of Wells
When water is pumped from a well, the water level around the well drops. The amount of this drop, and shape of the cone of depression formed depends on the rate and time of pumping, and the porosity of the aquifer. Figure 16 illustrates this phenomenon. The cone of depression is important for several reasons. If two wells are placed such that their cones overlap, the water level will be lowered more than during the pumping of only one well. This interference changes the yield and recharge rate of the wells.
When a well is pumping, the cone of depression changes the hydraulic gradient within its radius. In some instances the extent of this change is far enough to reverse the flow of contaminants from a soil absorption system located downhill from the well. Figure 17 shows how this could happen. How much effect this will have on water quality depends on the duration of pumping and the permeability of the aquifer.
«
EFFECT OF PUMPING ON CONE OF DEPRESSION â–  - Disdiwy

Ground Suffice
Figure 16. Cone of depression From EPA(44)
45


Figure 17. Cone of depression causing effluent to flow towards a well.
From Waltz(32).
46


Water Budgets
A water budget is an account of what happens to all the water that falls on an area. It includes the results of runoff, evaporation, transportation, percolation, storage and discharge. When man is introduced the budget must also include withdrawal from wells and recharge through sewer systems. Water budgets are generally considered in terms of withdrawal and recharge from a fixed amount of storage. This assumption is valid for years of average or above average precipitation, but breaks down during years in which the water â– table lowers through drought. In artesian aquifers, all the water drawn from wells is lost even if subsurface disposal systems are used. This is because the effluent cannot percolate through the impermeable upper layer that forms the aquifer.
Well Construction
Proper well construction is very important in protecting groundwater from contamination. The outside of the well bore provides an easy avenue for surface runoff to enter the water table with no treatment. Jones has demonstrated that proper well construction, including casing and grouting, improves well water quality in farm water supplies (47). In addition to protecting against surface contamination, wells drilled through impermeable layers should be sealed to the lowest layer encountered. This will prevent contaminated water in the upper layers from migrating down the bore. The Environmental Protection Agency lists well construction practices that will provide high levels of protection (44).


Water from Crystalline Rock Formations
The study of groundwater is very complex and broad generalities cannot be made for all situations. It is thereby helpful to examine in greater detail a specific type of groundwater strata.
Studies of groundwater in crystalline rock formations, such as those found in many mountain areas show that it has an extremely low porosity.
This is because the rocks are densely structured and contain little storage capacity. The vast majority of water stored in these formations is in the fractures which develop. Fractured igneous or metamorphic rocks have porosities ranging from 0.1 to 0.001 percent. This is opposed to sedementary rocks which may have between 5 and 35 percent porosity (45)„
The fractures in this material originate from near-surface stresses and therefore exist in greater numbers closer to the surface. In addition, the vast amount of pressure exerted by overlying material tends to close fractures with increasing depth into the formation (45). This means that permeability and storage capacity decrease with depth. In a study of the Colorado front range, Snow noted that porosity decreased logrithmicaly with depth (48).
It is suggested that if water is not found within 175 feet, that another site should be tried rather than drilling deeper (48).
Faults within crystalline formations may contain large quantities of water. This is because they contain large numbers of open fractures. Faults are hard to locate because of concealing soil and vegitative cover however.
In the front range, they are steeply inclined so the probability of intersecting
48


one does not improve appreciatively with greater drilling depth. Groundwater is also found in the aluvial material along valley floors. These have porosities several times greater than do the fracture aquifers. In Jefferson County, Colorado, these zones average 300 feet wide and five feet thick (49). Relatively few wells are located in this material, but those which are experience high yields.
Hofstra and Hall, in their study of water quantity and quality in Jefferson County, Colorado, estimate that storage in water-bearing fractures to be a depth of 300 feet is 4,000 cubic feet per acre (feet ^/acre), or 30,000 gallons. Approximately one-half of that water is recoverable by wells (49).
Well yields in this material vary according to several factors. Of primary importance is the amount of fracturing in the vicinity of the well. Figure 18 illustrates this principle. The greater the number of fractures which intersect the well, the higher the capacity of the well. In the Colorado front range, these fractures are on the order of 10 feet apart and inclined vertically. The typical mountain well of 100 feet probably intersects two of these and yields about one gallon per minute (50). Wells located in faults or aluvium usually produce more water than fracture wells.
Topography plays an important role with regard to well yield. A study of wells in crystalline rock in North Carolina found that valleys and broad ravines produced more water than wells near the crest of a hill and that wells on slopes or flat uplands produced quantities between these extremes. Figure 19
49


