Sources of sulfur in the Nandong underground river system, southwest China: A chemical and isotopic reconnaissance

Sulfur isotopic compositions (δ34S) and major ions were measured in groundwater and surface waters in order to elucidate the source(s) of elevated SO42- content in some of these waters in the Nandong Underground River System (NURS), located in an agricultural and urbanization-dominated area in SW China. The major chemical composition of analyzed groundwater and surface water from NURS was characterized by Ca–HCO3 or Ca (Mg)–HCO3 type in forested and grass land areas, and Ca–Cl (+NO3 + SO4) or Ca (Mg)–Cl (+NO3 + SO4) type in urban and agricultural areas, respectively. The anomalously high SO42- concentrations in waters collected from urban and agricultural areas indicated a likely anthropogenic origin for SO42-. The groundwater collected from forested and grass land areas was characterized by relatively low δ34S–SO42-, from −1.7‰ to +4.3‰, and low SO42- concentrations of less than 10 mg L−1, consistent with an origin for SO42- in these waters from a mixture of rainfall-derived SO42- and SO42- from oxidation of bedrock sulfide. High SO42- groundwater and surface water (>20 mg L−1) collected from urban areas was characterized by elevated δ34S–SO42-, from +8.3‰ to +12.1‰, compared to lower δ34S–SO42- in samples from cultivated land areas (−3.7‰ to +6.4‰), indicating that the SO42- in these two groups of waters resulted from different anthropogenic sources. The S isotopic composition of SO42- in urban areas is consistent with addition of SO42- from detergent/sewage, whilst in cultivated land areas isotope composition indicates that additional SO42- could be derived from fertilizers and/or increased oxidation of sulfide.

► Origin of sulfate in the NURS was studied by the combined use of S isotopes and hydrochemistrical data.
► Highly enriched δ34S was found in groundwater and surface water contaminated by sewage.
► Relatively low δ34S values with low SO42- concentrations were derived from the oxidation of sulfide minerals and rainwater.
► While relatively low δ34S values with high SO42- concentrations were derived from the chemical fertilizers.