Identification of dissolved sulfate sources and the role of sulfuric acid in carbonate weathering using δ13CDIC and δ34S in karst area, northern China

Abstract

Karst groundwater is one of the most important water resources for agriculture and industry in Fenyang City of Shanxi province, northern China. In the last few decades, excessive exploitation of groundwater and other human activities have caused serious environmental problems in this area. Hence, it is essential to identify the resources of pollutants that deteriorate the water quality in the area. A total of 16 water samples were collected to assess the hydrochemical characteristics from wells and springs in bare karst area, northern Fenyang and covered karst region, south of the city. The chemical composition of groundwater was characterized by a dominance of Ca2+, Mg2+, HCO3−, and SO42−, which accounted for more than 85 % of the total ion concentrations, indicating that limestone dissolution by weathering controls water chemical composition. SO42− in the groundwater showed high positive significant correlation with HCO3− and δ13CDIC, showing that carbonate dissolution by sulfuric acid has important contribution to groundwater chemical composition and δ13CDIC value. The relationship between δ34S and 1/SO42− revealed that the sources of SO42− are different in various types of groundwater. In the bare karst area, SO42− in springs mainly came from atmospheric precipitation, while in the shallow wells it mainly came from human activities; in the covered karst region, SO42− in quaternary pores water was mainly from anthropogenic inputs, while in the deep wells it was mainly from gypsum dissolution; in gypsum mine water, SO42+ was mainly from gypsum dissolution. Comparing with the bare karst area, human activities have significantly changed the chemical composition of groundwater in the covered karst region. Therefore, more control policies and measures should be formulated and applied to protect the covered karst region in the future.