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Changes in Concentrations of Selected Chemical Pollutants in Wet, Tile-drained Soil Systems as Influenced by Disposal of Septic Tank Effluents¹

ABSTRACT

Changes in P, MBAS, NH₄, NO₃, NO₂, Cl, pH, and Eh from septic effluent disposal were monitored in shallow ground waters between septic effluent disposal fields and an agricultural tile drainage system in an area composed of Typic and Aeric Ochraquults.

As effluent moved through the soil system toward the agricultural tile, P and MBAS decreased logarithmically with distance. As much as 87% of the observed variation could be attributed to distance for both P and MBAS. Phosphorus and MBAS in ground waters intercepted by the tile drainage system at two of the sites had been reduced to acceptable levels. At location 1 (Typic Ochraquult), P and MBAS moved further (12 m) than might be predicted and were present in ground waters intercepted by the agricultural tile. These concentrations of P and MBAS in waters intercepted by the tile at location 1 resulted from rapid movement of effluent via saturated flow through the larger soil pores and from an inadequate soil volume for effluent purification.

The concentrations of inorganic nitrogenous components in relation to the disposal area were similar at all three locations. The NH₄-N concentration was highest adjacent to the drainfield (20–30 µg/ml) and decreased logarithmically with distance. A maximum of 83% of the observed decrease in NH₄ could be attributed to distance. The NO₃ concentration increased to a maximum at approximately 5 m and then decreased with increased distance. This was substantiated by the NO₃/Cl ratios which indicated that NH₄ was nitrified and the NO₃ produced was subsequently denitrified to a large degree. The NO₃ accumulation was seasonal with the highest concentrations present during periods of rising water tables.

Key Words: phosphorus • methylene blue active substances • nitrogenous components • inorganic soil P • extractable NH₄ • seasonal water tables

¹ Contribution from the Dep. of Agronomy, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061. This work was supported by the Virginia Dep. of Health.

² Associate Professor of Agronomy.

Received for publication March 27, 1978.