reproduces those results. Fault and fracture zones are usually more pronounced in valleys and ravines and are attributed to these results.
The infiltration of rainfall produces a net flow of groundwater from the ridges to the valleys, with streams acting as drains. During dry periods with no surface runoff, this groundwater discharge is the only source of stream flow. Hofstra and Hall estimate the groundwater recharge rate in the mountain portions of Jefferson County, Colorado at 0.6 inches per year. This is from an average yearly precipitation of 18 inches, of which 1.4 inches is lost to surface runoff and 16 inches lost through evapo-transpiration. This recharge is equal to 16,296 gallons per acre per year.
The Jefferson County Planning Department has calculated several important variables which relate to the capacity of this groundwater to serve as a water supply (51). The average family of 3.5 persons requires 95,800 gallons per year. Most of this is returned to the groundwater table through the sewage system. Recharge through soil absorption systems is estimated at 90% of the total draw, so 9,580 gallons are lost through consumption each year. A water budget was calculated using these figures. The density of development which is possible to balance the water budget was calculated as follows:
1. When recharge equals consumptive loss = 1 family/.59 acres
2. When recharge equals withdrawal = 1 family/5.83 acres
The Planning Department utilizes the more conservative figure of one family per 5.83 acres as a planning tool. The reasoning behind using this figure is that for most cases an individual well is not able to capitalize on the total amount of storage or recharge available on a per acre basis.
50


WvH< to»l«
Sketch showing relation of wells, specific capacity and fracturing.
Figure 18. From Summers(60).
Cumulative frequency distribution of well yields with respect to topographic position, Statesville area. North Carolina
Figure 19. Water yields in relationship to topography. From Freeze, Cherry(45).
51


Chapter 3
INTERACTIONS BETWEEN WATER AND SEWER SYSTEMS
The preceding chapters dealt separately with sewer systems and groundwater,, This chapter will look more closely at the interrelationships between them. It seems clear that from an ecological point of view, the recycling of groundwater to the aquifer after treatment is a sound one. Cases, however, of groundwater pollution cast a shadow of doubt on this assumption. Is this pollution the cause of inherent shortcomings of onsite sewer systems? The answer to that question is no. The environment provides a remarkable capacity to cleanse itself, but it cannot be expected to accommodate man’s abuse or carelessness without producing negative effects.
Onsite water and sewer systems are intimately related. In discussing this relationship in Jefferson County, Colorado, the League of Women Voters accurately summarized the situation, "...possible contamination is determined in large part by bedrock fractures, which are the same geologic features that determine availability of groundwater" (52).
In order to avoid pollution of groundwater, a knowledge of what happens to sewage effluent below the soil absorption system is necessary. In most cases, some of the liquid is retained in the soil pores due to the moisture tension of the soil. Most of the effluent makes its way toward the water table by gravity flow. Treatment through the soil continues, but some substances such as nitrates are not removed. As the effluent contacts the uppermost level of groundwater, it will degrade its quality. This degraded
52


water does not remain static, but moves with the groundwater flow. Gradual mixing by molecular diffusion and hydrodynamic dispersion takes place. The mixing will reduce the concentration from the original contact point. If this process could be seen it would look like a fan or plume of degraded groundwater in which the levels of contamination decrease with distance from the absorption system (49).
Figure 20. Schematic of contamination plume downgradient from a point pollution source. FromBouwer(53).


Flow of Effluent Within the Groundwater System
The shape of the effluent plume within the groundwater system is represented in Figure 20. The mathematical equation describing this dispersion is complex and need not be considered here, but the relationships involved are important. The velocity at which the groundwater flows through the aquifer influences the spread of the plume. The faster the groundwater moves, the tighter the plume remains. The other important variable is the longitudinal spreads in the aquifer. This variable cannot be determined mathematically, but must be determined experimentally for a given soil or aquifer material. Depending on site conditions, the spread may be only a few degrees in granular materials or greater than 20 degrees in fractured rock (53).
Field data gathered in water quality monitoring experiments does not always follow this textbook explanation. Childes, et al, found that contaminated groundwater does not always move in a single plume, but may branch out into several fingers (54). This variation is controlled by differences in loading rates, local recharge, local hydrology, promimity to other waste sources and the texture and fabric of soil. Figure 21 illustrates this phenomonon.
Another investigation observed that a unique underground flow pattern concentrated effluent within flow channels that prevented dilution with groundwater (36). It has also been observed that accidental geologic features, such as fault running diagonal to a slope, may divert the groundwater flow in an unexpected direction (50).
54


The last chapter examined a situation in which this conclusion does not hold true. As was seen in Figure 17, the effect of pumping a well may actually change the hydraulic gradient of the water table, causing a flow of contamination toward a well located uphill from the wash source. This situation may be most likely to occur in areas underlined by crystalline rock. The low storage capacity of these aquifers means that the cone of depression produced by the wells will cover a much greater area than in aquifers of higher yields.
The plume of contaminated groundwater does not appear to mix vertically within the groundwater layer. Analysis have shown that high nitrate levels are confirmed to the upper layer of groundwater and that concentrations decrease rapidly with depth (40, 55). This suggests drilling deeper wells may avoid pollution of household water supplies located near the soil absorption system (40).
Figure 21. Vertical migration on nitrates in groundwater. From Childes(54).


