Impact of past mining activities on water quality in a karst area in the Cévennes region, Southern France

Abstract

Sampling and analysis of groundwater and surface water were conducted to assess the potential impacts of abandoned mines on water quality in a karst area in Southern France. The results of multivariate statistical analysis and geochemical mapping revealed that water quality is affected by contaminated drainage from abandoned mine sites. Acid mine drainage with very high concentrations of Fe, Mn, Al, Pb and Zn was identified in a few samples collected from mine openings and near waste dumps. In general, neutral drainage with elevated concentrations of Fe, Mn, Zn, As, Ni and Cd was observed due to buffering by carbonate dissolution. The contamination is spatially limited around abandoned mine sites, suggesting that metal(oid)s are sequestered in secondary phases that form under near-neutral and oxidizing conditions. However, the analysis of seasonal variations in trace metal concentrations showed that the transport of metal contaminants in water is highly variable according to hydrological conditions. During low flow conditions, trace metals are likely to be rapidly sequestered in Fe-oxyhydroxides and carbonate minerals in the karst aquifer and the river sediments, while low or no surface runoff in intermittent rivers limits the transport of contaminants in the environment. On the other hand, significant amounts of metal(loid)s can be transported under high flow conditions, primarily in dissolved form. Dissolved metal(loid) concentrations in groundwater remained elevated despite dilution by uncontaminated water, likely as a result of the increased leaching of mine wastes and the flushing of contaminated waters from mine workings. This work shows that groundwater is the main source of contamination to the environment and highlights the need to better understand the fate of trace metals in karst water systems.