Chemical fate and transport in a domestic septic system: Site description and attenuation of dichlorobenzene

Abstract

AbstractThe mobility and attenuation of dichlorobenzene (DCB) in a septic system groundwater contaminant plume with a very detailed monitoring network was evaluated by conducting a tracer experiment in which 450 ml of a plumbing line cleaner containing DCB and 1 kg NaBr were injected into the septic system. DCB concentrations of up to 3,460 μg/L were observed in the septic tank effluent, up to 650 μg/L in the unsaturated zone 0.45 m below the tile bed, and up to 13 μg/L at the water table at 2 m depth. Peak DCB concentrations arrived at the water table after 57 d at a source‐normalized concentration of 0.02, whereas peak Br− concentrations arrived at the water table after only 10 d at a normalized concentration of 0.61, thus demonstrating DCB attenuation and retardation during migration through the aerobic unsaturated zone at this site. Model simulations of DCB breakthrough at the water table provided an adequate fit with the field data when a retardation factor of 6.5 and decay half‐life of 15 d were used. Evidence for biodegradation of DCB was provided by the occurrence of a 7‐d lag (biota acclimation period), after which increased attenuation of DCB was observed in the unsaturated zone and by the preferential attenuation of two of the three isomers of DCB. Vigorous biodegradation of trace volatile organic compounds such as halogenated and substituted benzenes was indicated in the unsaturated zone below this septic system.