EVIDENCE FOR ENHANCED MINERAL DISSOLUTION IN ORGANIC ACID-RICH SHALLOW GROUND-WATER

Total concentrations of formate, acetate, and isobutyrate varied from less than 5 to greater than 9,000 mu mol/l over distances of <3 m in g round water from a shallow hydrocarbon contaminated aquifer. Laborator y incubations of aquifer material indicate that organic acid concentra tions were dependent on the amount of hydrocarbon loading in the sedim ent and the relative rates of microbial organic acid production and co nsumption. In heavily contaminated sediments, production greatly excee ded consumption and organic acid concentrations increased. In lightly contaminated sediments rates were essentially equal and organic acid c oncentrations remained low, Concentrations of dissolved calcium, magne sium, and iron generally were one to two orders of magnitude higher in organic acid-rich ground water than in ground water having low organi c acid concentrations. Carbonate and Fe(III)-oxyhydroxide minerals wer e the likely sources of these elements. Similarly, concentrations of d issolved silica, derived from quartz and k-feldspar, were higher in or ganic acid-rich ground water than in other waters, The positive relati on (r = 0.60, p<.05, n = 16) between concentrations of silica and orga nic acids suggests that the microbially mediated buildup of organic ac ids in ground water enhanced quartz/k-feldspar dissolution in the aqui fer, although it was not the only factor influencing their dissolution . A model that included organic acid microequivalents normalized by ca tion microequivalents significantly strengthened the correlation (r = 0.79, p<.001, n = 16) between dissolved silica and organic acid concen trations, indicating that competition between silica and cations for c omplexation sites on organic acids also influenced quartz/k-feldspar d issolution. Physical evidence for enhanced mineral dissolution in orga nic acid-rich waters included scanning electron microscopy images of h ighly corroded quartz and k-feldspar grains from portions of the aquif er containing organic acid-rich ground water. Microporosity generated in hydrocarbon contaminated sediments may adversely affect remediation efforts that depend on the efficient injection of electron accepters into an aquifer or on the recovery of solutes from an aquifer.