Oxidation-reduction And Characteristics Of Hydrogen Sulfide Release Of Contaminated Groundwater At Zengpiyan Site In Guilin

The convocation of the 19th National Congress of the Communist Party of China further enhanced the important strategic position of"ecological progress"."Lucid waters and lush mountains are invaluable assets.".It is the only way for sustainable development to protection nature resources in China.Zengpiyan is an important human site in the Neolithic period in southern China.It has rich cultural relics,so it is particularly necessary to protect cultural sites.Zengpiyan is located in the Guilin peak forest plain.There are abundant groundwater resources.With the development of urbanization,the stability of the site is threatened by groundwater dynamic erosion and chemical erosion.At the end of the 1950s,the waste water from the surrounding factories is discharged directly into the groundwater,causing serious pollution to the groundwater,but the groundwater pollution is subtle,complex,and difficult to reverse,with long-term impact.At present,the problem of sulphate pollution is the most serious index.According to long-term monitoring,it is found that some sites in the core of the site are characterized by reducing environment,and the smell of groundwater is very smelly.These problems need to be solved.Therefore,we selects Zengpiyan as the research object,and conducts long-term continuous monitoring,Based on the study of chemical characteristics,redox environment of the groundwater,and the enrichment characteristics of H₂S in aeration zone,to analyzes the migration and transformation of groundwater pollutants,the oxidative-reduction zone of groundwater,and the law of migration and transformation of sulfate in the reducing environment,to explore the impact of H₂S on the cultural layer of the site and provide scientific basis for protect human site.In this paper,21 monitoring sites were selected to monitor groundwater levels,hydrochemical parameters,and other indicators for the period from 2016 to 2017,and hydrogen sulfide was collected and analyzed from November 2016 to January 2018,it was used to study the characteristics of the accumulation of erosive gases and their effects on the stability of cave sites under the environment of sulfur-rich groundwater.The results show that,the groundwater flows along the overall runoff path from the Huaxia Art Park through the caves of Zengpiyan and continues discharged to downstream.The buried depth of groundwater in retort is shallow.The average buried depth of water level at each monitoring point is between 1.89 and 13.07 m,which belongs to shallow groundwater.The water level in the upstream is higher than downstream,and the water level amplitude of upstream about 1.7 m.the site of the cave and downstream is smaller,1.1 m and 1.3 m,respectively.The results of hydrochemical data analysis show that several characteristics of groundwater hydrochemistry in Zengpiyan are as follows○1 The hydrochemical types of groundwater in the site are complex.Although the spatial distance of each sampling point of site is close,the ion composition is quite different.The main types of hydrochemistry are HCO₃-Ca,HCO₃-Ca·Mg,and HCO₃·SO₄-Ca·Mg and HCO3·SO₄-Ca are also found in some months and sampling points.From the cationic mass concentration,Ca²⁺>Mg²⁺>Na⁺>K⁺.From the mean of the anion mass concentration,HCO₃^->SO₄^2->Cl^->NO₃^-.Ca²⁺,Mg²⁺and HCO₃^-mainly come from rock weathering,especially carbonate rock wearthing.○2 There are obvious seasonal variations in the mass concentrations of K⁺、Na⁺、Ca²⁺、Cl^-.K⁺and Ca²⁺showed higher concentration value in wet season than dry season,while Na⁺and Cl^-were opposite.○3Sulfate pollution is serious,the seasonal variation is not obvious,The variation of sulfate concentration has a tendency to decrease with the runoff direction of groundwater,The sulfur content in the upstream（Huaxia art garden）is higher than that in the downstream（the core area of the site）.The ZK10 borehole has obvious abnormal low value.Comparing the concentration values of sulfate,nitrate and ammonium in different sampling points with the initial pollution value,it is found that the concentration of sulfate and ammonium in groundwater is increasing,the concentration value of sulfate pollution high value area is increased by 5-8 times,and the concentration of nitrate is decreased.The hydrochemical characteristics of ZK10 borehole groundwater have obvious particularity.According to the redox sensitivity index and the reduction products,the monitoring points were classified.the results showed that,ZK10 is in sulfate reduction stage;ZK15,ZK20,ZK21,Y2,JD,and SD are in nitrate reduction stage,The remaining monitoring points are in the stage of dissolved oxygen reduction stage.Dissolved oxygen reduction stage and nitrate reduction stage belong to partial oxidation environment,sulfate reduction stage belong to partial reduction environment.The stronger the oxidizing ability,the greater ability to the final electron acceptor for the final electron,the easier it is to be consumed,the easier it is to be polluted and transformed into anaerobic reducing environment.In the reducing environment,the content of substances is relatively high;in the oxidizing environment,the content of oxidizing substances such as NO₃^-,SO₄^2-and DO is relatively high.The groundwater of the site ZK10 is relatively closed,the flow rate is slow,and groundwater will stay a long time in the aquifer.Strong sulfate reduction is the main factor affecting the low concentration of SO₄^2-in ZK10.After sulfate is reduced,the concentration of sulfate is reduced.Because of the supply from upstream groundwater that containing sulfate,Sulfate Reducing Bacteria can obtain continuous supply of sulfate.Sulfate reduction continues to generate hydrogen sulfide gas into the environment,which affects the preservation environment of the site cultural layer.The concentration of hydrogen sulfide in the core area of the site is very unstable and varies greatly.The maximum value of hydrogen sulfide appeared in September and November 2017.At the same time,the concentration of each monitoring point in the study area showed an increasing trend,and the other time remained relatively stable.The results of dissolution test show that,comparing the dissolution rates of the limestone samples in the air with those in water of ZK10 and ZK1,the former is faster than the latter,about 52%.Indicating that it is very likely that hydrogen sulfide is oxidized to generate sulfuric acid,in drilling air,which accelerates the dissolution rate of limestone samples.Therefore,if hydrogen sulfide is continuously enriched in the vadose zone or cave space,and co-participates in the dissolution of carbonate rocks in the cave,it will further exacerbate the chemical erosion effect on the cultural layer of the site.