Flow of Effluent Above the Groundwater Table
Up to this point, the discussion of the flow of contaminants has dealt with what happens once the effluent has reached the groundwater., In instances where the groundwater is a significant distance below the bottom of the absorption system, the properties of the intervening layer will have an effect on where the first contact with groundwater occurs.
The flow of effluent through evenly textured material will follow the contour of the water table. In this situation, a well located up gradient will not be intercepted by the effluent before it reaches the water table.
In sedementary or metamorphic rock the direction of the dip and strike will determine flow patterns. In crystalline rock the direction and rate of flow is controlled by fractures and foliation (34). Waltz has illustrated different conditions which effect the flow of effluent. Figure 22 shows effluent traveling away from a well located up gradient from a sewer system.
In Figure 23 the well is subject to contamination even though it is located up gradient from the point of discharge. This is due to fracture patterns directing effluent into the slope.
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Well
Figure 22. Effluent moving down gradient from a leach field. From Waltz(32).
Leach Field
Rock Fractures
Water Table
Saturated
Zone
Unsaturated
Zone
Water Table Drawdown From Well Pumping
Figure 23. Effluent moving toward a well through rock fractures.
From Waltz(32).
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Minimum Safe Distances Between Wells and Absorption Systems
Several studies have considered the distance required for the plume of wastewater to become diluted to acceptable levels. The results are useful in formulating guidelines for the installation of systems.
Walker sampled groundwater quality adjacent to absorption systems located in sandy soils (40). Figure 24 shows results of that study.
Walker concluded that approximately .5 acres down gradient from the absorption system was needed before the concentration of nitrates in the top layer of groundwater was diluted to less than the drinking water standard of 10 mg/1 of nitrates as nitrogen. As can be seen from the graph, nitrate levels were reduced to the drinking water level at about 70 meters (230 feet).
In other investigations, nitrate concentrations were reduced to within standards from 20 to 43 meters (16).
A study by Rajagopol, et al, found that nitrate levels could be statistically related to land use and groundwater conditions (2)„ In this relationship the concentration of nitrates varied directly with the number of residences within a 200 foot radius of the well and inversely with the depth of the column of water in the well and the depth of any intervening clay layers. These relationships may vary with other factors, so broad generalities cannot be made using the formula developed. The concept, however, can be used in other situations.
A study of minimum protective distances in Jefferson County, Colorado concluded that coliform bacteria concentrations were not statistically related
58


SYSTEM I
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Fig 1-Concentration of NH4-N and N03-N in ground water as a function of distance to the seepage bed and a top view with the locations of well points in system 1. Open circles represent NH4-N and solid square blocks represent N03 N in the upper 30 cm of the ground water. The triangles represent samples taken at a depth of 1.5 m into the ground water (NH4-N: open triangle, N03-N: solid triangle).
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Fig. 2-Concentration of NH4-N and N03-N in ground water as a function of distance to the seepage bed and a top view with the locations of well points in system 2. Open circles represent NH4-N and solid square blocks represent N03-N in the upper 30 cm of the ground water. **
Figure 24. Groundwater quality adjacent to an absorption system.
From Walker(40).
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to distance but that nitrate concentrations were (12). Figure 25 shows the findings of the study. In it the probability of a well exceeding the 10 mg/1 limit is shown as a function of distance from the nearest soil absorption system. The related study by Hofstra and Hall (49) indicates that protective distance is more important than location with relationship to the hydraulic gradient. This is probable because of the influence of rock fractures on the travel path of contaminants above the water table. The average depth to groundwater in the study area is 40 feet.
Hofstra and Hall identified other factors which appear to have some
relationship with water quality. Aquifers located in aluvium are low in
*
extremes of nitrate contamination but high in fecal and total coliform bacteria. This is probably due to the higher potential of aluvial material to dilute nitrates but the decreased efficiency in filtering bacteria.
Nitrate levels were elevated at the edge of cluster communities and low for strip communities. The results do not distinguish between wells down or up gradient from these cluster communities, but it would be a reasonable assumption that the highest nitrate levels occur down gradient. Topography also plays a role in determining water quality. Wells on hilltops were high in nitrates, whereas valley locations were low. Hillside locations showed nitrate levels in between.
It can be seen that safe distances between wells and absorption systems vary considerably with location. Variables which must be considered include the amount and concentration of effluent, the depth to the water table, the
60


