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 × 10⁹ moles of calcite and 2.6 × 10⁹ 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 × 10⁹ moles of chloride, 4.5 × 10⁹ moles of sodium, 1.4 × 10⁹ moles of sulfate, and 2 × 10⁹ 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 <10% of the aquifer volume, may yield as much as 20% of the solutes generated within the aquifer. Weathering rate of the aquifer framework of the eastern Snake River Plain is 14 (Mg/km²)/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/km²)/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.