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Nitrogen Transformations During Subsurface Disposal of Septic Tank Effluent in Sands: I. Soil Transformations¹

ABSTRACT

Soil physical and chemical studies of five subsurface septic tank seepage beds were conducted to determine the biochemical transformations of N and thereby its potential for ground-water pollution. Effluent was found to be ponded in all the seepage beds examined due to the presence of an impeding layer, a "crust", at the boundary between the gravel bed and adjacent soil. The crust reduced infiltration rates approximately from 500 to 8 cm/day. Soil atmospheric composition 5 cm below the crust averaged 19.6% O₂ and 0.66% CO₂. Nitrogen in the septic tank effluent occurred as NH₄-N (80%) and organic N (20%) with virtually no NO₃-N. Organic-N was largely concentrated in the crust zone. Nitrification of NH₄-N to NO₃-N was essentially complete and commenced in the unsaturated subcrust soil within about 2 cm of the crust. Nitrification did not occur and NH₄-N was absorbed by the soil below a seepage bed that was submerged in the ground water.

Key Words: soil crusting • unsaturated hydraulic conductivity • nitrification • nitrate • pollution

¹ Research supported in part by the College of Agricultural and Life Sciences, Univ. of Wisconsin, Madison; Geological and Natural History Survey, Univ. Ext., Madison; and by the Wisconsin Dep. of Natural Resources.

² Research Assistant, Associate Professors, and Post Doctoral Fellow, respectively, Department of Soil Science, Univ. of Wisconsin, Madison 53706.

Received for publication February 15, 1973.