hydraulic gradient of the water, the characteristics of intervening material and the volume of water in storage. Studies similar to those referenced above are necessary in order to develop guidelines in specific instances.
P
Plot of estimated percentage P of wells exceeding 10 mg NO>-N per liter water as a function of distance D from the nearest wastewater effluent, along with approximate 95% confidence band and scatterplot of observed percentages.
Figure 25. Relationship between water quality and distance to the nearest absorption system. From Ford(12).
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Other Sources of Nitrate Contamination
It may be useful to examine what other sources contribute to groundwater contamination. Because nitrates seem to be the limiting factor with regard to soil absorption systems, they will be discussed here.
McCarty estimated the concentrations of nitrogen compounds from various sources (56). The results are shown in Table 10.' One of the figures that stands out in this data is the runoff concentration from agricultural land. In many instances the contribution of nitrogen to groundwater is considerably higher for agricultural versus residential development. Runoff from non-agricultural land is characteriscally low in nitrogen content.
Walker estimates that under natural conditions, 10 kg of nitrogen per acre may reach the water table yearly in areas with high organic content soils (40). Thus, nitrogen derived from sewer systems will equal natural loading at a density of one dwelling per six acres. This is based on an estimated 63 pounds of nitrogen discharged yearly for an average family of 3.5 members. In areas with soils of low organic content, the relative contribution from residential development would be higher.
. Table 10. From McMarty(56).
Estimate of Nutrient Contributions From Various Sources
Nitrogen Phosphorus
Source 1,000,000 lb/year Usual Concentration in Discharge—ntg/l 1,000,000 lb/year Usu.i! Concentration n; Discharg-.—
Domestic waste 1400-1,600 18-20 200-500 3.5-9
Industrial waste Rural runoff: >1,000 0-10,000 t
Agricultural land 1,500-15,000 1-70 120-1,200 0.05 1.1
Xonagricultural land 400-1,900 0.1-0.5 150-750 0.04-0.2
Farm animal waste >1,000 t t t
Urban runoff 110-1,100 1-10 11-170 0.1 -1.5
Rainfall* 30-590 0.1-2.0 3 9 0.01-0.03
* Considers rainfall contributed directly to water surface, t Insufficient data available to make estimate.


Aerial Extent of Groundwater Contamination
Background nitrate levels in groundwater are typically how. In Jefferson County, Colorado, they are less than 0,2 mg/1 as nitrogen (49). Because of the scattering of dwellings normally found in rural settings, the additions of nitrates from onsite wastewater systems is very small when judged on a regional or watershed scale (40). The problem with nitrates, and other contaminants as well, is that of local contamination of groundwater supplies.
This conclusion holds up in a study of an intensely used mountain area of Jefferson County, Colorado. Nitrate levels in groundwater above the 10 mg/1 nitrate-nitrogen level forced the county Board of Health to declare a moratorium on additional sewer systems (57). The site consists of mostly residential development along a valley. Onsite sewer systems and a combination of onsite and community wells are utilized. The density of development is greater than one unit per acre. The topography of the valley is such that groundwater flow is directed to a bowl-shaped area within the valley.
Highest nitrate levels occur in the lowest areas and concentrations decrease with distance up the valley wall, even though housing densities remain high. While more groundwater is available for dilution in the valley floor, the combination of groundwater flow from several directions and the intensity of development do not allow for adequate dilution. High nitrate levels do not extend outside the area of higher density development.
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Accumulation of Contaminants Over Time
Contaminants do not appear to accumulate or build-up in unconfirmed aquifers over time. Rajagopal found that for a set of groundwater flow, soil, and land use conditions, there exists an equilibrium concentration of nitrates in groundwater (2). The time for this equilibrium to develop will vary with conditions, but probably is less than six months. Seasonal changes such as dry periods will cause a shift in the equilibrium.
The fact that groundwater is constantly recycling within the hydrologic cycle is of great benefit for the use of soil absorption systems. The greatest danger of groundwater pollution from onsite systems is that of wells being drilled in locations subject to inundation by contaminated groundwater plumes or that high density development will cause these plumes to intersect, causing broad reaches of poor quality groundwater.
While nitrate contamination constitutes the principal concern for most groundwater problems, other contaminants must be considered. As was noted in Chapter One, bacteria can travel great distances in fractured rocks or within the water table. In addition to bacteria, many chemical contaminants will be released in large quantities when the absorption system is too close to the groundwater table. Sewer systems which are overloaded provide less treatment of effluent is saturated subsoil conditions develop.
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Chapter 4
PRINCIPLES FOR WATER QUALITY PROTECTION
The proceeding chapters have focused on the performance and function of onsite water and sewer systems. From them it is evident that individual sewer systems which are properly designed, installed, and maintained can function in the environment without causing health hazards. This chapter will summarize the principles which may be used for protecting groundwater quality in cases where onsite sewage disposal systems are utilized. It will not be a detailed guide for the engineering of systems to fit specific conditions. Other publications are available for that purpose (1, 11, 58). The issues which are important in this regard are: site characteristics, selection of treatment alternative, system design, system installation, relationship to wells, operation, maintenance, and well construction.
Site Characteristics
Adequate site evaluation is the first step in insuring satisfactory performance from an onsite sewage disposal system. It will reveal what natural assets the land has that can be used to obtain the maximum treatment efficiency and the greatest protection of groundwater supplies. Iiwill also show conditions which hinder the operation of onsite systems. The following issues should be addressed during the site evaluation.
Topography. In general, moderately sloped sites, with drainage away from the absorption system should be chosen. Slopes which face the sun allow more evaporation than north slopes.
65


