Analysis of long-term thermal water abstraction and its impact on low-temperature intergranular geothermal aquifers in the Mura-Zala basin, NE Slovenia

• Geothermal aquifers in the Mura-Zala basin have been overexploited.
• Aquifers’ depletion is reflected in hydraulic and chemical changes.
• Water from the Middle Miocene sandstone indicates dilution and ceasing outflow.
• Upper Miocene sandy aquifer is being depleted at a drawdown rate of 0.53 m/year.
• Daily, seasonal, annual and multi-annual hydraulic trends were identified.

The already exploited geothermal resources in the Mura-Zala basin are planned to be further developed. In this study I investigated thermal water abstraction and its impact on both the fissured basement aquifers and the intergranular Neogene siliciclastic aquifers. Total abstraction of thermal water in north-east Slovenia summed to 3.1 million m3 in 2011, with very limited artificial recharge supplied through a single reinjection well. This exploitation has resulted in depletion of the aquifers, with decline in aquifer pressure, piezometric groundwater levels and discharge rates, as well as chemistry variation, being evident in many wells. A research monitoring network has been established in 2009 and has been taking hourly measurements in eight wells. These wells are up to 2 km deep and tap aquifers in the Upper Miocene sandy Mura Formation. Daily, seasonal and annual trends were interpreted, and the measured overall regionally declining static groundwater levels are alarming, reaching 0.53 m per year. Despite the changes in conditions in the aquifers, no change of production temperature has so far been perceived.