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Spatial and Temporal Variation of Fecal Coliform Movement Surrounding Septic Tank-Soil Absorption Systems in Two Atlantic Coastal Plain Soils¹

ABSTRACT

Field experiments involving residential, conventional septic tank-soil absorption systems (ST-SAS) were conducted to determine the extent, direction, and seasonal variation of fecal coliform movement away from drainfields placed in two poorly drained Coastal Plain soils with seasonally high fluctuating water tables. Monitoring wells at 152- and 305-cm depths were placed in and around two ST-SAS that had been in operation for about 2 years. Fecal coliform levels were measured one to two times per month from January 1977 to September 1979. One site had effective artificial drainage that created a water table gradient of 3.2% and resulted in lateral mass flow of bacteria, up to 10 m, in the direction of the gradient. At this site the predominate fecal coliform movement was horizontal, as indicated by an average concentration of <3 Most Probable Number (MPN)/100 ml in the deep wells within the drainfield. The lack of a discernible water table gradient at the second site, which employed only shallow surface drains, resulted in downward movement of fecal coliforms with the deep wells, averaging 10 MPN/100 ml compared with the 45 MPN/100 ml average in the shallow wells within the drainfield. Fecal coliform movement at both sites was almost entirely limited to periods when the water table was at or above the drainfield trench bottoms. Nonuniform distribution of septic tank effluent throughout the entire drainfield was apparent at both locations. The results indicate placement of conventional ST-SAS in high-water-table soils may result in contamination of shallow ground waters (305 cm).

Key Words: ground-water pollution • water table • onsite disposal • artificial drainage

¹ Contribution of the Dep. of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.

² Research Associate and Associate Professor of Agronomy, Virginia Polytechnic Inst. and State Univ.

Received for publication November 4, 1980.