Institute Of Food And Agricultural Sciences
University Of Florida
Florida Cooperative Extension Service

December, 1987 No. 31

Notes in Soil Science

On-Site Sewage Disposal - Influence Of System Densities On Water Quality

Importance of Scale, Studies Concerned with OSDS Densities, Conclusion, List of References Cited, Crediting Information

This is one of a series of NOTES IN SOIL SCIENCE addressing behavior of effluent and effluent constituents during on-site sewage disposal. This NOTE is intended to give environmental health officials, soil scientists, consulting engineers, and others interested in the environmental impact of septic tanks a basic understanding of the influence of on-site sewage disposal system densities on water quality.

Many regulatory agencies controlling on-site sewage disposal system (OSDS) installations depend on setback distance requirements between wells and OSDSs, minimum percolation rates, and/or absorption field sizing requirements to provide adequate dilution and attenuation of chemical and biological contaminants and thus prevent contamination of ground water and drinking water supplies (Perkins, 1984; Yates, 1985). In order to comply with established distance requirements for separation of OSDSs and private water wells, lots must have a minimum linear dimension greater than the minimum setback distance. If a municipal or community water supply exists, minimum lot size is commonly decreased and may be limited only by the area necessary to locate the dwelling and its OSDS. Municipal water supplies may alleviate the concern for contamination of private water supply wells by OSDSs, but can result in increased contamination of local or regional ground water by allowing increased OSDS density. Population density ultimately determines the effluent load per unit of land area and hence the concentration of contaminants in ground water.

Importance of Scale

The apparent importance of OSDS density (i.e., the number of OSDSs per unit land area) depends in part on the geographic scale that one employs to examine the question. Gainesville Regional Utilities (GRU) (1986) used water budgets to show that the contribution of septic tanks to ground water recharge depends on the size of the area being considered. If one considers the 52,000-acre Gainesville urban area as a whole, septic tanks contribute roughly 10 percent of the total recharge to the shallow aquifer, with the remaining 90 percent coming mainly from downward infiltration of rainfall and irrigation water. GRU noted that, since the rate of recharge (~0.8 inches/year) is small compared to other inflows and outflows in the regional water budget, the potential for pollution from OSDSs regionally is low.

However, when a water budget was generated for an individual 0.4-acre lot, rather than for the region as a whole, the potential for pollution from the OSDS was higher. At this level of consideration, GRU calculated that an OSDS would contribute nearly 60 percent of the total recharge to the shallow aquifer. A typical OSDS would contribute roughly 12 inches/year to total recharge beneath a 0.4-acre lot, and thus have a significant influence on the amount and quality of water reaching the shallow aquifer.

If one's interest were confined instead to the area immediately underneath the OSDS drainfield itself, the recharge rates from the OSDS could be of the order of several hundred inches/year, and thus overwhelm any other sources of groundwater recharge.

The GRU study gave some additional attention to the impact of OSDS densities on water quality (as opposed to quantity of recharge), by considering available data on the water quality of urban runoff. It was found that other urban pollution sources contribute more total loading, and are more significant than septic tanks, over the entire Gainesville urban area. On a localized basis, however, OSDSs would be expected to degrade shallow ground water significantly at densities of about 2 OSDSs/acre (0.5 acre/OSDS) or greater. These conclusions point up the importance of scale in evaluating OSDS densities. Average OSDS density in the Gainesville urban area is about 0.17 OSDS/acre (6 acres/OSDS). As shown above, however, if the entire urban area were made up of 0.4-acre lots served by OSDSs, roughly 60 percent of recharge to the shallow aquifer across the 52,000-acre area would come from OSDSs. Concern regarding the impact of OSDSs would be heightened in comparison with the concern generated at the current average density.

