Production Mechanism Of Corrosive Gas At The Water-air Interface Of Groundwater At Zengpiyan Cave Site In Guilin

As one of the important water sources of human production and life,the quality of groundwater is related to the sustainable development of economy and society.In the covered karst area of peak-forest plain,the surface has different thickness of covering,and the surfacing karst shape is lacked.The groundwater dominated by fracture media is difficult to obtain oxygen and forms a reducing environment.When the pollutants produced by human activities enter into the groundwater,their properties may change,resulting in poor water quality and other adverse consequences,even threatening human health.Zengpiyan cave site in Guilin is a typical representative cave site of the Neolithic Age in China.The cultural layer of the site is located in the seasonal variation zone of groundwater,which is not only affected by the hydrodynamic erosion caused by the fluctuation of groundwater,but also by the chemical erosion of groundwater,which may be intensified after the groundwater is polluted.Therefore,it is very important to protect the environment of the site.In this paper,the physical and chemical characteristics of groundwater in different karst medias,the temporal and spatial changes of sulfate and organic matter,the content changes and influencing factors of Sulfate-reducing Bacteria(SRB),the concentration characteristics of H2 S at the water-air interface of groundwater and CO2 and the diversity characteristics of microbial community were analyzed,to reveal the production mechanism of corrosive gas at the water-air interface of groundwater,so as to provide a basis for the study of site preservation environment.The results showed that:(1)The main physical-chemical parameters of water body had obvious temporal and spatial changes.The temperature of surface water fluctuated seasonally due to the influence of surface temperature,while that of groundwater changed stably due to reflect the temperature of karst aquifer.p H was weakly alkaline,and slightly lower in spring and summer than in autumn and winter.Ec was diluted by rainwater and appeared to be higher in winter than in summer.In addition to being more negative in autumn in terms of time,ORP was also quite different in space.The chemical types of water body were mainly HCO3-Ca type,followed by HCO3-SO4-Ca type,HCO3-Ca-Mg type and HCO3-SO4-Ca-Mg type.Due to the dilution effect of precipitation and the water rock interaction,the concentrations of main dissolved ions had obvious seasonal variations.The concentrations of K+,Na+,Cl-,NO3-,SO42-and Mg2+trended as winter > autumn > spring > summer.The concentrations of Ca2+and HCO3-were relatively low in summer.In space,the differences of physical and chemical properties between sample points were influenced by water storage media and human activities.(2)The groundwater in Zengpiyan area was generally polluted,and some indexes were not reach to the class ? groundwater quality standard of China.The water environment rich in sulfate and organic matter provided preconditions for Sulfate-reducing Bacteria to play a role.The spatial distribution of SO42-concentration was very uneven,the ZK10 sample point in the cave showed a very low value.There were significant differences between ZK10 and other sample points in various water quality indexes.ZK10 showed a strong reducing environment,with a pungent smell.Adjacent samples occasionally showed similar characteristics.In the dry season,Dissolved Organic Carbon(DOC)was significantly correlated with NO3-and ORP in groundwater.In the rainy season,the correlations were weak due to the unstable source of DOC and the strong conversion of organic matter components.?13CDIC fluctuated more in rainy season,that is to say,the carbon source was more complex in rainy season.The organic matter degradation of DIC played an important role in the formation of DIC in rainy season.(3)SRB was generally participated in sulfate reduction in Zengpiyan area.The content of SRB determined by temperature,precipitation and organic matter trended as rainy season > dry season,surface water > groundwater.In the environment with low sulfate concentration,SO42-concentration was the limiting factor of SRB growth,but when SO42-concentration was high,the limiting effect would be greatly weakened.The H2 S concentration of sulfate reduction product varied greatly in season,and was significantly affected by temperature.Both the SRB and the reduction degree of environment can promote the formation of H2 S and the organic matter degradation product of CH4.(4)The dissolution of carbonic acid combined with good hydrodynamic conditions promoted the dissolution of carbonate rocks in Zengpiyan area,while H2 S played an accelerating role to a certain extent.Therefore,if the production capacity of H2 S in the cave site is increased and diffused outwards,it will accelerate the erosion of carbonate rocks and eventually threaten the preservation of cultural layer of the site.(5)The composition and structure of water microbial community in Zengpiyan area were complex,and there were significant differences among fissure water,conduit water and surface water.The dominant phyla of fissure water were Proteobacteria,Patescibacteria and Actinobacteria;while the conduit water and surface water were both Actinobacteria,Proteobacteria and Bacteroidetes.The community composition of surface water and conduit water were similar at genus level,but the relative abundances of bacteria were different.The fissure water was different from the others.The degree of environmental oxidation-reduction and pollutants were important factors affecting the distribution of microbial community.In addition to SRB,which was mainly distributed in the ?-Proteus,the ?-Proteus related to sulfur reduction may also be an important microorganism that caused the formation of H2 S at the water-air interface of groundwater.