Potentiella miljörisker med geoenergisystems temperaturpåverkan i mark och grundvatten
Sammanfattning: The aim of this literature study was to answer whether the thermal impact of geothermal energy systems pose a potential environmental risk to groundwater quality and microorganisms in the sub surface and to above ground vegetation. This study shows that the thermal impact of high temperature systems operating with temperatures ≥25⁰C in groundwater could lead to adverse effects on groundwater chemistry and biodiversity, such as mobilization of arsenic and DOC. Elevated levels of DOC combined with a rise in temperature could lead to changes in microbial activity and a shift to anaerobic redox processes. Severe elevations of temperature in ground water might lead to reduced biodiversity and establishment of termophilic communities. However, many of the identified risks depend on site-specific factors, such as mineralogical composition of the aquifer, available amount of dissolved organic carbon and microbial communities. There is not enough data to state that the same effects occur with energy systems in bedrock. Small temperature differences can affect groundwater living organisms’ activity and biodiversity, though the effects vary with access to organic matter. Therefore, this should mainly concern horizontal systems, or ATES in the case the aquifer is polluted with organic matter. Vertical systems’ thermal exchange occurs below the ground water pressure surface. While microorganisms should occur at those depths, there are no studies confirming whether they are affected. Horizontal systems have been seen to lower the soil temperature resulting in reduced occurrence of earthworm and later flowering of vegetation. High temperature storage systems may elevate ground temperature levels. While no research has been made on such a site, persistent elevated temperatures in soil may affect the presence of soil living organisms and possibly lead to changed plant flowering pattern. Though, the response of both flora and fauna seem to be species and site dependent. The results of the literature review were compared to observed changes in temperature and groundwater chemistry at a HT-BTES site in southern Sweden. Changes in groundwater chemistry, such as elevated levels of iron, was in line with what could have been expected based on the literature review. To date no complete survey of the biological impact of the temperature changes have been done at the site and there are no reported visible effects on the vegetation at the site.
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