For high water table properties, the ideal type of leach field depends on whether you are installing a new leach field or replacing a failed one.  For a new drain field, an advanced enviro septic system can be sited within 24 inches (2 feet) of the seasonal high water table (SHWT), while new concrete chambers must be sited 48 inches (4 feet) from the SHWT.  However, replacement concrete chambers may be sited 24 inches from the SHWT, which is the same distance requirement as a new advanced enviro septic.