The Influence of Water Temperature on the Hydrogeochemical Composition of Groundwater during Water Extraction and Reinjection with Geothermal Heat

Abstract

This study presents a simulation of potential changes in groundwater in three wells within a Quaternary gravel aquifer in the city of Ljubljana when groundwater cooled by about 4 °C is reinjected into it. The research focuses on the mass transport of calcite in the vicinity of boreholes. According to our results, the impact of the changes in the geochemical composition of the water is relatively small (around 1%). Although the waters are approximately in equilibrium, calcite may be dissolved and sometimes precipitated within the aquifer when cooled water is reinjected into it. The amounts of precipitated calcite always decrease with decreasing temperatures of the reinjected water, which can lead to calcite dissolution if the temperature difference is large enough and the water is only slightly oversaturated. This novel finding is significant since previously published studies have mostly focused on understanding the scaling (precipitation) of calcite and not its dissolution. The mass transfer of calcite is relatively low, but in a scenario of long-term pumping for several years, such low values could lead to a dissolved or precipitated mass of several hundreds of kilograms of calcite per year (at a pumping rate of 46 L/s).