Biogeochemical Evolution of Domestic Waste Water in Septic Systems: 2. Application of Conceptual Model in Sandy Aquifers

Abstract

AbstractAqueous geochemical data from unconfined sand aquifers beneath two operating domestic septic systems are used to illustrate and evaluate a conceptual model of septic‐system geochemistry. This model emphasizes the changing redox and alkalinity conditions in the septic system and the subsurface. The septic‐tank effluents flow as distinct plumes downward through the unsaturated zones and then primarily laterally in the ground‐water zones. The composition of the effluent was measured at several points in each system. At each site, the septic‐tank effluent underwent aerobic oxidation in the unsaturated zone, which caused conversion of NH4+ to NO3−, organic C to CO2 and organic S to SCh42‐. At the first site, calcium carbonate dissolution in the unsaturated zone buffered the acidity released by the redox reactions. In contrast, the second system was poorly buffered and the pH dropped from 6.7 to 4.9 as aerobic oxidation occurred. Below the water table a small amount of aerobic oxidation occurred at each site. Nitrate‐N concentrations in the cores of both plumes were above 25 mg/1 as the plumes traveled from the septic systems. At the second site, the ground‐water plume discharges to a river at the edge of the property. As the effluent flowed through the organic C‐rich sediments of the river bed, NO3− disappeared and alkalinity increased, presumably due to denitrification. Differences in sediment composition at the two sites also led to different behaviors of Fe, Al, and possibly PO43‐. The conceptual model offers an organized approach to interpreting the major geochemical trends observed in the two systems, which are determined mostly by the well‐aerated unsaturated zones below the drain fields and the amount of buffering material present in the sediments.