Human Impacts on Karst Groundwater Contamination Deduced by Coupled Nitrogen with Strontium Isotopes in the Nandong Underground River System in Yunan, China

Abstract

With the rapid increase in population and economy, groundwater quality has degraded in the Nandong Underground River System (NURS), a typical karst underground river developed in carbonate rocks (limestone and dolomite), which is located in an agriculture-dominated area in the southeast Yunnan Province, China. Determining sources of contamination in the groundwater is an important first step toward us improving its quality by emission control. It is with this aim that we reviewed here the benefit of using a coupled isotopic approach (δ¹⁵N and ⁸⁷Sr/⁸⁶Sr) to trace the origin of contamination in groundwater. Thirty-six representative groundwater samples, which were collected at different aquifers and land use types, showed significant disparities for major element concentrations and Sr and N isotopic composition in the NURS. Nitrate, along with Cl⁻ and SO₄²⁻ and some Na⁺ and K⁺, pollution of groundwater is a significant problem in the NURS. The joint use of nitrogen and strontium isotope systematics in each context deciphered the origin of contamination in groundwater in the NURS as agricultural fertilizers and sewage effluents. Therefore, an increase in knowledge of groundwater geochemistry by means of hydrochemical and isotopic data will be helpful for understanding water−rock interactions and the influence of human activities on the hydrogeochemical environment of karst groundwater and provide a scientific basis for protection and rational utilization of groundwater resources in karst regions.