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Field Performance of Conventional and Alternative Septic Systems in Wet Soils¹

ABSTRACT

An 18-month field evaluation of 15 conventional and alternative septic systems was conducted to determine their effectiveness in soils with high water tables. The alternative designs included four low-pressure pipe (LPP) systems, two soil replacement systems, and two pressure-dosed mounds. Water tables were measured and groundwater samples collected monthly from shallow wells placed around each system. The water samples were analyzed for NO₃^–-N, NH₄⁺-N, total Kjeldahl N, total P, and fecal coliform bacteria.

All but one of the systems installed beneath the natural soil surface were saturated with groundwater for at least part of the year. Effluent treatment was poorest around systems that were continuously saturated with groundwater, with average levels of 7.3 mg/L NH₄⁺-N, 1.0 mg/L NO₃^–-N, and 1700 fecal coliform most probable number (MPN)/L found in the groundwater in wells 1.5 m from these systems. Systems that were only seasonally saturated generally performed better, with average levels of 2.3 mg/L NH₄⁺-N, 2.0 mg/L NO₃^–-N, and 560 fecal coliform MPN/L in the groundwater. Mean P levels in the groundwater exceeded 1 mg/L around six of the systems. The highest concentrations were associated with the most heavily loaded systems. Levels of all contaminants decreased with distance from the source. The greatest amount of lateral transport occurred when continuous saturation was accompanied by a steep groundwater gradient.

Improved treatment was obtained using the LPP alternative, provided site and soil conditions were acceptable. Soil replacement systems did not improve treatment, while one mound failed due to overloading and abuse.

Key Words: on-site sewage treatment • groundwater pollution • low-pressure pipe system • high water tables

¹ Paper no. 8817 of the Journal Series of the North Carolina Agric. Res. Service, Raleigh. Research supported by funds from the Coastal Plains Regional Commission.

² Research Associate and former Associate Professor, Dep. of Soil Sci., North Carolina State Univ., Raleigh, NC 27650. Second author is currently Extension Specialist, Dep. of Crop and Soil Sci., Texas A&M Univ., College Station, TX 77840.

Received for publication April 18, 1983.

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