Chemical fate and transport in a domestic septic system: Unsaturated and saturated zone geochemistry

Monitoring the septic system at a single family home provides quantitative information on geochemical evolution of septic tank effluent along its travel path In both the unsaturated and saturated (groundwater) zones, microbiological and chemical reactions occur that bring the effluent closer to equilibrium with the subsurface conditions In the unsaturated zone, 90% of the DOC is removed from the effluent, mostly by aerobic oxidation, and NH+4 in the effluent is almost completely ox ldized to NO−3 As these reactions occur in the tile field, the oxygen content of the soil gas is partially depleted Calcite dissolution from the soils in the unsaturated zone buffers the acidity of the oxidation reactions and increases Ca2+ concentrations In the saturated zone, organic carbon oxidation in the effluent plume apparently decreases dissolved oxygen concentrations to <10 mg/L Potassium is retarded relative to other cations, and dissolved PO43 from the effluent is detectable in the ground water directly beneath the tile field but appears to be advancing very slowly, if at all, within the plume In addition NO3− concentrations remain above drinking-water standards (10 mg/L NO3 N) in the groundwater plume