Soil Characteristics. Soils should be examined for texture, structure,
color and saturation. The best soils for absorption systems are medium textured and well drained, but without large voids on mottling. At least two feet of soil must be available above the bedrock surface.
Hydraulic Conductivity. Soils with moderate percolation rates, between 10 and 60 minutes per inch, are most suitable for absorption systems. Soils which percolate too fast may not allow for sufficient treatment of effluent. Slowly percolating soils may not be able to transmit effluent efficiently.
Depth to Water Table. At least two feet of soil must exist between the effluent distribution system and the highest seasonal groundwater level. This is to allow for adequate treatment through the soil before contacting the groundwater.
Evaporation Potential. If evaporation is to be the sole method of disposal, the effective evaporation rate must exceed the precipitation rate.
Selection of Alternative Treatment Systems
From the information obtained at the site evaluation, the most suitable type of treatment system can be chosen. The best alternative is the one which provides the highest level of treatment, requires the lowest maintenance, and is cost effective. Table 5 is a guide to selecting between alternatives.
System Design
Systems must be designed according to site and wastewater characteristics In sizing the absorption system, the importance of maintaining unsaturated flow beneath the distribution system is the primary concern.
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Installation
The installation of onsite systems must be in accordance with their design. Distribution lines must be properly sloped in order to achieve even loading and help maintain unsaturated subsoil conditions. Soils with high clay content are easily smeared during construction. This reduces the infiltrative capacity of the soil. Machinery should not be allowed on the working face of the absorption system during construction if clay soil is present.
Relationship to Wells
The relationship between the well and absorption system is very important in controlling pollution potential. The onsite well should be located hydraulically up gradient from the absorption system, at a far enough distance so that the cone of depression will not cause a reversal of effluent flow to the well. The absorption system should be as far away from down gradient wells as possible. Minimum safe distances vary with location. In crystalline rock at least 200 feet is needed. The system should also be located laterally away from the expected travel path of the effluent plume. In crystalline rocks, fracture patterns must be accounted for in estimating the direction of flow toward the water table.
Operation of Systems
Homeowners should be advised as to the limitations imposed by the use of onsite sewage disposal systems. Large amounts of chemical solutions, such as bleach or cleansers should not be discharged, otherwise a dieoff of the treatment bacteria may result. Chemicals not normally associated with


in-house use, such as gasoline or insecticides should not be allowed to enter the system.
Onsite systems are sized to accommodate a reasonable quantity of flow. Homeowners should be aware that wasteful water use practices or over-occupancy may overload the system.
Maintenance
Each alternative disposal system has different maintenance requirements. For septic tanks this consists of pumping the sludge that builds up over time. This must be accomplished before it builds up to the point of discharging into the absorption system.
Well Construction
Proper well construction can prevent surface contaminants, and many times subsurface contaminants, from entering the groundwater in the vicinity of the well. Wells should be grouted and sealed to prevent infiltration down the bore.
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Chapter 5
LAND USE IMPLICATIONS
Background
This chapter will discuss how land use issues relate to water quality in cases where onsite sewage disposal systems are utilized. It will draw primarily from the previous data regarding sewer and water systems, and their interaction in the environment.
It has been suggested that the design and installation of onsite sewer systems is an engineering function, and that land use decisions should be made on other criteria. The author disagrees with this suggestion for two reasons. First, the provision of safe water supplies and sewage disposal is an integral part of supplying basic human needs associated with land use. Secondly, engineering alone does not insure safe drinking water supplies. Given that absorption systems contribute contaminant levels to the groundwater, the relative arrangement of wells and sewer systems will play an important role in determining water quality. This relationship is controlled to a large degree by land use patterns.
It seems obvious that in order to make reasonable land use decisions, a good knowledge about how water and sewer systems function in the environment is necessary. Unfortunately, information about new technologies for onsite wastewater treatment are not widely disseminated. Onsite systems have a bad name, which
69


