Sources Of Sulfate,Nitrate And Its Water-rock Interaction Process In Lihu Underground Water

Groundwater is one of the major sources of fresh water in the world, because of the relatively abundance and excellent quality, the karst groundwater has been considered as an important source of water supply, especially in the karst area of southwest China. But, in recent years, the continuous reports about the water body polluted by nitrate and sulfate have attracted world-wide attention, the governance of groundwater deterioration caused by excessive nitrate, sulfate is urgent. Before that, the most important thing is to clarify the sources of nitrate and sulfate so as to reduce the input of pollution. However, due to the double-layer structure and the fast switching of “three-water” in karst area of southwest China, the karst groundwater response to environment sensitively. At the same time, because of the randomness of nitrate and sulfate pollution, the complexity of the process of mechanism, the uncertainty of the ways and pollutants of discharging, the far-distance migration, tracing its source become more difficult, moreover, using a single indicator to determine the source of pollution is still not targeted and scientific, so it’s necessary to combine multiple isotopes to trace the source. The sulfuric acid and nitric acid which input into the underground water system by human activities participated in the water- rock interaction, caused the temporal and spatial variation of the water- rock interaction, changed the groundwater composition and geochemical cycle, and interfered with the carbon sink effect.This paper, take Lihu underground river as example, combining sulfur isotope of sulfate, nitrogen and oxygen isotopes of nitrate with hydration method to analyze the groundwater composition and its influencing factors, and reveal the sources of nitrate and sulfate, meanwhile, calculate the proportion of sulphuric acid and nitric acid dissolving carbonate rocks by hydration method. The results shows that:The groundwater is mainly characterized by HCO₃-Ca chemical composition, impacted by the weathering of carbonate rocks, anthropogenic discharge of pollution material and the self purification of groundwater, the ion concentration in dry season is higher than in rainy season and shows a decrease trend from the upstream to the downstream.The δ³⁴S-SO₄^2- values range from-4.12 ‰ to-0.93 ‰, it indicates that the ground water is discharged by the emission of domestic sewage, acid rain and chemical fertilizer, in the farming season the value is higher than in the dry season, it means that it has mixed with more chemical fertilizer in the underground water, because the chemical fertilizer has higher δ³⁴S-SO₄^2- value. The δ¹⁵N of nitrate range from-0.05‰ to 18.15‰, with an average value of 8.20 ‰, the δ¹⁸O of nitrate range from-6.71‰ to 77.04‰, with an average value of 12.58 ‰, this reveals that the main source of nitrate varies from season to season. It is complicated in January 2015, in May, October in 2014, the NO₃^- mainly effected by the mixing of synthetic fertilizer, organic fertilizer and sewage, soil nitrogen in different degrees, while the main source of nitrate is synthetic fertilizer in July 2015, and the δ¹⁸O-NO₃^- value is influenced by the atmospheric precipitation which with higher δ¹⁸O-NO₃^-value.The δ¹³C-DIC values range from-10.74 to-4.34‰ with an average of-7.36‰, the carbon isotope composition of groundwater is effected by both the natural dissolution of carbonate and the sulfuric acid, nitric acid dissolution of carbonate. Compared with Guili, Nanshan, Wujiang river, the carbon isotope composition is more positive, it indicates that the δ¹³C-DIC is affected by the DIC which with high δ¹³C-DIC value and caused by sulfuric acid, nitric acid dissolve carbonate. Equivalent rations of [Ca²⁺+Mg²⁺]/[HCO₃^-] are between 1.12 and 1.56, equivalent rations of [SO₄^2-+NO₃^-]/[HCO₃^-] are between 0.11 and 0.50, exposing that the sulphuric acid and nitric acid have took part in the water- rock interaction.Integrated water hydrochemical data, using hydration method, has calculated that the proportions of carbonic acid dissolving carbonate rocks range between 28.04% to 78.55% with an average of 50.01%, and the proportion is greater in dry season than in rainy season, in spatial, the carbonic acid weathering carbonate rocks plays a key role in the downstream Xiaolong cave; the proportions of sulphuric acid and nitric acid dissolving carbonate rocks vary from 21.45%- 71.96% with an average of 49.71%, disturbed by agricultural activities, the dissolution of carbonate rocks by sulphuric acid and nitric acid was obvious in May in 2014, under the comprehensive influence of human activities and the self purification of groundwater, the dissolution intensity of carbonate rocks in descending order are the midstream agricultural region, the upstream urban and industrial region, the downstream region with less human activities.