Changes of groundwater chemistry caused by the flooding of iron mines (Czestochowa Region, Southern Poland)

Abstract

Underground mining always causes disturbances of the hydrogeological environment, primarily due to the dewatering of mine workings. The flooding of abandoned mines contributes to the rebound of the cone of depression developed previously, but it also leads to significant pollution of groundwater in the mined areas. The Middle Jurassic aquifer in the Czestochowa region of the flooded iron mines has been investigated with respect to patterns of water movement and groundwater chemistry. Basing on eight years of studies including field measurements, data collection and physicochemical analyses of groundwaters, two models has been developed: a hydrological model of water flow and a hydrochemical speciation model. Additionally, in order to validate the results, stable isotopes of hydrogen and oxygen, together with tritium and radiocarbon, have been analysed. Four distinct hydrochemical zones has been identified within this aquifer based on the results of these studies: Zones I and II — where the groundwater is influenced by iron mining, and Zones III and IV — where the groundwater in the Middle Jurassic aquifer is uncontaminated. The results indicate that the changes in groundwater chemistry within the flooded iron mines area are significant. Maximum SO 4 2− contents reach 1103.0 mg l −1 and are 6 times higher than the acceptable levels for human consumption of water (Polish Drinking–Water Limits), Mn 2+ contents reach 6.61 mg l −1 and are 70 times higher, and Fe tot reach 271.4 mg l −1 and are 500 times higher. Water flow modelling and hydrogeochemical interpretations suggest that waters in the zone III, in spite of their occurrence within the area of the cone of depression caused by the iron mining, are not influenced by the polluted mine waters. This is corroborated by the isotopic investigations. The age of waters from this zone has been determined as Early Holocene, which strongly suggests that these waters had recharged before the dewatering began. The results of the hydrochemical studies suggest that fifteen years after the flooding, waters stored in the mine workings may be defined as quasi-stagnant. Middle Jurassic aquifer water supplies located in Czestochowa are not polluted by mine waters today, but the predicted contamination of waters will occur in the first supplies at the end of the 21st century. However, it may last for 300–500 years. The general conclusion is that if the formation of the polluted acid and neutral waters in such mined sedimentary ores as in the Czestochowa region is not prevented, the residual pollution of groundwaters in the flooded mines may persist for several centuries.