has been earned because of a lack of knowledge about how they can be safely utilized. The following sections will outline a philosophy for the use of onsite systems in such a way as to compliment their ability to provide high levels of effluent treatment and avoid the limitations they possess.
The previous chapters discussed a wide range of factors which must be considered when public sewers are not available. These range from specific detail about the size of a septic tank to concerns about nitrate loading on a regional level. This suggests that different scales may be considered when approaching the subject of land use - water quality relationships. The approach taken here will be to consider regional, community and site specific issues separately. Regional scale here refers to areas larger than what a single developer would normally control. It could range from a few sections to an entire stream basin. Community scale will be limited to a single subdivision or built up area with similar land use development. The site specific scale refers to those issues related to a single lot or building site. It is agreed that specific issues do cross over into each scale of planning.
The approach is useful however in organizing data in a logical order for making land use decisions.
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Regional Issues
Background. Regional land use planning involves a wide range of issues, of which groundwater quality is one. This section will show how decisions on the regional scale can be made on the basis of water quality concerns. Before proceeding with this issue, it will be helpful to put the concept of regional land use into perspective.
Many different kinds of uses occur on a regional scale. This is because of the wide variety of human needs which must be provided in proximity to where people live. The normal progression of land use is commonly thought of in economic terms. The first users to establish in the region are the base, or export oriented, industries. These industries bring in dollars which are available for development of other activities.
Residential development establishes in response to the base industry. Their pattern is dependent on transportation facilities and the mode of travel of the workers. The automobile has played an important role in determining these patterns. Peoples choices on where they wish to live is based in part on the travel time to work and on income available for transportation expenses. Modern freeway systems have contributed to the popularity of low density residential development outside of the urbanized area. Density decreases rapidly as distance and travel time to employment centers increase.
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The services including commercial and public uses locate in response to residential development. Efficiency in providing services is one of the major locational factors considered in their development. The services are more concentrated in high density areas than in suburban or rural areas. Some have very large service areas and will locate only in large metropolitan areas. Others, such as a convenience store or post office, are found in all areas of development.
The progression of regional land use patterns are important to the topic of this paper. It is clear that not all development in a large region will be suitable for onsite sewage disposal systems. High intensity uses that are located in close proximity to other built environments are best suited for central sewage collection and treatment facilities. The urbanized portion of a region should be included within a sanitation district providing centralized treatment facilities. It is important to include enough land for the necessary services within the area planned for public sewer. This will help avoid the problem of high density development locating on the fringe of the urbanized area requesting the use of individual sewer systems.
As the density of residential development decreases to a point of around one unit per acre central sewer systems will no longer be cost effective. At this density onsite sewer systems offer an environmentally sound method of sewage disposal. Small
72


clusters of service oriented development will locate in response to the low density residential development. Depending on the scale of these developments, onsite sewer systems may be feasable.
In larger developments a small "package plant" utilizing surface discharge will be more appropriate. One way to approach this issue is to limit the amount of commercial development in a single location to an intensity which will produce a like amount of sewage expected from residential development on the same ground.
In this way, enough open space will be provided around the development to protect groundwater supplies from contamination.
If the expected sewage flow is significantly greater, then subsurface disposal is probably not the best option.
On a regional scale the contamination of groundwater by onsite systems is not a problem. This is because of the large amounts of open space associated with flood plains, hazard areas, steep slopes, ecologically sensitive zones agriculture and recreation.
The major issue on a regional scale is directing development to those areas most suitable for the use of onsite sewer systems.
7 3


Designation of Areas Suitable for Onsite Systems
The technology which has developed for onsite systems in the past ten years has considerable implications for land use management on a regional scale. Ten years ago, it was estimated that up to 68 percent of the land in the United States poses severe limitations for the use of septic systems. The criteria for these limitations are soil permeability, shrink-swell potential, water table levels and slope. Alternative onsite systems have been developed which are suitable for use in cases when many of these limitations are present. A map showing severe limitations for onsite sewers today would indicate a much smaller area because of the technology.
Chapter One discussed the performance of onsite systems that rely on subsurface disposal. It was shown that soil can be a very effective medium for the treatment of sewage. In those cases where conditions are not as favorable, such as mountainous terrain with thin soils, treatment efficiency is reduced. This suggests that given a choice, sewage disposal systems should be installed in those locations offering the highest degree of suitability first and in areas of lower suitability which require more advanced systems, second. In other words, every advantage should be taken of the lands natural ability to accomodate the type of development proposed. In a way it is like having a safety factor which states that a high level of site suitability is favorable to engineering solutions. Of course, many other issues are involved in making


land use decisions. In those cases where development is directed toward less ideal areas, alternative technologies can be chosen to compensate for constraints imposed by the land.
Techniques for ranking land according to its natural suitability for various uses is well developed. They consist of identifying relevant natural features and ranking them according to the opportunities and constraints they pose for various uses. The following information is offered as an example of this kind of technique.
The most important natural characteristics for the proper functioning of onsite sewer systems are soil cover, depth to groundwater, and slope. An area in Jefferson County was chosen to illustrate how these features can be used to rank lands according to their relative suitability for individual sewer systems. The area is in a rapidly growing portion of the County, located between a mountain community and a major interstate highway used for commuting to Denver employment centers.
The first feature considered is soil cover. Map 1 shows the relative amounts of soil in the region. As is typical of mountain areas, very little land contains the mostly soil classification.
This zone contains the fewest constraints for onsite sewer systems.
A larger amount of area is characterized as soil with rocks. This area is suitable for standard absorption systems in local areas where bedrock is not close to the surface. In other areas, systems in fill
75


