Carbon And Nitrogen Isotope Geochemistry Of Karst Groundwater In City

In recent years the application of stable isotopes is till growing in environmental studies. Stable isotope composition of carbon and nitrogen have been used extensively to provide information on the solution of a wide variety of environmental problems, such as the affects of contamination to the carbon and nitrogen biogeochemistry, the sources of greenhouse gases, eutrophication, nitrate pollution in groundwater and so on. Using of multi-isotope to trace contaminant transportation and transformation is one of new frontiers in stable isotope geochemistry.The objective of this study was to use stable isotope method (13C, 15N, 18O) and water chemistry approach to (1) evaluate the carbon and nitrogen deposition of rain in Guiyang; (2) understand carbon evolution and the relation with contamination in groundwater system in Guiyang and Zunyi; (3) ascertain the sources of nitrogen pollution in groundwater system in Guiyang and Zunyi. The main conclusions have been summarized as follows:1. The average concentrations of DOC (dissolved organic carbon) and POC (particulate organic carbon) in rain of Guiyang are 3.76mg C/L and 0.54 mg C/L, respectively. The DOC values have negative correlation with pH, which indicates that organic acid contributes to the acidity of rain in Guiyang. δ13C-POC of rain ranges from -27.0 to -24.0‰ and shows no distinct seasonal variations, suggesting that POC mainly comes from local atmospheric dust.2. The mean values of NH4+ and NO3- of rain in Guiyang are 0.81mg and 0.51 mg N/L, respectively. The average of δ15N-NH4+ is -4.7‰, indicating that volatilization and selective washout of N-bearing material are the major factors. The average of δ15N and δ18O of nitrate are +1.5‰ and +34.2‰, respectively. There are no distinct seasonal variations of isotopes of rain in Guiyang. Nitrate in the precipitation mainly comes from incomplete combustion of fossil fuels and vehicle exhaust. Meanwhile, meteorological conditions play an important role in controllingisotope fractionation in the precipitation.3. In ground water Ca2+ and Mg2+ are the dominant cations, and HCO3- and SO42-are the dominant anions, suggesting the important contributions principally of carbonate weathering and oxidation of FeS and/or dissolution of gypsum strata to the solutes of ground water. Concentrations of K+, Na+, Cl- and NO3- in some of ground water samples are relatively high, probably due to anthropogenic pollution.4. Concentrations of DIC (dissolved inorganic carbon) are generally lower in summer than in winter, reflecting the dilution effects of the heavy monsoon rains; The δ13C values of DIC in ground water higher in winter than in summer, which suggests that biogenic CO2 makes a larger contribution to the DIC in summer than in winter. The calculated results show that more than 50% of the DIC in ground water is from dissolution of carbonate rocks, which is possible that sulfuric acid plays an important role in weathering of carbonate bedrock.5. The concentration of DOC and POC in most of groundwater samples are lower than 2.00 mg C/L and 0.50 mg C/L, respectively. The δ13C-POC values are higher in summer than in winter, suggesting some particulate organic carbon from C4 plant in summer. There is positive relationship between TOC (DOC+POC) and anthropogenic ions (K++Na++Cl-) in surface water, which indicates that the concentration of organic carbon was affected by anthropogenic pollution.6. δ13C of DIC in ground water might be a useful tracer to monitor ground water pollution and the change in ecology of ground water due to anthropogenic pollution. For example, Ground water samples collected from the urban, as well as the northeast and west suburbs of Guiyang city are influenced by contamination according to δ13Cdic contour and water chemistry distribution.7. Nitrate is the major dominant specie of nitrogen in most water samples. But high concentrations of ammonium were detected in sewage effluent and contaminated waters, showing ammonium has become the major species of nitrogen pollution in som...