Date of Award

1972

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

J.K. Neel

Abstract

The nutrient contribution of septic tanks to a lake was evaluated at Lake Sallie, in glacial outwash terrain of northwestern Minnesota. Groundwater entering the lake was collected by covering 0.258 m2 of lake bed with a bottomless cylinder vented to deflated plastic bag. Inflow velocity ranged from 0.01 to 2.5 micrometers per-second along 30% of the lakeshore. Groundwater inflow along an 800 m segment of shore amounted to 4.50 x 105 m3/year, and was uniformly distributed along the shore, but decreased exponentially away from shore.

Effluent from a heavily-used lakeside septic tank fanned out along the surface of the water table, and entered the lake within 9 m of shore. Phosphorus was fixed in the soil-near the septic tank but. 40% of the effluent nitrogen reached the lake. Sampling lakeward and landward of five septic tanks indicated that nitrate and possibly ammonium ions travel in groundwater and that nitrogen is contributed to groundwater by septic tanks.

Seepage entering the lake contained as much as 3.67 mg soluble orthophosphate/liter but no pattern was apparent regarding land use and phosphorus content of groundwater. A nearby eutrophic lake was a suspected source of both phosphorus and nitrogen in groundwater inflow. Along the only lakeshore adjacent to cropland, seepage was nitrate-rich (2.19 to 50.4 mg nitrate nitrogen/liter) within 8 m of shore and nitrate-poor (less than 0.022 mg nitrate nitrogen/liter) farther off.

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