material, or mounded systems, are appropriate. This zone is classified as having moderate constraints for onsite systems.
The next ranking is for rocks with soil. This type of land offers severe constraints to onsite systems, although alternative systems can be designed in specific instances. Greater separation between wells and absorption systems is needed to protect water quality in these areas. A fourth category which does not occur in this particular analysis is mostly rock. These areas consist of rock outcrops on ridges or cliffs. For obvious reasons this type terrain is unsuitable for onsite sewer systems.
The aluvial soil, shown in blue on Map 1 has acceptable filtering ability, but is constrained by high groundwater tables during spring months. For this reason aluvial soils are classified as having severe constraints for onsite systems.
Map 2 illustrates the slope analysis. Slopes of less than 15 percent offer the fewest constraints for onsite sewers. The next interval is between 15 and 30 percent. These areas have a moderate constraint because of construction difficulties and possible short circuiting of effluent to the surface. Slopes of greater than 30 percent are classified as severe constraints. These slopes typically hold very little soil, may be unstable if cuts are constructed downgradient and are prone to effluent surfacing at the toe of the system.


Map 3 is a composit of the soil, water table and slope constraints. It illustrates the relative suitability of land within the study area for onsite systems. This information can be used in conjunction with other relevent factors to make land use decisions. Development in areas of few or moderate constraints may be appropriate at moderate densities of one to two acres per unit. In those cases where a community water system, utilizing common wells is utilized, densities may be increased even further.
If development is directed toward areas of high constraints than lower densities, on the order of five acres per dwelling should be required. This is necessary to compensate for the greater probability of contaminating groundwater supplies. The alternative systems required in these areas are more expensive to install than those systems suitable for areas of lower constraints.
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MAP 1 - SOIL ANALYSIS
Mostly Soil Soil with Rock Rock with Soil Alluvial Soil
Few Constraints Moderate Constraints Severe Constraints Severe Constraints
-Wah Keeney
A m
l)
V
& \ \\ '2




- SLOPE ANALYSIS
â–º Few Constraints Moderate Constraints Severe Constraints
MAP 2
Less than 15%
15 - 30%
Greater than 30%
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Community Issues
The greatest potential for protecting groundwater from contamination exists at the community level. It is often easier for a landowner to protect his own well from contamination than that of his neighbor. Analysis of proposed subdivisions may adequately determine the constraints imposed by the property but community wide relationships between wells and absorption systems are often overlooked.
Chapter Three described the findings of a study of protective distances in Jefferson County in which a distance of 200 feet between wells and absorption fields was found to provide significant protection against nitrate contamination. This separation distance is based on a relatively uncontaminated source of ground-water up gradient from the point of discharge. In a residential subdivision of equally sized lots, effluent plumes up gradient will contribute to nitrate levels in wells and lower elevations. The farther down gradient one moves, the higher the contamination levels become.
If dwellings are clustered and share absorption fields, wells can be located up gradient from sources of contamination. In this way, all homes will receive high quality water. Figure 26 illustrates this principal. In both examples, the groundwater flow is assumed to be in one direction. Groundwater quality is the same at the
81


lower boundary for both examples.
Topography plays an important role with respect to groundwater quality. Concave slopes, or bowl shaped depressions, may serve to concentrate effluent from several sewer systems into one area.
The same density of dwellings on convex slope would have less effect on groundwater quality because the effluent plumes would tend to dispense in different directions. Figure 27 shows how this principal works. The arrows represent direction of flow within the groundwater table.
Topography can also be used to determine the locations of groundwater basins. In general, ridges and valleys form the boundaries of groundwater basins. Because very little interbasin transfer of water takes place, development located on these ground-water divides has less impact on one basin. This makes the necessary dilution much more effective. Figure 28 shows how groundwater divides can be used to segregate the flow of effluent plumes.
Soils, slope and depth to the groundwater table were utilized on the regional scale to rank land according to its relative suitability for onsite sewer systems. These criteria are also important on the community scale. In general, higher densities and shorter protective distances are possible in areas of low constraints. In areas of moderate or severe constraints, protective distances must be increased and densities lowered. Lotting arrangement plays an important role in this regard. In mountain areas, deposits of high quality soils occur in small pockets even though very little soil
82


exists overall. Instead of requiring large lots to compensate for poor site conditions, development can be clustered to take advantage of those areas which offer the fewest constraints for onsite sewer systems.
The major issue on a community scale is how to arrange absorption systems to obtain the maximum amount of treatment by the soil with the least amount of interaction with wells. Table 11 lists general principals by which land use and site conditions influence water quality on the community scale. It is apparent that controls on the installation of onsite sewage disposal systems which do not allow for consideration of the lands natural features are less effective than the use of planning principals which shape development to fit the land.
83


