Effects of recharge events below on-site wastewater drain fields as related to soil type.

This study evaluated septic effluent as it was affected by recharge through different soil types. The methodology included a column study involving three soil classifications (types); Type Ib soils (sands and loamy sands), Type II soils (sandy loams and loams) and Type III soils (sandy clay loams). Loading rates followed ANSI/NSF Standard 40 design loading and local regulations for wastewater volume allowed per drain field area per soil type. All soil types effectively removed CBOD (>85% reduction). The soils were less effective at removing TSS with removal rates ranging from 52% to 87%. Total inorganic nitrogen (TIN) removal efficiency for Type Ib soils was only 10% while Type III soils removed 66% of TIN. A 5-year, two-hour storm diluted the effluent leaving the soil by decreasing concentrations without increasing mass flux. The larger recharge events appeared to flush the columns and increased the mass flux of contaminants below the soil.