Subsurface temperature trends in response to thermal water exploitation in the Jiashi Hot Spring, northeastern Taiwan

•Repeatedly measured borehole temperature-depth curves provide important information regarding temperature changes.•The Mann–Kendall trend test is a good tool for depicting trends in subsurface temperature changes within a geothermal field.•We obtained borehole temperature-depth curves for ten monitoring wells within the Jiashi Hot Spring in Taiwan.

Temperature monitoring provides important information for the sustainable management of a geothermal field. Previous studies indicate that a decline in aquifer pressure is an obvious indicator of thermal water overexploitation. However, many thermal water producing aquifers display a subtle temperature change rather than a pressure decline. Due to the importance of monitoring temperature change, temperature detection is an important topic in the sustainable management of geothermal fields. In this study, we used borehole temperature-depth curves, obtained from boreholes measured over half-year intervals from 2011 to 2014, as well as the Mann–Kendall method, to determine trends for subsurface temperature within the Jiashi Hot Spring in Taiwan. Our results indicate that trends for subsurface temperature are related to hydrogeology and the flow field of groundwater. Groundwater/thermal water flow directions are impacted by the exploitation of thermal water in production wells, according to their depths and distributions, but pressure declines are not a dominant feature. Repeatedly measured borehole temperature-depths and their resultant curves provide important information for understanding trends in subsurface temperature change within a geothermal field.