Effects of an abandoned Pb-Zn mine on a karstic groundwater reservoir

Abstract

The release and transport of metals in active mines pose a threat to water quality when mine sites become abandoned. Karst aquifer systems are especially vulnerable due to their ability to quick transport polluted water over large distances through surface and subsurface routes. Surface and groundwater samples collected from the Guilin-Yangshuo Basin in southwestern China were analyzed for δ18O and δ2H of water, major and trace elements, including Sr, pH, temperature, and electrical conductivity to gain an understanding of how an abandoned Pb-Zn mine and its tailing deposits influence the groundwater quality of a downgradient karst carbonate aquifer. The δ18O and δ2H values of the water samples indicate that the karst aquifer is dominantly recharged by local precipitation during the wet season and additionally by water from upstream mountainous areas during the dry period. Severe heavy metal contamination (9154, 1081, 37 and 55 μg/L of Zn, Mn, Ni and Cd, respectively) was detected in the mine drainage water. Ninety-one percent of the downstream surface samples and 67% of the groundwater samples collected during the dry season exceeded the limit value for As (10 μg/L) according to the China Standards for Drinking Water Quality. Factor analysis was applied, and showed the river and groundwater were affected by the Pb-Zn mine and its tailing deposits and by carbonate dissolution. The Sidi River plays an important role in trace element transport, which supported by a strong correlation between sulfate and strontium (with r2 = 0.85 in the dry season; r2 = 0.94 in the wet season). The strong acid generated during sulfide oxidation of the mine tailings is neutralized by carbonate rock dissolution, resulting in neutral to alkaline water which hinders the mobility of trace elements. However, the increasing sulfate contents and the elevated total trace element concentrations (from undetectable to 1503 μg/L except for the mine drainage with 10,350 μg/L) confirm the release of toxic metals into the water system. In addition, adsorbed arsenate anions may exchange with dissolved carbonate anions and be released into the groundwater.