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

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 SO42-), and N and O (for NO3-)), remote sensing, and socio-economic development. We identify groundwater SO42- and NO3- sources by combined use of δ34S and Ca2+/Na+ molar ratio and δ15N and NO3-/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 NO3- and SO42-. The elevated concentrations of NO3- and SO42- 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 NO3- in regional groundwater systems. Vegetated/forested land area decreases as constructed land area increases, which results in declining biogenic CO2 production and inputs into the aquifer. Although the local government has attempted to control SO2 emissions, substantial increases in fossil fuel utilization and fertilizer consumption as well as population growth may have increased atmospheric HNO3 deposition and induced increasingly severe contamination of groundwater with NO3- and SO42-. Our results improve the understanding of urbanization impacts on water quality and are important for water resource management in karst regions.