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, seasonal-use, domestic septic system located on poorly buffered (CaCO₃ equivalent content ≤ 1.6 wt.%) lacustrine silt, has revealed the development of an acidic ground-water plume. Acid, generated by the partial oxidation of effluent NH₄⁺ dissolved organic carbon (DOC), and possibly sulfide minerals present in the sediment, has resulted in a distal plume core zone with pH values in the range of 4.4 to 5.0. The acidic zone, where NH₄⁺ does, however, persist (> 2 mg/1, as N) and where DOC remains elevated (6–13 mg/1), is associated with high average concentrations of the trace metals Fe (4.7 mg/1), Al (1.9 mg/1), and Mn (3.6 mg/1). Attenuation of nitrogen along the plume core flowpath is indicated 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 can be at least partly attributed to reduction of plume NO₃⁻ by oxidation of reduced S present in the sediment. PO₄³⁻ has not migrated significantly beyond the infiltration bed gravel layer, demonstrating that PO₄³⁻ mobility is limited in these sediments (retardation factor > 10).