Unsaturated flow during soil treatment of septic tank effluent

Subsurface soil disposal of septic tank effluent is governed by physical flow processes in unsaturated soil. The capacity of a soil to accept, conduct, and purify effluent cannot be expressed by percolation tests or by hydraulic conductivities at saturation alone. Field tests are available now to measure complete K-curves and application to many soils in Wisconsin has shown that four different types of curves can be distinguished. Effects of biological clogging or mechanical compaction, or puddling on infiltration into the soil are examined, considering different levels of the water table and using a simple one-dimensional flow theory. Results of in situ monitoring with tensiometers of 12 seepage systems in different soils and design criteria, some tentative, for innovative systems in a wide range of soils are considered. Use of soil survey information for purposes of extrapolation of research results from tested to untested sites is recommended.