There shall be at least 4 feet of soil between an absorption bed, trench, or seepage pit bottom, the lowest construction joint on a septic tank, or any other component of a subsurface absorption system, and the seasonal high ground water table, ground water table, rock formations, or impervious soil strata. Absorption systems shall not be constructed in soils rated as having severe or very severe limitations for underground dispersal by the Soil Conservation Service, U.S. Department of Agriculture, unless that limitation is not present, as shown by field investigation or unless prior written approval is granted by the Secretary.