Table 1. Qualitative overview of risks of low-temperature (< 30°C) underground thermal energy storage on groundwater systems.

Negative effect of underground thermal energy storage Probability Consequence Risk§
Hydrological impacts
Changing water levels and fluxes ++ Desiccation, water logging, settlements ±
Changing other well’s capture zone ++ Increasing vulnerability, pollution ++
Poorly sealed boreholes + Cross-aquifer flow ++
Thermal impacts
Changing water temperature ++ Temperature, reaction kinetics +
Chemical impacts
Mixing processes and chemical reactions ++ Salinity, IMIPO, OMIPO ++
Reactivation of otherwise stable groundwater pollution plumes ± IMIPO, OMIPO ++
Oxidation of organic matter ± Nutrients, DOC, color +
Oxidation of iron sulfides ± Fe, SO4, As, Ni, Co, Zn +
Dissolution/precipitation of carbonates - Ca, HCO3, Sr ±
Dissolution/precipitation of silicates - SiO2 ±
Leaching from installation materials ± Cd, Cu, Cr, Ni, Pb, VC +
Leaking anti-freeze fluids or additives ± Glycol, biocides, corrosion inhibitors ++
Microbiological impacts
Introduction or mobilization of pathogens - Pathogens ±
Increasing biodegradation rate ± Nutrients, IMIPO, OMIPO - or +
Changing microbiological population + Unknown ?

Probability of occurrence is small (-), moderate (±), high (+), or almost always (++).
IMIPO = inorganic micro-pollutants, OMIPO = organic micro-pollutants, DOC = dissolved organic carbon, VC = vinyl chloride.
§Probability of risk is negative, resulting in opportunity (-); none (±); low (+), high (++).