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

•We find a trend of decreasing 87Sr/86Sr with groundwater age in a sandstone aquifer.•We consider Sr from silicates in HAc leachate of rocks to obtain 87Sr/86Sr of calcite.•Sr isotopes reveal that groundwater has higher degree of weathering than most rivers.•Low 87Sr/86Sr of old groundwater reveals deep aquifers can be important carbon sinks.•The ratio of silicate to calcite weathering decreases with content of dissolved CO2.