The Florida Department of Environmental Regulation (1979) determined county-wide OSDS densities for the forty-one counties in Florida that were not included in designated Section 208 planning areas. The number of OSDSs in each county was obtained from either 1970 census data (U.S. Department of Commerce, 1970) updated with data from the Florida Department of Health and Rehabilitative Services (1979), or from survey estimates submitted by county sanitarians, whichever was larger. Land areas were adjusted to developable land acreages by excluding wildlife management areas, national parks, wildlife refuges, etc. On-site sewage disposal system density ranged from 0.303 OSDSs/acre (3.3 acres/OSDS) in Monroe County to 0.0057 OSDSs/acre (175 acres/OSDS) in Hamilton County. These densities are below those recommended by Betz {1975), Holzer (1975), Peavy and Brawner (1979), and Starr and Sawney (1980) to prevent deterioration of ground water by OSDSs. These counties, however, generally represent the rural areas of the state and lack major population centers. Considerably higher county-wide OSDS densities exist in Dade, Broward, and Hillsborough counties, and probably in other areas as well. The Florida Department of Environmental Regulation (1979) identified Duval, St. Johns, Dixie, Marion, Lake, Highlands, Indian River, Martin, and Monroe Counties as areas in the non-designated Section 208 regions where OSDSs may impact ground water quality, based on OSDS density and septic tank soil suitability ratings (Soil Conservation Service, 1978) for the soil associations found in the counties. Some of the counties identified by the Florida Department of Environmental Regulation as having a potential to contaminate ground water are located in high recharge areas for the Floridian aquifer (Stewart, 1980), and therefore may have particularly high potential for affecting water quality.

Studies Concerned with OSDS Densities

Several researchers across the U.S. have studied the influence of OSDS densities on water quality. Studies have involved actual measurements and/or computer modeling. Results are summarized in the following paragraphs.

Woodward et al. (1961) reported a correlation between rural population density and well contamination near Coon Rapids, Minnesota. An area with a population density of 0.54 persons/acre (7.4 acres/OSDS) had two percent of its private water wells contaminated with nitrate, while an area with a population density of 2.7 persons/acre (1.5 acres/OSDS) had more than 29 percent of its private water wells contaminated with nitrate.

Miller (1972, 1975) recommended that house lot size requirements in Delaware be increased from 0.5 acre to 2.0 acres after a water quality survey indicated that 25 percent of the water wells in the shallow water table aquifer had nitrate-nitrogen concentrations of 4.5 mg/liter (twice background levels). Nitrate-nitrogen concentrations in ground water from areas with well drained soils and lot sizes ranging from 0.25 to 0.5 acres were as high as 31 mg/liter.

Walker et al. (197Sa, 1973b) estimated that a maximum allowable OSDS density of 2 OSDSs/acre (0.5 acre/OSDS) would be necessary to insure ground water dilution of nitrate-nitrogen to concentrations below 10 mg/liter (the EPA standard) in loamy sand soils at four sites in Wisconsin. Nitrate contributions from OSDS effluent to ground water in sands were estimated to be approximately equal to those from natural sources (i.e., rainfall and decomposition of organic matter) when one OSDS was located on six acres of land.

Morrill and Toler (1973) indicated that the contribution of OSDS to dissolved solids load or to soluble salt concentrations in streams draining seventeen small drainage basins near Boston, Massachusetts could be predicted on the basis of OSDS density. OSDS density ranged from 0 to 900 OSDSs/square mile (0.7 acre/OSDS). In the range of housing densities observed, the dissolved solids concentration in stream flow was found to increase by 10 to 15 mg/liter per 100 houses per square mile.

Pitt (1974) and Pitt et al. (1975) monitored ground water quality near Homestead, Florida in an area with OSDS densities of 4 OSDSs/acre (0.25 acre/OSDS) and l OSDS/acre (1 acre/OSDS). Slightly higher concentrations of sodium, total coliforms, fecal coliforms, and fecal streptococci were detected in ground water at the higher OSDS density.

Geraghty and Miller (1978) collected 865 ground water samples from 54 wells on Long Island, New York and correlated nitrate concentration with OSDS density. A nitrate-nitrogen concentration in ground water of 10 mg/liter or more was detected in fifty percent of the ground water samples when OSDS density exceeded 2.8 OSDSs/acre (0.36 acre/OSDS). Where densities were less than 1.25 OSDSs/acre (0.8 acre/OSDS), less than ten percent of the ground water samples contained nitrate-nitrogen concentrations of 10 mg/liter or more.

