Anomalous arsenic concentrations in the Ďurkov carbonate geothermal structure, eastern Slovakia

Concentrations of As in carbonate reservoirs in Slovakia are relatively low, with an average value of 0.72 mg/l in typical waters of Ca–Mg–HCO₃ type. In factor analysis, arsenic is in Factor 1 with Na and Cl, separated from carbonate aquifer species Ca and Mg in Factor 2. A principal exception is the Ďurkov site in eastern Slovakia, where As concentration reaches 36.7 mg/l (median 30 mg/l) in geothermal waters of the Na–Cl type. The δ²H and δ¹⁸O isotopes indicate a primary meteoric origin of the water. Low δ²H and ¹⁴C reflect infiltration of surface water during a warmer climatic period in the Early Holocene, meanwhile positive δ¹⁸O shift (about 5 ‰) suggests enhanced water–rock interactions. The conceptual model of As-rich water formation includes the following steps: (a) Infiltration of meteoric waters and dissolution of evaporates, especially halite, in the overlying Neogene formation (b) reaction of the highly mineralized water with Hg–As–Sb type of mineralization, related to the Neogene volcanism, and, (c) accumulation of As-rich geothermal water in Triassic carbonates with an additional input of CO₂. Geochemical modeling of the reaction between water and arsenopyrite reproduced measured As concentrations for a typical range of reservoir temperatures. Analogous to similar geothermal sites around the world, concentrations of arsenic in carbonate reservoirs in Slovakia are low except for cases where As is enriched by previous water–rock interaction related to Neogene volcanic activity outside the carbonate reservoirs, which then serve exclusively for a secondary accumulation of As-rich waters.