CRUDE-OIL IN A SHALLOW SAND AND GRAVEL AQUIFER .1. HYDROGEOLOGY AND INORGANIC GEOCHEMISTRY

Changes in the distribution of inorganic solutes in a shallow ground w ater contaminated by crude oil document a series of geochemical reacti ons initiated by biodegradation of the oil. Upgradient of an oil body floating on the water table, oxidation of oil to carbonic acid dissolv es carbonate minerals in the aquifer matrix. In this oxidized zone pH is depressed approximately 1 pH unit, and the concentrations of Ca, Mg and HCO3- increase to more than twice that of the native ground water . In the anoxic zone beneath the oil body concentrations of dissolved SiO2, ST, K, Fe and Mn increase significantly. Here, Fe is mobilized b y microbial reduction, pH is buffered by the carbonate system, and sil icates weather via hydrolysis and organic-acid-enhanced dissolution. F arther down-gradient the ground water is reoxygenated and Fe precipita tes from solution, possibly as iron hydroxide or iron carbonates, whil e SiO2 precipitates as amorphous silica. Other solutes, such as Mg, ar e transported more conservatively down-gradient where contaminated and native ground waters mix. The observed changes in inorganic aqueous c hemistry document changes in water-mineral interactions caused by the presence of an organic contaminant. These organic-initiated interactio ns are likely present in many contaminated aquifers and may be analogo us to interactions occurring in other organic-rich natural waters.