| Title | Effects of Regional Production of Thermal Water on Low-Temperature Geothermal Aquifers in North-East Slovenia |
|---|---|
| Authors | Nina RMAN, Andrej LAPANJE, Joerg PRESTOR, Michael J. O’SULLIVAN, Mihael BRENCIC |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Mura-Zala basin, Pomurje, over-exploitation, Upper Miocene, intergranular aquifer, production monitoring, groundwater drawdown, chemical change, Mura Fm. sand, available resources |
| Abstract | Low-temperature geothermal aquifers in the Neogene Mura-Zala sedimentary basin in northeastern Slovenia have been exploited for direct use for over 40 years. This basin is one of the western sub-basins of the large Pannonian basin with very favorable geothermal conditions. Here, the most important geothermal aquifer is the Pannonian to Pontian delta front sand of the Mura Formation. Its over-exploitation due to regional thermal water abstraction has been assumed and three hypotheses were set: a) hydraulic and b) chemical state of this aquifer has changed due to exploitation, plus c) current thermal water abstraction exceeds its recharge. The hypotheses were tested by interpreting time-series of archive chemical analyses and new hydraulic datasets acquired from a research monitoring network of 11 geothermal wells which is being constantly upgraded since 2009. The results point out that the current exploitation is not sustainable because hydraulic and chemical state of the aquifer has been continuously deteriorating; however, no temperature changes have been measured at the wellheads. The regional drawdown rate in observation wells is approximately 0.5 m annually, while very rough average value for abstraction wells is 3.1 m per year. This rate is twice as high locally. Total regional drawdown caused by 40 years of abstraction is above 15 m and even higher in the vicinity of the abstraction sites. The numerical model set up in the AUTOUGH2 code confirmed the analytical results. Both research approaches imply that current abstraction of thermal water from the Mura Fm. (75.5 l/s) and hydraulically connected Ptuj-Grad Fm. (7.0 l/s) geothermal aquifers severely exceeds the recharge rate. Based on the modelling scenarios we have estimated that the available groundwater resources sum up to approximately 41 l/s, which do enable sustainable exploitation. We expect that these findings will affect the concession granting process in Slovenia by improving optimization of exploitation and implementation of reinjection. The experience gained by establishing the research monitoring system will be used to develop a harmonized transboundary monitoring of the sharing ‘Transboundary Thermal Groundwater Body Mura-Zala’ between Slovenia and Hungary. |