The geochemical mass transfer model WATEGM-SE is used to illustrate by example potential chemical reactions that can occur at a hypothetical low-temperature thermal energy aquifer storage facility. Important processes that control the chemistry include heating and cooling of the water, equilibration of the pumped water with the atmospheric partial pressure of CO2 and O2, and the mixing of the injection water with the native groundwater during the injection, storage, and recovery cycles. For the given example, 0.3 mmol/L of calcite would be precipitated under closed system pumping and heating from 10 to 50 °C while a total of 1.9 mmol/L would be precipitated under the open condition. If this calcite were to form scale within the facility's piping then considerable lengths can be affected depending on the pumping rate. Alternatively, if the precipitate is kept in suspension it will be transported to the injection well and will be filtered out by the aquifer itself. This filtration can result in significant decreases in porosity and hence permeability in the immediate vicinity of the injection well. Mixing of the injection water with the native groundwater changes the water chemistry and can result in mineral supersaturation or undersaturation depending on the composition of these waters and the proportions in which they are mixed. The effect of mixing on the given example is undersaturation with respect to calcite and supersaturation with respect to amorphous Fe(OH)3. The pe values in the simulations of mixtures of an oxidizing injection water and a reducing native groundwater yielded some results with significantly higher pe values than the oxidizing injection water. The use of equilibrium geochemical mass transfer models tempered by an understanding of their limitations may prove to be an effective tool for evaluation of potential chemical reactions associated with low-temperature aquifer thermal energy storage facilities. Key words: thermal energy storage, geochemical equilibria, groundwater, simulation, scale formation, mixing.
Low-temperature thermal energy storage program annual operating plan