EFFECTS OF RECHARGE ON GROUNDWATER BELOW ON-SITE WASTEWATER DRAIN FIELDS AS RELATED TO SOIL TYPE

Most rural homes and many suburban homes in the United States use “on-site” wastewater treatment systems (OWTS). These OWTS are designed traditionally as septic tanks with absorption systems termed “drain fields” where wastewater effluent is introduced into the soil for further treatment and dispersal. This effluent may eventually migrate into shallow groundwater systems. Wastewater contamination is a recognized problem worldwide but little is known about the effectiveness of the soil treatment phase in OWTS. In addition to the potential for increasing carbonaceous biochemical oxygen demand (CBOD) and nutrients, recent studies have shown that emerging contaminants (EC) such as pharmaceuticals are frequently being discovered in groundwater as a result of wastewater dispersal.

This study focuses on septic effluent dispersal in soil and the potential for recharge events to affect wastewater contribution to groundwater. Specifically this study evaluates septic effluent as it is affected by recharge through different soil types. The recharge effects are of particular interest because precipitation can result in a statistically significant increase in either the concentration or the mass of potential contaminants.

The methodology includes a column study designed to evaluate the effect of wastewater dispersal and recharge on three soil classifications (types). Those “types” as defined by the USDA soil classification system include; type Ib soils which are sands and loamy sands, type II soils which are sandy loams and loams and type III soils which are more varied and include sandy clay loams. Loading rates and volumes were based on the American National Standards Institute formula to simulate household usage and local regulations for wastewater volume allowed per drain field area per soil type. Each soil was tested in triplicate. In total, nine columns, six inches in diameter, were constructed and packed with care taken to eliminate bypass flow. Tracer testing confirmed uniform discharge. Effluent draining from the soil columns was analyzed for CBOD, total suspended solids, nitrogen and EC following each precipitation event and during periods of no recharge which served as control data. The results provide information that can improve the siting and design of OWTS and therefore enhance ground-water quality.