Mechanisms of well iron clogging in groundwater heat pump systems: Insights from video imaging, hydrogeochemical analysis, and geochemical modeling

Abstract

Groundwater heat pump (GWHP) systems are increasingly popular as low-carbon and environmentally friendly technologies, but well clogging induced by iron remains a significant issue. This study investigated the clogging characteristics and biogeochemistry of three typical wells (pumping, injection, and observation wells) in an operating GWHP system using video imaging, sampling, and analysis of hydrogeochemical and microbial data. The results revealed that iron-induced well clogging is a complex process involving physical, chemical, and microbial factors. Pumping wells experience clogging due to water mixing with varying redox conditions, resulting in hematite-based iron oxide deposits. Injection wells exhibit higher clogging severity, with transformed oxidation and accumulation of reduced iron minerals at the solid-liquid interface, resulting in darker colored clogs with magnetite. Clogging in both extraction and injection wells is closely related to iron-rich aquifer sections, where severe clogging occurs. Shallow clogging due to iron oxide is limited and attributed to the oxidation of zero-valent iron in well casing material. Iron-oxidizing bacteria and iron-reducing bacteria were detected in the consolidated deposits of clogged wells, indicating their involvement in the clogging formation process. Moreover, a strong correlation was observed between the presence of nitrate-reducing bacteria in the water phase and the severity of clogging, suggesting a possible link between iron oxidation and nitrate reduction in the system. Geochemical modeling results further supported the observed clogging severity in GWHP systems and confirmed varying clogging mechanisms in different wells and depths. These findings contribute to the understanding of clogging in GWHP operations, aiding in robust water utilization and energy-saving efforts, and supporting global carbon reduction initiatives.