Multiple isotope geochemistry and hydrochemical monitoring of karst water in a rapidly urbanized region.

Abstract Karst water is an important resource for drinking water supply. To determine the impacts of urbanization on karst water quality, we performed a case study in the rapidly urbanized Guiyang-Anshun region, Guizhou province, southwestern China. We interpret data from regional reconnaissance and long-term monitoring related to major ion chemistry, stable isotopes (Sr, C, S (for SO 4 2−), and N and O (for NO 3 −)), remote sensing, and socio-economic development. We identify groundwater SO 4 2− and NO 3 − sources by combined use of δ34S and Ca2+/Na+ molar ratio and δ15N and NO 3 −/Na+ ratio, respectively. We find that carbonate, sulfide, silicate, and gypsum weathering, anthropogenic inputs, and hydrodynamic conditions account for karst water composition and its seasonal variations. Atmospheric N and S deposition, nitrification of soil N, and sulfide oxidation control the background levels of groundwater NO 3 − and SO 4 2−. The elevated concentrations of NO 3 − and SO 4 2− at residential sites in rural and urban areas mainly arise from domestic sewage. Nitrification and fertilizer application are major reasons for the high levels of NO 3 − in regional groundwater systems. Vegetated/forested land area decreases as constructed land area increases, which results in declining biogenic CO 2 production and inputs into the aquifer. Although the local government has attempted to control SO 2 emissions, substantial increases in fossil fuel utilization and fertilizer consumption as well as population growth may have increased atmospheric HNO 3 deposition and induced increasingly severe contamination of groundwater with NO 3 − and SO 4 2−. Our results improve the understanding of urbanization impacts on water quality and are important for water resource management in karst regions. Graphical abstract Unlabelled Image Highlights • Combined use of δ34S-Ca2+/Na+, δ15N-NO 3 −/Na+ may identify water SO 4 2−, NO 3 − sources. • Domestic sewage mainly accounts for high levels of NO 3 −, SO 4 2− in residential sites. • Elevated NO 3 − loads in regional groundwater arise from nitrification and fertilizers. • Urbanization may have induced groundwater NO 3 − and SO 4 2− contamination. • Rise in constructed land area decreases biogenic CO 2 inputs into the groundwater. [ABSTRACT FROM AUTHOR]