Dissolved organic matter tracers reveal contrasting characteristics in the concentrated flow zone and matrix-with-fractures zone of a sulfate-contaminated karst aquifer in South China

Abstract

Dissolved organic matter (DOM) is of great importance in biogeochemical processes, such as sulfate reduction, in groundwater. H2S is generated from microbial-mediated sulfate reduction fueled by DOM and erodes carbonate. However, understanding the transformation and fate of DOM in sulfated-contaminated karst aquifers is still poor. Here, fluorescence spectroscopy was used to characterize the DOM pool in the sulfate-contaminated (from human activities) karst aquifer of the Zengpiyan Site, South China, by investigating the groundwater flow system. A comparison of fluorescence DOM (FDOM) characteristics between the concentrated flow zone (CFZ) composed of highly permeable small conduits and fracture networks and the matrix-with-fractures zone (MFZ) made of low permeable fractures and matrix rock aquifers was done. The relative groundwater residence times for all sample sites were estimated using γCa/γMg, which are consistent with those of the groundwater tracer test. The FDOM in groundwater in the CFZ was composed mainly of terrestrial-derived fulvic acid (FA)- and humic acid (HA)-like components, with relatively small contributions of microbially derived tryptophan (TPH)-like components. Compared with CFZ groundwater, the MFZ groundwater showed a longer residence time and reducing conditions, more bioavailable DOM, dominant TPH-like components with relatively small contributions of FA-like components, and fewer humus and hydrophobic structures. Hydrogeological conditions indirectly altered DOM composition by altering the biotic and abiotic activity conditions. The coexistence of relatively strong reducibility conditions and the higher proportion of bioavailable DOM in the MFZ would promote sulfate reduction to H2S gas, possibly amplifying carbonate dissolution, leading to the erosion of cultural relics. It is hoped that this study will contribute to knowledge on DOM behavior in karst aquifers and provide a scientific basis for cave site protection.