Geothermal drilling in an Alpine karst aquifer and its impact on downstream springs – A case study from Finkenberg, Tyrol, Austria

Abstract

A borehole heat exchanger array was installed in an Alpine karst aquifer for the first time in Europe. It is composed of nine 400 m deep boreholes in the dolomitic marble of the Hochstegen Formation. New approaches were developed for this unprecedented application and potential risks were checked throughout the sinking of the initial exploratory wellbore. As Finkenberg is located in a tourist area, drilling operations had to be executed within only a few weeks. Several springs downstream from the drilling site are sources of drinking water and tap water supply. Given that the degree and distribution of the Hochstegen Formation karstification was unknown, a spring monitoring program was mandatory in order to obtain drilling permission from public authorities. A practicable and affordable spring monitoring program including several physical and chemical parameters was developed and implemented to document the potential impact of the drilling activities on the spring water. A temporary and locally limited impact of the drilling works on groundwater quality was detected by turbidity measurements. This paper reports the implementation of geothermal drilling in a karst aquifer with a focus on karstification as well as planning, implementation and the results of the spring monitoring program. Such a program can be recommended for comparable projects or questions, adapting the characteristic parameters to be collected and the measurement intervals in consultation with the relevant authorities. It might also increase the chances of gaining approval for geothermal projects in karst and could enhance public acceptance. Key words: Pneumatic down hole hammer drilling, Borehole heat exchanger, Groundwater monitoring, Turbidity, Hochstegen Formation. Geotermalno vrtanje v alpskem kraskem vodonosniku in njegov vpliv na dolvodne izvire - studija primera Finkenberg, Tirolska, Avstrija V alpskem kraskem vodonosniku je bilo prvic v Evropi vzpostavljeno polje geosond. Vkljucuje devet 400 m globokih vrtin, izvrtanih v dolomitnem marmorju Hochstegenove formacije. Razvili smo nove pristope za to aplikacijo brez primere in potrdili morebitna tveganja v casu potopitve prvotne raziskovalne vrtine. Ker je studijsko obmocje Finkenberg na turisticnem obmocju, je bilo treba opraviti vrtanje v samo nekaj tednih. Vec izvirov od mesta vrtanja so viri oskrbe s pitno vodo. Glede na to, da stopnja in porazdelitev zakraselosti Hochstegenove formacije nista bili znani, je bilo treba za pridobitev dovoljenja za vrtanje nujno opazovati izvire. Da bi dokumentirali vpliv vrtanja na izvirsko vodo, je bilo pri spremljanju izvirov treba upostevati vec fizikalnih in kemijskih parametrov. Zaznali smo obcasen in lokalno omejen vpliv vrtanja na kakovost podzemne vode. V prispevku smo predstavili izvajanje geotermalnega vrtanja v kraskem vodonosniku s poudarkom na identifikaciji zakraselosti ter nacrtovanju ter na izvedbi in spremljanju kakovosti voda na izviru. Tak program se lahko uporabi pri primerljivih projektih ali iskanju odgovorov na podobna vprasanja s prilagojenim programom opazovanih parametrov in intervalov merjenja. Prav tako bi lahko povecal možnosti za pridobitev soglasja za geotermalne projekte na krasu in povecal ozavescenost javnosti. Kljucne besede: vrtanje s pnevmatskim kladivom, geosonda, spremljanje podzemne vode, motnost, Hochstegenova formacija. A borehole heat exchanger array was installed in an Alpinekarst aquifer for the first time in Europe. It is composed of nine400 m deep boreholes in the dolomitic marble of the HochstegenFormation. New approaches were developed for thisunprecedented application and potential risks were checkedthroughout the sinking of the initial exploratory wellbore. AsFinkenberg is located in a tourist area, drilling operations hadto be executed within only a few weeks. Several springs downstreamfrom the drilling site are sources of drinking water andtap water supply. Given that the degree and distribution of theHochstegen Formation karstification was unknown, a springmonitoring program was mandatory in order to obtain drillingpermission from public authorities. A practicable and affordablespring monitoring program including several physical andchemical parameters was developed and implemented to documentthe potential impact of the drilling activities on the springwater. A temporary and locally limited impact of the drillingworks on groundwater quality was detected by turbidity measurements.This paper reports the implementation of geothermaldrilling in a karst aquifer with a focus on karstification aswell as planning, implementation and the results of the springmonitoring program. Such a program can be recommended forcomparable projects or questions, adapting the characteristic