Aqueous geochemistry and diagenesis in the eastern Snake River Plain aquifer system, Idaho

Abstract

Water budget and isotopic analyses of water in the eastern Snake River Plain aquifer system confirm that most, if not all, of the water is local meteoric in origin. Solute mass-balance arguments suggest that ∼5 × 10 9 moles of calcite and 2.6 × 10 9 moles of silica are precipitated annually in the aquifer. Isotopic evaluations of calcite and petrographic observation of silica support the low-temperature origin of these deposits. Approximately 2.8 × 10 9 moles of chloride, 4.5 × 10 9 moles of sodium, 1.4 × 10 9 moles of sulfate, and 2 × 10 9 moles of magnesium are removed annually from the aquifer framework by solution. Proposed weathering reactions are shown to be consistent with mass balance, carbon isotopes, observed mineralogy, and chemical thermodynamics. Large quantities of sodium, chloride, and sulfate are being removed from the system relative to their abundances in the rock. Sedimentary interbeds, which are estimated to compose 2 )/yr or less than half the average of the North American continent. This contrasts with the rate for the eastern Snake River basin, 34 (Mg/km 2 )/yr, which is almost identical to the average for the North American continent. Identification and quantification of reactions controlling solute concentrations in ground water in the eastern plain indicate that the aquifer is not an “inert bathtub” that simply stores and transmits water and solutes but is undergoing active diagenesis and is both a source and sink for solutes.