AbstractDetailed monitoring of groundwater in a shallow sand aquifer impacted by a large-flux septic system showed that water quality in the shallow water table zone below the tile bed was similar to that of smaller-flux septic systems in similar hydrologic settings where effluent residency in the unsaturated zone was of similar duration. During residency of about one week in the 4-m thick unsaturated zone, effluent NH+4 was largely oxidized to NO3, about 75% of DOC was biodegraded, and acidity produced by the above reactions was neutralized by dissolution of calcite. Beneath the tile bed and extending laterally downgradient a distance of 80 m to the Lake Erie shoreline, a distinct plume of impacted groundwater was easily distinguished by elevated levels of electrical conductance, Cl, NO3, HCO3, NA+, Ca2+, and K+ and by depressed levels of pH and dissolved oxygen. High NO3 levels that occur below the tile bed disappear, however, in the anaerobic plume core zone 10 to 70 m downgradient, apparently as a result of denitrification. The rich reserve of solid-phase organic carbon in the aquifer sediment (2.5%) probably provides much of the organic carbon for heterotrophic denitrification. This condition is in contrast to other septic system plumes in sand aquifers where high NO3 levels persist and where aquifer organic carbon values are much lower. Although NO3 is attenuated in the plume core, persistence of NO3 along the aerobic upper fringe of the plume demonstrates the ability of septic systems to cause significant water-quality degradation of sand aquifers when the conditions favorable for denitrification do not exist. The sharp boundary between the plume water and non-impacted water adjacent to and overlying the plume in the area 50 to 100 m downgradient from the tile bed, and the undiluted nature of non-reactive solutes such as Cl throughout the core of the plume, demonstrates that dispersion has only a weak influence on the plume. This is consistent with dispersion studies in many other sand aquifers.