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Movement of Coliform Bacteria from Septic Tank Effluent through Selected Coastal Plain Soils of Virginia¹

ABSTRACT

Movement of total and fecal coliform bacteria form septic tank effluent through three Virginia Coastal Plain soils was monitored in situ over a 2-year period. These soils were considered to be marginally suited for sanitary disposal of domestic wastes because of fluctuating seasonal water tables and/or restricting layers. Since septic effluent moved predominantly in a horizontal direction in these soils as a result of slowly permeable subsurface horizons, series of piezometers were installed to collect samples at selected distances and depths from the source (subsurface drainfield) in each of the soils studied. Generally, the most-probable-numbers (MPN) of both total and fecal coliforms decreased significantly with horizontal distance and depth. At the Varina location total and fecal coliform counts were observed to decrease from 11 and 1.3 x 10⁶/100 ml in the distribution box to 11 and 2.5 x 10³/100 ml at 6.1 m, respectively, above a relatively impermeable plinthic horizon. At the Goldsboro and Beltsville sites large reductions in both total and fecal coliform counts were noted within 13.5 m. Few detectable fecal coliform were present below the restricting layers at all three sites. On the basis of these results, it seems unlikely that coliform bacteria would move into the permanent ground water system.

Key Words: total coliform • fecal coliform • biological contaminants

¹ Contribution from the Department of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061. This work was supported by the Virginia Department of Health. Presented in part before Div. A-5 of ASA at Miami Beach, Fla., 1 Nov. 1972.

² Assistant and Associate Professors of Agronomy, respectively.

Received for publication December 9, 1973.