Use Of Multiple Environmental Isotopes(δ³⁴S,δ¹⁵N,⁸⁷Sr/⁸⁶Sr Andδ¹³C_DIC) To Trace Sulfate And Nitrate Contaminations Of Karst Groundwater In An Agricultural Area

Karst landform has unique dualistic hydrogeology, where dynamic changes of groundwater are highly related to surface water and they always show quickly mutual conversions. The distribution of groundwater is uneven. Due to thin soil layers or lack of soil, the capacity of leaching and purging surface water is limited, leading to fragile underground hydrological system, which may easily suffer interference from anthropogenic activities. Especially, in some mountainous area and agricultural region where karst groundwater is used as drinking or domestic water, problems of water resources are becoming increasingly serious.Qingmuguan subterranean stream, located in Chongqing, is a typical karst subterranean stream that develops at a karst trough in Southwest of China. The study area is dominated by agricultural activities. Also, there is some quarry and discarded coal kiln in the internal trough. Jiangjiaquan, at the exit of the subterranean stream, is an important water resource provided for nearby residents. So, the water quality is directly connected to human health. In this thesis, dynamic changes, influencing factors and sources of contaminants of groundwater were studied to explore temporal and spatial variations and laws of groundwater, which will help to utilize karst groundwater and to control contamination.In this thesis, a combined method of field sampling, situ monitoring, analyzing of hydrochemistry and isotopic compositions was used to analyze surface water and groundwater in Qingmuguan subterranean stream watershed. The sampling activities had been taking place from April,2010 to May,2011.4 (once every month)。Items of analysis included Temperature Specific Conductivity, pH, Ca2+, HCO3-, K+、Na+、Ca2+、Mg2+、Sr2+、HCO3-、NO3-、SO42-、Cl- et al. Based on above items analyzed, general characteristics of hydrochemical variations were obtained. We collected samples in high density from the end of May to the beginning of June when high-dense agricultural fertilization activities had been occurred. In this case, we studied the variational laws and influencing factors of groundwater quality. We analysed C, N, S and Sr isotopic compositions of Qingmuguan subterranean stream watershed to distinguish the effects of natural factors and anthropogenic activities.In conclusion, there were significant seasonal variations of groundwater quality in Qingmuguan subterranean stream watershed. During the period of densely agricultural activities, the groundwater quality of Jiangjiaquan was strongly impacted by anthropogenic inputs, which leads to a high risk of contaminations. In the low-flow period, the groundwater was mainly recharged by lateral and split water where water-rock interaction was dominant factor.The range of carbon isotopes was large and the calculated values were corresponding to monitored values. The range of monitored carbon isotopes was between -8.17‰and -9.70‰with an average value of -9.25‰during the fertilizing period. However, during the period of July and February of next year (the low-flow period), the range of monitored carbon isotopes was between-11.5‰and -13.68‰with an average value of -12.13‰. It is obvious that carbon isotopes in fertilizing period were likely to be more positive than other hydrological periods, suggesting effects from dissolution of carbonate rocks by sulfuric acid and nitric acid.The Sr isotopes were within a narrow range, between 0.708 and 0.710. Specificly,87Sr/86Sr ratios from Jiangjiaquan had a range of 0.70836 and 0.70910. Sr concentrations and isotopic compositions showed similar values in paddy fields and Yankou sinkhole that locate in upper stream, which revealed that the both surface waters had the same sources that affected by fertilizer contamination. Theδ15NNO3 proved that groundwater of Qingmuguan was disturbed by anthropogenic activities.δ15NNO3 of Jiangjiaquan showed lower values in rainy seasons, indicating that nitrate in groundwater was mainly from mineralization of soil organic nitrogen, and parts of nitrate might came from remaining fertilizers in paddy fields. In contrast, the principle sources of nitrate were mixed with soil organic nitrogen, domestic sewage and manure. The average sulfate concentrations of paddy fields and Yankou sinkhole were 63.03mg/L and 67.60mg/L, respectively, withδ34S of 9.95‰and 8.97‰. It indicated that sulfate could have come from anthropogenic activities, such as fertilizing and domestic sewage. Another interpretation could have originated from dissolution of gypsum. The average SO42- concentration and its isotope were 63.22mg/L and 7.76‰, suggesting a combined contribution from precipitation and anthropogenic inputs.