Environmental isotopes (δ18O, δ2H, 222Rn) and hydrochemical evidence for understanding rainfall-surface water-groundwater transformations in a polluted karst area

Abstract

The vulnerable karst water systems and the serious pollution issues in Southwest China have been facing tremendous challenges, especially when controlling processes of transformation in rainfall, surface- and groundwater (SW and GW) are poorly understood in such areas. In this study, transformation processes have been identified based on physical–chemical indicators (T, DO, EC, HCO32−, SO42−, Ca2+, and Mg2+), δ2H and δ18O isotopes, and 222Rn activities in different water forms. The δ2H and δ18O isotopic signals of the river are depleted in the upstream non-karst area due to the altitude effect. The karst aquifers are dominantly recharged by local rainfall in summer and by river water from the non-karst area in winter indicated by the seasonal variation of stable isotopic compositions. A 222Rn mass-balance model was constructed to estimate GW-SW exchange during the autumn period. Net gaining and net losing sections (including a segment of conduit flow) were identified to be fluxes of 3.6 × 10−3 and −4.0 × 10−3 m3 s−1 m−1 respectively. The bank storage water contributed critically in the net gaining section, deduced by the inconsistent responses of stable isotopic and chemical tracers to the GW influent, which resulted in a relative identical amount of influent and effluent water (5.0 × 10−3 and −1.4 × 10−3 m3 s−1 m−1 respectively). In the net losing section (influent and effluent are 2.8 × 10−4 and −4.2 × 10−3 m3 s−1 m−1 respectively), interactions through the segment of surface flow are sluggish, which is standing in stark contrast to the well-mixed flow in the conduit segment. The results reveal various signs of sulfides oxidation from the mine and tailings, along with liberated metals could be introduced and travelled across in the transformations of rainfall-SW-GW. It needs to put more efforts into the research of water quality and transport of metals hereafter.