Effects of human activities on karst groundwater geochemistry in a rural area in the Balkans

Abstract

Abstract The geochemistry of three drinking water sources (wells, springs, and tap) in the Vratza region of northern Bulgaria was investigated to gain an understanding of how human activities influence natural geochemical processes in a carbonate aquifer system. Numerous villages have been identified within this highly agricultural area as endemic for the environmental disease Balkan endemic nephropathy, with some suggesting a link to the geochemistry of the region. We observe that water quality varies significantly as a function of source. Hydrochemical facies analysis reveals trends not typical for limestone systems, with cation trajectories falling along the Ca2+–Mg2+ axis and the anion trajectories lying along the HCO 3 - – SO 4 2 - axis. R-mode factor analysis reveals: (i) an increasing dominance of a Cl−–Na+– SO 4 2 - – NO 3 - association from tap to spring to well waters, (ii) a strong association between Mg2+ and U, and (iii) a lack of associations between Ca2+ and Mg2+ and between Ca2+ and HCO 3 - that is atypical for limestone systems. These observations are interpreted to indicate that human activities have influenced all sources across the Vratza region and that Mg and U concentrations result from the dissolution of limestone, which is enhanced due to the input of anthropogenic chemicals. Geochemical modeling indicates that Ca2+ concentrations are in equilibrium with calcite, but that Mg2+ concentrations remain conservative, resulting in the decoupling of a Ca2+ and Mg2+, association. Uranium concentrations also appear to be conservative, aided by a poised redox state due to high concentrations of NO 3 - and the formation of CO 3 2 - and PO 4 3 - ion pairs. The similar source and conservative nature of both Mg2+ and U can account for their association. Finally, cluster analysis indicates that the pattern of water geochemistry in the endemic area differs from the surrounding non-endemic area, may be explained by differences in the underlying geology. This study has shown that the general water quality in this rural area, whether in endemic or non-endemic villages, is poor and that agricultural activities have not only added chemicals to the groundwater system, but that these chemicals have likely disrupted geochemical processes. More work is required to understand the specific details of anthropogenic influences on geochemical processes on water quality in karstic terrains.