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Columns Representing Mound-Type Disposal Systems for Septic Tank Effluent: I. Soil-water and Gas Relations¹

ABSTRACT

Columns were designed to represent the vertical dimensions of a mound-type disposal system for receiving septic tank effluent on problem soils. The columns were filled with gravel (representing creviced bedrock), silt loam (representing the original topsoil), a sand or sandy loam till (fill material), gravel (the seepage bed), and another layer of silt loam (the mound cover). The columns were dosed with septic tank effluent at the rate of 2 cm every 6 hours. Until crusting caused permanent ponding at the fill-gravel interface, the fill was aerobic and the silt loam at the bottom of the column was anaerobic. Redox potentials were higher in the fill (350 to 600 mV) than in the silt loam (200 to 410 mV). Moisture tension fluctuations after a 2 cm addition were greatest near the fill-gravel interface and decreased with depth in the column. After continuous ponding, tension fluctuations almost ceased, the subcrustal soil became anaerobic, and the redox potentials decreased to around –250 mV. In a separate experiment, simulating field conditions, aerobic conditions were maintained in the subcrustal fill, by perforating column walls. The moisture tension regime and the rate of crust formation were similar to nonperforated columns.

Key Words: soil crusting • unsaturated water flow • redox potentials • crusting • methane

¹ Contribution from the Soil Science Dep., Univ. of Wis., and the Geological and Natural History Survey, University-Extension Madison, Wis. 53706. This research was part of the Small Scale Waste Management Project, funded by the State of Wisconsin and the Upper Great Lakes Reg. Comm.

² Postdoctoral Fellow, and Associate Professors of Soil Science, respectively, Univ. of Wis. Senior author is presently Assistant Professor of Plant and Soil Science, University of Vermont, Burlington, 05401.

Received for publication June 22, 1973.