Konikow and Bredhoeft (1978) developed a computer simulation model to evaluate the effects of OSDS density on water quality of the Rio Grande alluvial aquifer in New Mexico. They concluded that steady state levels of nitrate in ground water may not be reached for many decades, and that the effect of lot size on nitrate concentrations in ground water is not necessarily a linear function. Predicted nitrate-nitrogen concentrations of ground water after 10 years of OSDS effluent applications in the Rio Grande Valley were 60 mg/liter below 0.25 acre house lots and 35 mg/liter under 1.2 acre house lots. Nitrate concentration in ground water was dependent on lot size, ground water mixing, street orientation with respect to ground water flow direction, and ground water velocity.

Ford et al. (1980) reported that nitrate contamination of ground water was associated with increased housing density in unsewered residential areas of Jefferson County, Colorado. Contamination of ground water with nitrate-nitrogen concentrations exceeding 20 mg/liter was associated with OSDS densities exceeding 1 OSDS/acre and with well setback distances of 100 ft or less.

Duda and Cromartie (1982) and Everette (198Z) related closure of shellfish harvesting beds to density of OSDSs along the coast of North Carolina. They examined the bacteriological quality of surface water from tidal estuaries and tributary freshwater creeks with different OSDS densities in four coastal watersheds. No industrial or point-source discharges were located in the watersheds, and all residential developments utilized OSDSs. The watersheds ranged in size from 0.2 to 1.35 square miles. On-site sewage disposal system density ranged from 0.08 to 0.52 OSDSs/acre (12.5 to 1.9 acres/OSDS). A highly significant correlation was found between bacterial levels in surface water and increasing density of OSDSs. On-site sewage disposal system densities greater than 0.17 OSDSs/acre (5.9 acres/OSDS) resulted in closure of shellfish harvesting beds in the watersheds examined. Forty-five to seventy percent of the OSDSs were estimated to be located in soils with severe limitations for on-site sewage disposal.

Trela and Douglas (1978) developed a model to estimate OSDS density which would prevent nitrate-nitrogen concentrations in ground water from exceeding 10 mg/liter below sandy soils in the New Jersey Pine Barrens. The minimum land area or lot size was 0.2 acre per capita, or 0.8 acre per household, assuming a family of four.

Brown (1980) and Tateman and Lee Associates, Inc. (1983) modified the model proposed by Trela and Douglas (1978) and calculated a minimum land area or lot size needed to prevent nitrate-nitrogen concentration in ground water from exceeding 10 mg/liter. Brown (1980) determined that a minimum land area of 0.34 acres/OSDS was necessary in Texas. Tateman and Lee Associates, Inc. (1983) calculated that a land area of 0.25 acres/capita or 1 acre/OSDS was necessary to achieve the same result in Delaware.

Holzer (1975), Peavy and Brawner (1979), and Starr and Sawhney (1980) recommended that OSDS density should not exceed an average of one system per acre (1 acre/OSDS) on well drained soils, and Olivieri et al. (1981) suggested that maximum overall OSDS density should be one OSDS per 1.4 acres (1.4 acre/OSDS) in order to maintain high-quality ground water and protect public health.

Bauman and Schafer (1985) employed a simplified model and sensitivity analysis to show that potential for ground water contamination with nitrate depends heavily on OSDS densities along with several other factors. These other factors include hydraulic conductivity of the aquifer and gradient of the ground water (which influence velocity of ground water), natural rates of recharge to the ground water, and concentrations of effluent nitrate reaching the ground water. Areas with high velocities of ground water flow have greater dilution of nitrate than areas with low velocities Sensitivity analysis showed ground water nitrate levels increasing with increasing densities, with a particularly high rate of increase at densities above about 0.7 OSDS/acre (1 to 1.5 acres/OSDS). In "high velocity" ground water systems (e.g., where saturated hydraulic conductivity of the aquifer is 0.01 cm/sec and gradient, or slope, of the water table is 0.01), lot sizes of less than one acre may cause no pollution with nitrate. On the other hand, "low velocity" systems (e.g., where saturated hydraulic conductivity is 0.001 cm/sec and gradient is 0.001) may have nitrate levels in excess of drinking water standards with lot sizes of 5 acres or even greater.

