Site Evaluation and Design of Seepage Fields

A reevaluation of previous work by others indicated that soil can absorb septic tank effluent indefinitely if the application rate is kept below a certain level, which is a function of soil permeability. This long-term acceptance rate is independent of whether the soil is continuously or intermittently flooded, and varies from approximately 0.3 gpd/sq ft (0.01 m/day) for clay loam to approximately 0.8 gpd/sq ft (0.03 m/day) for sand. A study of the ground-water flow pattern below a seepage field showed that it is, in many cases, the hydraulic conductivity of the ground surrounding the field, as determined by the external water table, soil permeability, and impervious strata, that controls the size of the field required. Reliable techniques for site evaluation of soil permeability, depth to water table, and depth to any impervious strata are presented, and a chart is given for designing a seepage field based on this information. Design examples are included.