Impact of Underground Thermal Energy Storage (UTES) on groundwater composition – Results from the Dutch national research program

Authors: Hartog, Drijver, Dinkla, Bonte
Keywords: aquifer thermal energy storage, ATES, groundwater quality, geochemistry, microbiology, temperature effects, mixing.
Conference: European Geothermal Conference Session: 9 Shallow (SG)
Year: 2013 Language: English
Abstract: In the Netherlands, the total number of aquifer thermal energy storage (ATES) systems (or open loop underground thermal energy storage systems, UTES ) has been growing exponentially during the last two decades, with a total of 2000 systems by the end of 2012. The exact number of borehole thermal energy systems (BTES, or closed loop UTES) is not clear, but the number of boreholes associated with these type of systems is about 200 times higher and also shows an exponential growth trend. Furthermore, current national and municipal ambitions in reducing CO2 emissions, combined with rising energy prices, provide a driving force for further rapid expansion. With up to 10,000 additional ATES systems expected in the coming decade, concerns have been raised on their impact on overall groundwater quality, as this may adversely affect other groundwater uses such as drinking water production. As a consequence, several research programmes were initiated to assess the impact of UTES on groundwater composition. In particular, field monitoring campaigns provided insight in the processes that cause groundwater quality changes. Here, we provide an overview of the insight gained from the monitoring of the main processes that were observed to affect of field groundwater monitoring performed at UTES field sites for chemical and microbial parameters. To be able to acquire broad insight, the selected UTES sites consist of different types of UTES systems (borehole heat exchangers, doublet systems, monowell, recirculation, low and high temperature storage) in different geochemical circumstances (fresh/salt water, different redox conditions). The results indicate that the impact of mixing of stratified groundwater from different depths in the aquifer is the predominant factor affecting groundwater quality changes in low temperature ATES systems, with various site-specific aspects contributing to the mixing extent. Redox, salinity and pH- and alkalinity stratifications have identified and described as the main gradients over which mixing can impact groundwater chemistry. Temperature effects on mineral equilibria and reaction kinetics are generally minimal in thermally-balanced systems at temperatures below 25 ºC, because the impact on the most sensitive parameters in the warm zone is largely compensated by the opposite effect in the cold zone. Of the chemical equilibria, particularly adsorption reactions are sensitive. With increasing temperatures the impacts on groundwater quality are progressively more pronounced, also because maintaining a thermally balanced system with respect to natural groundwater (~11°C) is no longer feasible. At higher temperatures the potential for organic matter mobilisation and mineral precipitation increases. Large temperature increases can have an impact on the composition of the microbiological population, with shifts to mesophile and thermophile species at elevated temperatures. These effects may also be reversed on a functional level as observed after the abandonment of a high temperature ATES site, where the lowered temperature, resulted in a population similar (but not equal) to the original population, performing the same functions. No indications were found for risks of multiplication of pathogens. Biodiversity, as the number of different species appeared not to be affected at the studied sites (temperatures up to 39 ºC).
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