| The karst groundwater resources in South China are rich,but the karst groundwater system is fragile and the groundwater is easy to be polluted.Peak forest plain is the main runoff and discharge area of karst groundwater system,and also the area with high intensity of human activities.With the development of urbanization and economy,groundwater in peak forest plain is gradually polluted by human beings,especially nitrate pollution.Zengpiyan site is a representative site of the early Neolithic Age in South China.The protection of the site is of great significance to the study of ancient Chinese civilization.As the cave site is located at the foot of mountain in peak forest plain,the pollution left by factories and domestic sewage discharged into the underground water system of peak forest plain around the site will cause chemical erosion to the relics and ancient relics,especially the destruction of the site by the pollution of three nitrogen.Therefore,in-depth study of the mechanism of nitrogen transfer and transformation in karst groundwater is of great significance for the protection of karst groundwater and Zengpiyan site.Guilin Zengpiyan was chosen as the research site,combining with drilling and geophysical data analysis,the source and transformation of nitrate pollution are identified by hydrochemistry and nitrogen and oxygen isotope technology.The main results are as follows:?1?The water chemistry of the Zengpiyan groundwater is controlled by the wate-rock interation,and the groundwater ions are mainly derived from the weathering of carbonate rock.The groundwater water chemistry type of the Zengpiyan site is dominated by HCO3-Ca water.Furthmore,the groundwater redox environment of Zengpiyan is complex,the groundwater of kasrt conduit medium and surface water is in the stage of DO reduction;the groundwater of karst fissure medium is in the stage of NO3-reduction;the groundwater with undeveloped fissure medium is in the stage of Fe3+-SO42-reduction;the groundwater of closed environment is in the stage of SO42-reduction.?2?The spatial and temporal distribution of nitrate in groundwater of Zengpiyan rock is highly heterogeneous.The concentration of NO3-in the underground water of Zengpiyan conduit medium is higher than that in the fissure medium;the concentration of NO3-in the underground water is higher in rainy season than in dry season,and the amount of nitrate input into the underground water is increased when the rainwater replenishes the underground water,resulting in poor water quality.The potential influencing factors of nitrate pollution in groundwater of Zengpiyan including human activities,karst water bearing medium,groundwater redox environment,DO content,NH4+content and rainfall.Pollutants produced by human activities are carried into water bearing media such as conduit or fissure by rainwater.The higher concentration of do and NH4+,the oxidized groundwater environment is conducive to the transformation of NH4+into nitrate by nitrifying bacteria.?3?The range of?15N value of nitrate in the study area is-0.17‰45.13‰,the average value is12.62‰,?18O value is-5.82‰16.47‰,the average value is 7.96‰.The nitrate isotope data show that the source of nitrate in the Zengpiyan groundwater is mainly feces and sewage,and a small amount comes from NH4+in rainfall and soil organic nitrogen.Controlled by the heterogeneity of the karst medium,the NO3-concentration,?15N-NO3-and?18O-NO3-in the groundwater of Zengpiyan all show significant spatial variability.?4?The transformation process of nitrate in the groundwater of Zengpiyan is complicated and controlled by the seasons dynamics and heterogeneity of karst media.The general rule of nitrate transformation in groundwater of Zengpiyan is that nitrification is the main process in the conduit medium.In the fissure medium,the nitrate transformation of groundwater is mainly nitrification in rainy season and denitrification in dry season.Clarifying the source and transformation of nitrate can provide a scientific basis for the control of nitrate pollution in groundwater of Zengpiyan. |
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