Mixing of endogenous CO2 and meteoric H2O causes extremely efficient carbonate dissolution.

Karst corrosion of carbonate rocks by water with dissolved gases proceeds in most cases along two major scenarios: (i) meteoric water absorbs CO2 from soil and atmosphere, or (ii) ascending water of deep circulation carries with it dissolved endogenous gases, mainly CO2 and H2S. We have observed a peculiar variant where meteoric water absorbs ascending endogenous gases at a natural gas vent on a travertine mound in Slovakia. Carbonate dissolution's extreme effectiveness is demonstrated by mineralization of rainwater ponded at a gas vent, rising to 3.2 g/L of dissolved solids shortly after the rainfall. One liter of water ponded at the vent and mixing with the venting gas, dissolved up to 800 mg of calcium at a rate exceeding 5.8 mg/L·min. Limestone tablets placed at the vent show signs of significant corrosion, at rates up to 126 mm/ka. The rate is comparable to those in coastal karst, where freshwater is mixing with seawater and to those in sulfuric acid speleogenesis (SAS), both the highest hitherto known rates of karst corrosion in carbonates. The geomorphic effects of the process described are depressions on the surface of travertine near the vents of endogenous CO2. This type of corrosion seems to be universal and probably occurs everywhere where endogenous CO2 is exhaled to the surface from carbonate rocks.

Selected trace elements and metals in groundwater within Permian sediments near Olkusz (Zn-Pb ore mining region, S Poland).

The extensive mining of Zn-Pb ores in the Olkusz region resulted in significant changes of both water table level and chemical composition of water in all aquifers in the area. This was caused by intensive dewatering of mining excavations and development of a thick aeration zone reaching 150 m in a central part of the area. That created favorable conditions for oxidation of metal sulfides occurring in the ore-bearing dolomites (Middle Triassic) and started the process of forming readily soluble hydroxysulphates which then migrated to lower aquifers, including the Permian. As a result of those processes, various metals and other elements toxic to the water environment appeared in leaks observed in mine galleries. Changes in concentrations of selected elements (Fe, Mn, Zn, Pb, Cu, Ba, Ni, Co, As, Cr, Hg, Tl, Ag, Cd, B) in mine waters over the period of the last nearly 50 years were described. Water samples were collected from exploratory boreholes, piezometers, and wells located in investigated area inflows and seepages occurring in shafts and drifts excavated in Permian conglomerates. Mean concentrations of metals (Pb, Cd, Cr, Hg, Tl) and other toxic elements were surprisingly low; Pb, 3.94 µg/L; Cd, 0.2 µg/L; Cr, up to 2.26 µg/L; Hg, 0.25 µg/L; Tl, 3.59 µg/L; and As, 6.31 µg/L. However, the observed concentrations varied significantly over time, reaching respectively up to 190 µg Pb/L, 60 µg Cd/L, 15.6 µg Cr/L, 2.67 µg Hg/L, 81.3 µg Tl/L, and 155 µg As/L.

Impact of Zn-Pb mining in the Olkusz ore district on the Permian aquifer (SW Poland).

Long-term extensive mining of Zn-Pb ores in the Olkusz area resulted in significant changes of water table levels and chemical composition of water in all aquifers in this area. Within the Permian aquifer, hydrochemical type of water evolved in two general stages. Short-term effect was freshening in the zones of contact with overlying the Triassic limestones and dolomites. Long-term effect was a change in flow pattern and, as a consequence, an inflow of naturally altered and antropogenically contaminated water from the Triassic aquifer into the Permian complex. This was especially intensive in densely fissured and fault zones. As a result of all these processes, hydrochemical type of water shifted from multi-ion types with various combinations of ions towards higher shares of sulphates, calcium and magnesium.