Figure 26
Subsurface effluent plumes from checkerboard and cluster lots (shown in red).
By clustering, all wells draw good quality water. Water quality is the same at the lowest level in both examles. A buffer zone, or groundwater divide, should be provided down gradient from the development.
84


Figure 27
The Effect of Topography
Concave slopes can serve to concentrate effluent into a smaller area, causing serious contamination of down-gradient wells. On convex slopes, effluent plumes scatter and do not concentrate down gradient.
85


Figure 28
Groundwater Divides as Boundaries Between Basins
Ridges and valleys form groundwater basin boundaries and can be utilized to segregate effluent plumes.
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Table 11
Principles for Water Quality Protection on the Community Scale
Natural Features
Land Use Factors Soils Slope Groundwater Topography
Density Density can be increased with increasing quality of soil to about one unit per acre. Moderate density in areas of low slopes to low densities in areas of steep slopes. Moderate densities for groundwater over 15 feet deep to very low densities in areas of shallow ground-water . Densities should be low in areas which may concentrate effluent in down gradient locations. Densities can be increased in areas which overlap ground water basins if suit able buffer zones are provided below.
Lotting Arrangement Checkerboard lots suitable for good soils. Cluster dwellings and share absorption systems to take advantage of good soil deposits . Checkerboard suitable for low slopes. Cluster on flat benches in areas of steep slopes. Locate development on high ground away from areas of shallow groundwater tables. Allow low lying areas to be buffer zoned. Take advantage of the shape of the land. Avoid concave slopes and utilize groundwater divides.
Protective Distance Between Wells and Absorption Fields Separat ion distances must be greater in poor soils or Wells should be located up slope from sources of contamination. Separation distances must be greater in areas of high ground-water. Both surface topography and sub-; surface fractures or strata must be
soils underlined by fractured bedrock . ! considered in predicting the trave path of effluent.


Site Specific Issues
Technical engineering issues predominate when considering water quality implications of a specific site. . If a sewage disposal system is not designed in accordance with the constraints present on the lot, planning at the regional and community scales will be of little value. Planning at larger scales should not be thought of as compensating for the failure of onsite systems, but rather as the arrangement of land uses that are compatable with properly designed sewer systems.
The elements of site analysis have been discussed in the first three chapters of this report. The following information is an example of how site specific issues should be addressed. It will begin with site analysis and proceed through the design and operation phases. The example is typical of conditions in the foothills of the Front Range of Colorado. Design requirements will vary according to state and local codes, but the general principal is applicable to other situations.
Figure 29 shows a plot plan and soil map of a two acre lot.
The site contains a large area of moderately sloped land in the northern portion. Due to possible groundwater constraints, the aluvial soil will not make as suitable a filtering material as the area marked mostly soil. The absorption system should be located away from the dry gulch to avoid infiltration and erosion during storms, The best place to install the system is in the location


shown on Figure 30.
The soil profile data indicates that partly weathered gneiss is present at 4^ feet below the surface. Because this does not provide a uniform filtering medium, an absorption system in fill material is called for.
It is anticipated that this partly weathered rock will break down during the excavation process and produce a suitable filtering material with percolation characteristics similar to that of the upper layer of mostly weathered rock. In this case there will be no need to import fill material, and the specifications call for replacing the native rock.
The maximum peak load anticipated on the system is 675 gallons per day. This is utilized in the formula to calculate the absorption area required. Figure 30 shows these calculations.
The effluent distribution lines are placed in a one foot layer of clean gravel. They are placed level to insure even loading over the field area. A 1,000 gallon septic tank, with internal baffels, is called for between the house and absorption field.
Figure 31 illustrates a report on the subsurface geology and is used to predict the flow of effluent from the absorption system toward the water table. It is assumed that the general flow of groundwater is to the north and west in the area of the system. The report indicates that the dip and strike configuration of the fracture
89


patterns will tend to direct the effluent toward the northeast until it reaches the water table. The safest location for the well is in the southwest corner of the property at a distance of at least 200 feet from the absorption field. In this location it is hydraulically upgradient from the sewer system and out of the path of effluent travel within the fracture patterns.
This example does not illustrate the only possible configuration for a well and sewage disposal system on the lot. A mounded absorption system could be installed in the aluvial soil along the stream, thereby designing around the possible high groundwater problem. It would also be possible to design an evaporative system although a southern exposure would be preferable. The point the examples attempt to make is that the best solution to locating the well and sewer system is the one which takes advantage of the natural opportunities present on the site rather than recycling or engineering solutions to overcome the constraints in other areas of the lot.
The importance of proper use of these systems is not specifically a land use issue, but it cannot be ignored if acceptable performance is expected. It is suggested that the owners of all new systems receive a data sheet describing the type of sewer system installed and instructions that will help insure its long life. Figure 32 shows a possible way of doing this.
90