When hot water is injected into an aquifer, the water will react with the minerals present in the aquifer. The most important processes are cation exchange, precipitation/dissolution of carbonates, and dissolution/precipitation of silicates. If the redox condition of the water is changed during the process (e.g., because of contact with oxygen in the air or because of mixing of water with a different redox status) redox reactions will be important as well.
In many cases, the water will have to be treated to prevent carbonate scaling during heating. This treatment will influence the reactions in the aquifer. Because the water is (at least partly) reused each cycle, the reactions in the aquifer may cause an increase or a decrease in necessary treatment in subsequent cycles. Also, the treatment and the reactions in the aquifer may have an influence on the environmental impact of the store because the quality of the water lost from the store is changed.
To be able to assess the relevance of these processes, a model has been developed that calculates the chemical reactions during transport of groundwater. An example of use of the model is given for the Utrecht University ATES system. At this site, a split treatment of Ca-Na softening is used to prevent carbonate sealing. The calculations have helped to make decisions on how to proceed with water treatment.