| Abstract |
Low enthalpy geothermal aquifers in sedimentary basins are predominately exploited for direct use and this is characteristic also for the Slovenian part of the Neogene Mura-Zala basin (NE Slovenia), which is one of the western sub-basins of the Pannonian basin with very favourable geothermal conditions. The Pliocene Ptuj-Grad Fm. contains lukewarm water, while the most important regional and transboundary geothermal aquifer is formed by the Pannonian to Pontian delta front sands of the Mura Fm. The turbidites of the Pannonian Lendava Fm. as well as the Sarmatian to Carpathian Špilje and Haloze Fms. often form rather limited aquifers with thermomineral water rich in gases or organic compounds. Over-exploitation of these Neogene aquifers was assumed for some time, being attributed to inadequate monitoring and lack of management strategies. The preserved geoscientific and production information was interpreted by the time-series trend analyses on the basis of a linear regression method. The results clearly indicate that not much systematic hydraulic measurements were done in the last 15 years. Also, the water composition was analysed unsystematically and only for about half of the 43 identified production wells. The chemical trends were estimated on basic chemistry datasets from eleven wells. New hourly datasets of piezometric groundwater levels, air pressure, water temperatures and daily abstraction rates have been acquired from a research monitoring network of 9 geothermal wells in the Mura Fm. It has been in operation since May 2009 and is managed by the Geological Survey of Slovenia. The total amount of abstracted thermal water from the Neogene aquifers in the period 1960-2011 is estimated to 64.9 million m3, of which 1% appertains to the Lendava Fm., 13% to the Ptuj-Grad Fm., 19% to the Špilje Fm. and 67% to the Mura Fm. Constantly increasing production reached 3.0 million m3 in total in 2011. The existent data on aquifer pressure decrease, yield and water mineralization indicate that the current abstraction from all geothermal aquifers exceeds their natural recharge rate. Moreover, the 14 active abstraction wells tapping the Mura Fm. caused its over-exploitation, which is locally mitigated by a reinjection well operating since 2009 in Lendava. Hydraulic, chemical and isotopic changes are evident, indicating induced recharge of less mineralized Pleistocene paleo-meteoric water. Daily, seasonal and annual groundwater level and temperature trends were confirmed in the observation wells Do-1 in Dobrovnik, V-66 in Petanjci and Fi-5 in Renkovci, plus in the abstraction wells Mt-6, Mt-7 and Mt-8g in Moravske Toplice and Sob-2 in Murska Sobota. The regional trend of decreasing water levels is app. -0.53 m annually, while near the abstraction wells a few meters drops are observed. The results imply a strong need for long-term feasible management strategies. The most important measures include: enhanced thermal efficiency simultaneously with reduced abstraction rates, reinjection wells in areas with the greatest thermal water withdrawal, establishment of a sound regional monitoring network and well-specific application of operational monitoring system. |