Mine water composition in abandoned copper mines of the UNESCO Global Geopark Ore of the Alps

The central theme of the UNESCO Global Geopark “Ore of the Alps” in Salzburg’s Pongau region is prehistoric and historic copper mining in the Eastern Alpine Greywacke Zone. Much is known about the history and geology of the ore deposits. However, to date the effects of mining on groundwater quality is not investigated yet. From January to July 2024, monthly samples were taken from mine water outlets and analysed for field parameters, main and trace elements as well as sulphate isotopes. The quality of mine water was compared with reference stations sampled at the same time. The measurements show that the pH values of the mine water are in the slightly alkaline range, meaning that the heavy metal concentration is mostly harmless, although the mine water has significantly higher concentration than the reference water. The sulphate levels in the mine waters are significantly higher than those of the reference waters. Isotope analyses indicate that the increased sulphate levels are due to sulphide oxidation and thus to mining. Although systematic discharge measurements were not possible observations in one of the mines indicate a positive correlation between discharge and mineralisation. The study shows that historical mining continues to affect water composition. However, as the main ore is chalcopyrite and the mineralisation of the ore veins is bound to carbonates, the effects of sulphide weathering in the aquifer are buffered. The mine water remains pH-neutral to slightly alkaline, so it has only a low potential to dissolve harmful heavy metals. The study provides an overview of the current groundwater situation in the ‘Ore of the Alps’ UNESCO Global Geopark and confirms that there is no groundwater contamination risk from historic mining facilities. It thus contributes to a holistic understanding of this specific geo-heritage.