Russell and Axon, Inc. (l979, 1980) studied potential health hazards in the Loxahatchee River Environmental Control District (southeast Martin County and northeast Palm Beach County, Florida). They estimated that, in part of the study area, densities greater than 2 units/acre (0.5 acre/OSDS) were exceeding the assimilative capacity of the shallow aquifer and thus causing ground water contamination.

Harkin et al. (1979) indicated that nineteen mound systems monitored in Wisconsin supplied considerably less nitrate to ground water than did conventional OSDSs, due to enhanced nitrification in the mounds and subsequent denitrification in the underlying anaerobic soil. Harkin et al. (1979) calculated that mound system densities of greater than 1.3 mounded OSDSs/acre (0.8 acre/mounded OSDS) would be necessary to reach an overall nitrate-nitrogen concentration of 10 mg/liter in ground water.

Kaplan (1987) provided a brief commentary on some of the pitfalls in modeling the effects of OSDS densities on water quality. He pointed out that oversimplification may limit the usefulness of some mathematical models. For example, a model might show that an area as a whole does not suffer from nitrate pollution from OSDSs, even though some individual wells within the area might in fact be affected.

A high proportion of the above studies have used nitrate and other chemicals as the key variables in estimating acceptable OSDS densities. Few models exist that include computation of the behavior of bacteria and viruses, which generally are the more important causes of waterborne diseases (Yates, 1985). In one such study, Yates et al. (1986) used geostatistics and measured decay rates of MS-2 coliphage in ground water from 71 drinking water supply wells to estimate setback requirements necessary to protect ground water in a 26 by 16 km (16 by 10 mi.) area in Arizona. They found wide variations in survival times of viruses in water from the different wells, due mainly to different temperatures of ground water at the various well sites. This phenomenon, along with variations in hydraulic gradient and hydraulic conductivity throughout the study area, resulted in wide variations in the setbacks required between OSDSs and drinking water wells. Setback distances thus generated ranged from 15 m (49 ft) to over 150 m (490 fit).

The Leon County Public Health Unit (19S7) and several other agencies studied poor OSDS performance in a subdivision of dominantly 1/4-acre lots. Water quality seemed not to have been reduced by OSDS practices in the subdivision, but hydraulic failure rates of OSDSs were high, even for soils that would ordinarily be considered well suited for OSDSs. The failures were found to be due to the artificial elevation of the water table by high housing densities and a sheetflow type of stormwater collection system, which together put more water into smaller areas of land than would naturally be available for infiltration.

Research currently underway on behalf of the Environmental Health Program Office, Florida HRS, is aimed in part at assessing the environmental impact of various OSDS densities under the different hydrogeologic conditions that exist around the state. It is hoped that the effort will improve understanding of the environmental risks associated with differing OSDS densities in soils and geologic strata that vary in permeability, in ability to retard movement of contaminants to groundwater, in tendency to move and disperse contaminants in groundwater, and in other inherent attributes that influence contaminant transport.


Several studies employing measurements and/or modeling have demonstrated a positive correlation between water contamination and OSDS density. Most of the studies estimated that the minimum lot size necessary to ensure against contamination is roughly one-half to one acre. Some studies, however, found that lot sizes in this range or even larger would cause contamination of ground or surface water.

Most studies of density have been confined in scope to nitrate related phenomena, and most were done in climatic/soil/geologic conditions different from those of Florida. Additional research is necessary to improve estimates of appropriate densities for the various soil, water table, geologic, and hydrologic conditions that exist in Florida.

List of References Cited

Bauman, B. J., and W. M. Schafer. 1985. Estimating ground-water quality impacts from on-site sewage treatment systems. p. Z85-294. In On-site wastewater treatment. Proceedings of the Fourth National Symposium on Individual and Small Community Sewage Systems, held Dec. 10-11, 1984, New Orleans, LA. ASAE Publication 07-85. Am. Soc. A9. Eng., St. Joseph, MI.

