Geochemical Characteristics Of Snowfall And Its Influence On Groundwater In Southwest Subalpine Karst Area

With the acceleration of China's modernization process,the problem of atmospheric pollution has become increasingly serious.Substances in atmospheric,like pollutants and aerosols,are mainly removed by atmospheric wet deposition,so snowfall can better indicate the regional atmospheric environment and participate in surface biogeochemical cycles.At the same time,snowmelt water is an important global and regional freshwater resource.The special structure of karst area?both over-ground and under-ground?caused the coexistence problems of groundwater contamination and groundwater resources utilization difficulties,which is particularly evident in sub-alkaline karst areas in China.In view of the vulnerability and importance of karst groundwater,it is imperative to launch the research about geochemical characteristics of snowfall and its impacts on groundwater chemistry in sub-alkaline karst areas,quantitatively assess the contribution of snowfall to groundwater resources for protecting the karst groundwater quality,rational developing and utilizing the groundwater resources.Also,it is an exploration and supplement to the evolution process of the karst dynamic system under the influence of snowfall.Selecting the karst groundwater basin in Jinfo Mountain as the research area,this study has sampled the groundwater and other water bodies periodically and with high-resolution from March 1,2016 to April 3,2017.The snowfall samples were collected 9 times,from November 2016 to March2017.Environmental indicators,isotopes,inorganic ions,and trace elements in water bodies were tested to study the isotopic characteristics and influencing factors of snowfall,the material composition features,changes and sources.The thesis also analyzed the isotopic and hydrochemical characteristics of groundwater with the influences of snowfall,and quantitatively assessed its contribution to groundwater discharge.Results showed that:The concentrations of?¹⁸O and?D of snowfall,rainfall,and soil water fluctuated greatly,with less fluctuations in sewage and groundwater.The seasonal changes of?¹⁸O and?D for snowfall and rainfall were obvious,which showed"negative temperature effect"and"precipitation effect"together.The?¹⁸O and?D in soil water changed significantly in the summer half year,and stabilized in the winter months,while the overall change of sewage and groundwater were stable.The slope and intercept of the?D-?¹⁸O evaporation equation line for snowfall(?D=8.41?¹⁸0+24.24),precipitation and soil water were commonly greater than those of the GMWL,and the sewage and groundwater were significantly smaller.The pH value of snowfall was acidic,and its conductivity was low and existed large variation.In snow,the contents and contents variation of each trace elements were significantly different,and the concentration and concentration fluctuations of anions were greater than those of cations.The concentration of SO₄^2-in snow were much higher than other water bodies,and with higher Cl^-,HCO₃^-,NO₃^-and Na⁺.The concentration of NO₃^-,K⁺and Si in soil water were significantly higher than other water bodies;HCO₃^-,Cl^-,Na⁺and Fe in the wastewater were the highest,whereas its concentration of NO₃^-was the lowest;The concentration of Ca²⁺in the groundwater was highest,secondly were the HCO₃^-,Cl^-and SO₄^2-.The seasonal changes of chemical ions and trace elements in snow were significant,and they were positively correlated with the change of snowfall.The origin,characters and migration paths of the water vapor that formed the snowfall in Jinfo Mountain were the fundamental reasons that affected the composition of isotopes,inorganic ions,trace elements,and its seasonal changes.Due to the effect of southwestern warm-humid air masses,the?¹⁸O and?D in snowfall were significantly lighter,and the contents of inorganic ions and trace elements were obviously lower at the end of the fall.In winter,the air mass originated from the oceanic warm-humid air masses to the continental dry air masses.Thus,isotope values and materials composition of snow increased continuously.At the end of the winter and the beginning of the spring,under the control of oceanic warm-humid air masses and continental air masses,the above-mentioned indicators decreased as a whole,and there was also a slight increase under the control of continental air masses.High concentrations of Cl^-,SO₄^2-and NO₃^-in snow were all derived from the pollution of fossil fuels,automobile exhaust and other human activities.Ions of K⁺and Na⁺in snow all came from the soil,weathering of sodium-bearing rocks,rock salt minerals;Ca²⁺mainly originates from CaCO₃dust produces by silicate industry and construction industry activities in the research area and surrounding;Mg²⁺was affected by the dual effects of ocean aerosol,soil and rock weathering;Al and Si were mainly derived from soil and rock weathering.Man-made pollutants?like metal smelting,etc?had a certain influence on the contents of Ba,Sr,Fe and Mn;Li,Cr,Cu and Zn were affected by fossil fuel combustion,metal smelting,and traffic pollution;Garbage incineration and landfill,heavy oil combustion contributed significantly to the Cd,Pb and Co contents in snow.The large fluctuations of?¹⁸O and?D in snowfall had little effect on groundwater isotopes,as well as rainfall and soil water isotopes.Sewage had a significant impact on the mixing of groundwater,and it covered up the isotope information of groundwater under natural conditions to a certain extent.The ablation and infiltration of snow were slow,which facilitated the more sufficient water-rock effect of the groundwater,so the concentration of Ca²⁺,Mg²⁺,and HCO₃^-in groundwater increased.And the infiltration of Cl^-and SO₄^2-in snow had a direct supplement to the corresponding ions and conductivity of groundwater.Acidic snowfall and acidic ions in the soil carried by snowmelt water reduced the pH value and increased the electrical conductivity of groundwater.Large snowfall and rapid snowmelt ablation have a significant dilution effect on main ions and conductivity of groundwater.The sewage discharge brought by tourism activities after snowfall led to a significant increase of main ions concentration and conductivity of groundwater,and the decrease of pH values.The groundwater flow changed significantly with snowfall and ablation speed of snowcover.Isotope hydrography separation results of Shuifang Spring under the natural condition indicated that the contribution of interflow and rainfall to runoff during non-snowfall periods were 34.9%⁸6.2%,13.8%⁶5.1%.During snowfall,the contribution ratio of snowmelt,interflow,and rainfall were4.4%⁶6.5%,30.3%⁷1.4%and 0.2%²4.2%.Therefore snowmelt had a prominent contribution to runoff,while the contribution of interflow reduced slightly,and contribution of rainfall reduced greatly.The contribution of snow to groundwater runoff varied obviously due to the differences of snowfall and the snow melting rate.Due to the complicated hydrogeological conditions in the study area,and the strong influence of sewage on the geochemical characteristics of groundwater,some groundwater hydrography separation results were non-ideal.The applicability of isotope hydrography separation method in karst spring area remains to be further verified.