Biological sulfate reduction in an epigene karst aquifer and its impact on cave environment

Abstract

Exogenous sulfate (produced by anthropogenic activities) in karst aquifer does not only induce groundwater pollution, reduction products of sulfate may also acid cave environment. However, studies have not explored whether sulfate reduction can occur in pure limestone aquifer under low temperature. In this study, it was observed that sulfate concentrations of groundwater were high (up to 131 mg/L) and irregularly distributed outside the cave, while it dropped to low values of 3.96 mg/L on average inside the cave. Influenced by the heterogeneity of karst development and hydrological conditions, the groundwater environment exhibited oxidation or reduction, and had a strong reducing condition in the saturation zone of the cave, with the oxidation–reduction potential (ORP) ranging from −99 mV to −344 mV. Microbial community analysis revealed the presence of anaerobic bacteria; in particular, sulfate reducing bacteria (SRB) was prevalent in the groundwater. Generally, H2S could be detected at the groundwater table inside the cave, with possible values ranging from 0.056 to 0.444 mg/m3. The findings of this study revealed that sulfate reduction occurred specifically in the epigene karst aquifer in the presence of SRB. The difference in karst development and hydrological conditions, as well as the increasing contaminant enrichment of the cave environment, are the adduced reasons for the difference in water chemistry and the strength of sulfate reduction in the study site. The volatilization of H2S into the moisture on the cave wall precipitated the oxidation of H2S to H2SO4, which then participated in carbonate dissolution, inducing damage to cultural relics from the cave site. The results of this study have contributed to enhancement of knowledge on H2S production in epigene karst aquifers and provided scientific support to management strategies in cave sites.