MAJOR ION AND TRACE-METAL GEOCHEMISTRY OF AN ACIDIC SEPTIC-SYSTEM PLUME IN SILT

Four years of detailed ground-water monitoring at a newly installed, s easonal-use, domestic septic system located on poorly buffered (CaCO3 equivalent content (1.6 wt.%) lacustrine silt, has revealed the develo pment of an acidic goundwater plume, Acid, generated by the partial ox idation of effluent NH4+, dissolved organic carbon (DOG), and possibly sulfide minerals present in the sediment, has resulted in a distal pl ume core zone with pH values in the range of 4.4 to 5.0. The acidic zo ne, where NH4+ does, however, persist (> 2 mg/l, as N) and where DOC r emains elevated (6-13 mg/l), is associated with high average concentra tions of the trace metals Fe (4.7 mg/l), Al (1.9 mg/l), and Mn (3.6 mg /l), Attenuation of nitrogen along the plume core flowpath is indicate d by a decrease in the N/Cl- ratio from an effluent value of 1.7, to a plume value of only 0.5 after 4 m of subsurface flow. Increased SO42- levels observed in the zone of N depletion suggest that attenuation c an be at least partly attributed to reduction of plume NO3- by oxidati on of reduced S present in the sediment. PO43- has not migrated signif icantly beyond the infiltration bed gravel layer, demonstrating that P O43- mobility is limited in these sediments (retardation factor > 10).