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Septic System Performance on a Coastal Barrier Island

Western Washington Res. and Ext. Ctr., Puyallup, WA 98371-4998 (formerly Dep. of Soil Sci., North Carolina State Univ.)

L. M. Hajjar

Minnesota Pollut. Control Agency, St. Paul, MN 55155 (formerly Dep. of Soil Sci., North Carolina State Univ.)

C. L. Moe and M. D. Sobsey

Dep. of Environ. Sci. and Eng., Univ. of North Carolina, Chapel Hill, NC 27514.

^* Corresponding author.

ABSTRACT

A field experiment was conducted on a Corolla fine sand (Aquic Quartzipsamment) to evaluate wastewater treatment by septic systems under conditions typical of Atlantic coastal barrier islands. The effects of loading rate and water table depth on wastewater treatment were studied. Two absorption fields were installed and were designated the upper and lower fields based on their relative topographic locations. The trenches in the lower field were 30 cm closer to the water table than the trenches in the upper field. Each field consisted of nine absorption trenches. Three loading rates (1, 4, and 16 cm d^–1) were each replicated three times in a completely randomized design. Groundwater samples around the trenches were collected biweekly from shallow wells (1.5–1.9 m deep) between May 1982 and December 1983, and were analyzed for NO^–₃-N, NH⁺₄-N, soluble P, Na⁺, Cl^–, K⁺, and fecal coliform bacteria. During the first year the water table was within 30 cm of the lower field trenches 20% of the time, and anaerobic conditions prevailed. Groundwater samples collected adjacent to the trenches contained predominantly NH⁺₄-N, fecal coliform counts averaged over 25 000 MPN (most probable number) L^–1, and the redox potential averaged <300 mV. During the second autumn the lower field showed aerobic tendencies due to drier weather and a deeper water table. The upper field remained aerobic throughout, with almost complete nitrification occurring. Average fecal coliform counts were reduced to 60 MPN L^–1, and the mean redox potential was >500 mV. The wastewater loading rate had a significant effect on the levels of all constituents, but the loading rate effect was smaller than the water table effect. Phosphorus movement appeared to be related to pore-water velocity.

Paper no. 9287 of the Journal Series of the North Carolina Agric. Res. Service, Raleigh.

Received for publication April 6, 1987.