Sizing onsite wastewater treatment systems using soil characteristics as compared to the percolation test

This study is a direct comparison of sizing filter fields based upon percolation test results and soil morphology. Eighteen sites were selected to compare soil morphology and percolation tests. Soil profile descriptions were written using the Natural Resources Conservation Service (NRCS) soil description guide and percolation tests were performed in strict accordance with the Arkansas Department of Health rules and regulations in areas 2 to 3 m adjacent to the pits. Percolation test were performed in August, 1996 and were repeated for 13 of the sites during the Winter and Spring of 1997. Shallow 1.5 m monitoring wells were installed at 7 of the sites, and soil samples were taken at 8 of the sites to determine Atterberg limits. The results of this study show that percolation rates vary from season to season, with wet season results indicating some sites as unsuitable even though dry season tests would result in selecting the minimum soil absorption area for that site. The study also indicates that soil morphology, although not completely accurate, is a reasonable indicator of seasonal water table depth from the surface during wet seasons. Sizing the soil absorption system based upon indicators of seasonal water tables, soil texture and structure results in a somewhat more conservative design.