Betz, J. V. 1975. Proposed standards: Land disposal of effluents. Civil Eng. 45(5):77-79.

Bicki, T. J., R. B. Brown, M. E. Collins, R. S. Mansell, and D. F. Rothwell. 1984. Impact of on-site sewage disposal systems on surface and ground water quality. Report to Fla. Dept. of Health and Rehabilitative Services under contract number LC170. Soil Sci. Dept., Inst. of Food and Agric. Sci., Univ. of Fla., Gainesville.

Brown, K. W. 1980. An assessment of the impact of septic leachfields, home lawn fertilization, and agricultural activities on groundwater quality. K. W. Drown Assoc., College Station, Texas.

Duda, A. M., and K. D. Cromartie. 1982. Coastal pollution from septic tank drainfields. J. Env. Eng. Div. Am. Soc. Civ. Eng. 108:lZb5-1279.

Everette, G. 1982. The impact of septic tanks on shellfish waters. North Carolina Div. of Env. Mgt., Shellfish Sanitation Unit, Dept. of Human Resources.

Florida Department of Environmental Regulation. 1979. Septic tank nonpoint source element. State water quality management plan. Tallahassee, FL.

Florida Department of Health and Rehabilitative Services. 1979. Septic tank installations by counties. Tallahassee, FL.

Ford, K. L., J. H. S. Schoff, and T. J. Keefe. 1980. Mountain residential development minimum well protective distances -- Well water quality. J. Env. Health 43:130-133.

Gainesville Regional Utilities. 198b. Onsite systems for wastewater treatment in the Gainesville Urban Area. Prepared by the Strategic Planning Department of Gainesville Regional Utilities for the Water Management Advisory Committee to the Gainesville City Commission. Gainesville Regional Utilities, Gainesville, FL.

Geraghty, J. J., and D. W. Miller. 1978. Development of criteria for wastewater management policy related to population density. Manuscript prepared for Suffolk County, NY. R. S. Kerr Env. Res. Lab., Ada, Oklahoma.

Harkin, J. M., C. J. Fitzgerald, C. P. Duffy, and D. G. Kroll. 1979. Evaluation of mound systems for purification of septic tank effluent. Water Resources Center Tech. Rep. WIS WRC 79-50. Univ. of Wisconsin, Madison.

Holzer, T. L. 1975. Limits to growth and septic tanks. p. 65-74. In Jewell, W. J., and R. Swan (ed.) Water pollution control in low density areas. Univ. Press of New England, Hanover, NH.

Kaplan, O. 8. 1987. Septic systems handbook. Lewis Publishers, Inc., Chelsea, MI.

Konikow, L. F., and D. J. 8redehoeft. 1978. Computer model of two dimensional solute transport and dispersion in groundwater. Techniques of Water Resources Investigations, U.S. Geological Survey. U.S. Env. Prot. Agency Rep. No. 024-0001-03130-2.

Leon County Public Health Unit. 1987. Killearn Lakes waste disposal study. Prepared for the Board of County Commissioners. Tallahassee, FL.

Miller, J. C. 1972. Nitrate contamination of the water table aquifer in Delaware. Delaware Geol. Surv. Rep. Inv. No. 20.

Miller, J. C. 1975. Nitrate contamination of the water table aquifer by septic tank systems in the coastal plain of Delaware. p. 121-133. In Jewell, W. J., and R. Swan (ed.) Water pollution control in low density areas. Univ. Press of New England, Hanover, NH.

Morrill, G. 8., III, and L. G. Toler. 1973. Effect of septic tank wastes on quality of water, Ipswich and Shawsheen River Basins, Massachusetts. J. Res. U.S. Geol. Surv. 1:117-120.

Olivieri, A. W., R. J. Roche, and G. L. Johnston. 1981. Guidelines for control of septic tank systems. J. Env. Eng. Div. Am. Soc. Civ. Eng. 107:1025-1034.

Peavy, J. S., and C. E. 8rawner. 1979. Unsewered subdivisions as a non-point source of groundwater pollution. Presented at National Conf. on Env. Eng., July 9-11, San Francisco.

