Analysis of septic tank soil absorption systems in South Carolina's coastal zone and generation of design aids for field use

The State of South Carolina is reviewing its criteria to determine site suitability for placement of septic tank soil absorption systems (STSAS). The current standards criteria that a minimum of a six-inch vertical separation between the bottom of the absorption field trench and the seasonal high water table, and a minimum of fifty feet separation between the absorption field and the nearest drinking water well or waterway. Both criteria are independent of soil type or aquifer characteristics.;This study used two finite element flow models to address the six-inch rule and a viral transport model to investigate the fifty-foot rule. The results indicated a strong dependency on soil type and aquifer depth for both mound growth and viral transport, with hydraulic gradient also influencing transport.;Two design aids were created for South Carolina Department of Health and Environmental Control use. A mounding design aid predicts the mound growth under a trench based on the soil type and aquifer depth. A viral transport design aid, with inputs of soil type, aquifer depth and hydraulic gradient, predicts the longitudinal distance traveled from an absorption trench by active virus (risk assessed value of 2 ;This study also revealed that the classification of soils and, hence, their level of acceptance for absorption field sites, is not supported by flow and transport modeling. The sequencing of the eleven soils classified by the South Carolina Department of Health and Environmental Control and some other state agencies are reordered in this work. The present classification series for soils is based on clay content. The new ordering is based on the predicted mounding and viral transport for these soils. The result is a new classification sequence of soils (seven of eleven soils were reordered). It is believed that this new ordering will better represent the actual behavior of these soils and yield more valid STSAS site acceptability determinations.