Sustainability assessment of hydrochemical and microbiological disturbances in shallow aquifer by thermal utilization

Abstract

The use of geothermal systems with shallow groundwater to achieve global carbon neutrality has increased substantially. However, they not only have hydraulic and thermal effects on the environment, but also induce hydrochemical and microbial changes that can worsen the sustainability of water resources. This study focused on evaluating groundwater mixing and pollution in a surrounding shallow aquifer induced by operation of a groundwater heat pump (GWHP) system. To explain these phenomena, hydrogeochemistry, multiple isotopes (O, H, and Sr), Rn, and microbial data were combined with geochemical modeling of the test system. Few studies have been conducted on field measurements using these combined approaches. The hydrogeochemical and microbial data showed specific characteristics accompanying the thermal use of groundwater. The calculated saturation index values and PHREEQC modeling results indicated a high probability of clogging effects in wells located near the boundary of the thermal plume. There was also a decrease in the microbial diversity index. Rn and stable isotopes indicated active mixing between circulating water and ambient groundwater along the main groundwater flow direction during the operation. This study demonstrated that subsurface hydrological and microbial environments, such as those of the GWHP system, responded to impacts of moderate intensity. Because the applied data were able to derive possible environmental and operational problems of shallow geothermal systems, hydrological and microbiological signatures should be considered for sustainable management of groundwater in geothermal sites. Our study can have implications for energy reuse from storage within an aquifer, such as aquifer thermal energy storage (ATES) based on field tests, using combined analysis.