Geological carbon cycle in a sandstone aquifer: Evidence from hydrochemistry and Sr isotopes

Abstract

Sr isotopes have been widely used to quantify silicate versus carbonate weathering reflected in rivers, but there is no such study in groundwater, which is restricted by the difficulty of accurately determining the Sr contents and isotopic ratios of the two endmembers in silicate-dominated aquifers. In a sandstone catchment, we find rainwater, modern groundwater in the recharge area, and old groundwater in the discharge area have contrast 87Sr/86Sr ratios, with mean values of 0.710784, 0.711448, and 0.710269, respectively. If we follow previous studies and use the acetic acid leachate of sandstone to represent the calcite fraction, the resulting 87Sr/86Sr ratio cannot explain most groundwater samples. By considering the simultaneous dissolution of calcite and silicates in the acetic acid leachate, we find the typical 87Sr/86Sr ratio of calcite equals 0.709113, which well explains the 87Sr/86Sr ratios of all groundwater samples. The 87Sr/86Sr ratios of groundwater in the recharge area with abundant dissolved CO2 are contributed by three endmembers (rainwater, silicate and calcite), whereas those of old groundwater in the discharge area with low dissolved CO2 as well as low Cl− and SO42− can be explained by two endmembers (calcite and groundwater in the recharge area). Based on strontium isotopic budget, we find the equivalent concentrations of CO2 consumption by silicates and calcite in groundwater are both larger than those in river waters studied in previous studies, and the majority of dissolved CO2 in groundwater can be fixed by carbonate weathering as long as the residence time is long enough. This study enhances understanding of the roles of silicate and carbonate weathering during long-time groundwater circulation.