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 (++).