Chemical fate and transport in a domestic septic system: Application of a variably saturated model for chemical movement

Abstract

AbstractA detailed mathematical modeling study of the impact of a single‐family septic system on groundwater quality is described. The work served to improve the conceptual understanding of subsurface processes associated with septic systems and lent insight into the fate of septic‐system‐derived chemicals. Two‐dimensional saturated‐unsaturated simulations of the chemical plume associated with the septic system were successful in closely matching field observations. Results indicate that the plume has been subject to limited dispersion (or spreading) reflecting the homogeneous nature of the aquifer materials. The sharp concentration gradients along the upper fringe of the plume near the watei table indicate weak vertical mixing and imply limited dilution by recharge water entering the aquifer downgradient of the septic system. Calibrated dispersivities were 10 cm for the longitudinal value (αL) and 1 mm for the transverse vertical value (αT). The movement of reactive chemical constituents present m consumer detergent products entering the septic system was simulated to assess the influence of sorption and biodegrada‐tion. The migration of the detergent compounds linear alkylbenzene sulfonate (LAS) and nitrilotriacetic acid (NTA) was found to be controlled mainly by these processes, and the limited movement predicted by the model is consistent with that observed in the field. For a half‐life equal to 15 d and a distribution coefficient (Kd) value equal to 1.5 L/kg in the unsaturated zone, LAS was predicted to move <0.5 m from the tile drains. However, chemicals having a significantly longer half‐life or those which exhibit limited partitioning to the soil matrix are likely to migrate much further.