Perkins, R. J. 1984. Septic tanks, lot size and pollution of water table aquifers. J. Env. Health 46:298-304.

Pitt, W. A., Jr. 1974. Effects of septic tank effluent on groundwater quality, Dade County, Florida -- An interim report. U.S. Geol. Survey Open-File Rep. 74010.

Pitt, W. A., Jr., H. C. Mattraw, Jr., and H. Klein. 1975. Groundwater quality in selected areas serviced by septic tanks, Dade County, Florida. U.S. Geol. Survey Open-File Report 75-607.

Russell and Axon, Inc. 1979. Loxahatchee River environmental control district. Septic tank study: Phase 1. Russell and Axon, Inc., Daytona Beach, FL.

Russell and Axon, inc. 1980. Loxahatchee River environmental control district. Septic tank study: Phase 2. Russell and Axon, Inc., Daytona Beach, FL.

Soil Conservation Service. 1978. National soils handbook -- Notice 24 USDA, Washington, DC.

Starr, J. L., and 8. L. Sawhney. 1980. Movement of nitrogen and carbon from a septic system drainfield. Water, Air, and Soil Poll. 13:113-128.

Stewart, J. W. 1980. Areas of natural recharge to the Floridan Aquifer in Florida. U.S. Geol. Survey and Florida Dept. of Nat. Res., Burl of Geol., Map Series 98. Tallahassee.

Tateman and Lee Associates, Inc. 1983. Nitrate movement in groundwater. Supplement to Working Paper No. 2. Tateman and Lee Associates, Inc., Wilmington, Delaware.

Trela, J. J., and L. A. Douglas. 1978. Soils, septic systems and carrying capacity in the Pine Barrens. Proc. and Papers of First Res. Conf. N.J. Pine Barrens: 37-58.

U.S. Department of Commerce. 1970. Census of population and housing. Advance estimates of social, economic, and housing characteristics -Florida Bureau of the Census, Washington, D.C.

Walker, W. G., J. comma' D. R. Keeney, and F. R. Magdoff. 1973a. Nitrogen transformations during subsurface disposal of septic tank effluents in sands: I. Soil transformations. J. Env. Dual. 2:475-479.

Walker, W. G., J. 90uma, D. R. Keeney, and P. G. Olcott. 1973b. Nitrogen transformations during subsurface disposal of septic tank effluent in sands: II. Groundwater quality. J. Env. Dual. 2:521-525.

Woodward, F. L., F. J. Kilpatrick, and P. 9. Johnson. 1961. Experiences with groundwater contamination in unsewered areas in Minnesota. Am. J. Public Health 51:1130-1136.

Yates, M. V. 1985. Septic tank density and ground-water contamination. Ground Water 23:586-591.

Yates, M. V., S. R. Yates, A. W. Warrick, and Charles P. Gerba. 1986. Use of geostatistics to predict virus decay rates for determination of septic tank setback distances. Appl. Environ. Microbial. 52:479-483.


This fact sheet is one a series of NOTES IN SOIL SCIENCE addressing on-site sewage disposal, and draws heavily on a major literature review entitled "Impact of On-Site Sewage Disposal Systems on Surface and Ground Water Quality" (Bicki et al., 1984). Inquiries as to the availability of that report should go to Mr. Eanix Poole, Director, Environmental Health Program Office, 1317 Winewood Blvd., Tallahassee., FL 32301 (904-488-4070; SUNCOM 278-4070).

For further information regarding the scientific literature on soils and on-site sewage disposal, contact Randy Brown at the address and phone number indicated below.

Prepared by:

R.B. Brown, Professor, Extension Specialist in Soils and Land Use, Soil Science Department, G-159 McCarty Hall University of Florida, Gainesville, FL 32611
T.J. Bicki, Former Visiting Assistant Professor, Currently Extension Specialist in Pedology and Soil Management, University of Illinois

This NOTE is published as part of a project entitled "Determining Soil-Water Seasonal Movement," supported in part by funds from the U.S. Geological Survey through the Florida Water Resources Research Center, University of Florida, Gainesville.

Published on the web with permission. 1997.

Purdue On-Site Wastewater Disposal