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Disposal of Septic Tank Effluent in Mound and Sand Filter-Trench Systems on a Clay Soil¹

ABSTRACT

Small-scale mound systems and conventional sand filter-trench systems for disposal of septic tank effluent were established on a clay soil. During a wet year (1976), major differences in soil moisture patterns were observed between the two systems. Unsaturated flow of effluent occurred through the gravel/sand interface of the mound systems, although a slight pressure-head buildup occurred at the bottom of the mound fill. Conventional sand filter-trench systems were under water throughout most of the 1976 monitoring season. However, when the dry conditions of 1977 allowed the water table to drop below 1.2 m, unsaturated conditions occurred below the gravel/sand interface of both the mound and sand filter systems. Water tension patterns below the gravel/sand interface indicated development of the early stages of clogging (but not permanent ponding) in the mounds, while those within the sand filter-trench system indicated the development of a restricting layer. Rejuvenation of the mound infiltrative surface occurred during the warm and dry 1977 spring and early summer.

Temperatures below the gravel/sand interface dropped to about –3°C during the winter in the mound systems, but stayed just above freezing in the sand filters. A lack of snow accumulation on top of the mounds probably allowed the subzero temperatures.

Key Words: clogging • waste disposal

¹ Vermont Agric. Exp. Stn. J. Article no. 407. Partly funded by USDI through the Univ. of Vermont Water Resour. Res. Center.

² Graduate student and Associate Professor, respectively, Dep. of Plant and Soil Science, Univ. of Vermont, Burlington, VT 05405. Senior author is now Assistant Professor, Dep. of Plant and Earth Sci., Univ. of Wisconsin, River Falls, WI 54022.

Received for publication July 20, 1978.