The C-S Coupling Cycle Processes Of Karst Critical Zone And Its Weathering Effect To Carbonate Rocks Under The Influence Of Tunnels Construction

The material cycle,energy flow,climate,geology and human activities on the karst critical zone and its response are one of the important contents of current research.China is the country with the largest number of tunnels and underground projects in the world currently,whose construction of the tunnels has had an important impact on the local socioeconomic development and ecological civilization,especially in the karst critical zone.The construction of the tunnels disturbed the karst aquifer,changed the groundwater flow field and circulation process.It also caused a series of ecological and environmental problems such as changes in soil properties,plant types,loss of surface water,and intensified rocky desertification,which seriously affected the water cycle process and the C-S cycle process of the karst critical zone.Therefore,it is great scientific significance to study the influence of tunnels construction on the C-S coupling cycle in karst critical zone.It is great significance for understanding the effects of tunnel construction on the hydrological and ecological environment deeply and improving the design and construction level of tunnels and dealing with the adverse effects of tunnel construction on the karst ecological environment scientifically.In this paper,the Longfeng trough valley in Zhongliang mountain,Chongqing,which is affected by the construction of three tunnels?Beibei tunnel of Rail line 6,Beibei tunnel of G75 Lanhai expressway,and Shijialiang tunnel of G5001 ring expressway?,was selected as study area.at the same time,Qingfeng trough valley,which is not affected by tunnel construction,was used as a control area.The monitoring and sampling works including basin meteorological data,the hydrochemistry and?²H,?¹⁸O,d¹³C_DIC,?³⁴S-SO₄^2-and?¹⁸O-SO₄^2-isotopes has been carried out from May 2017 to April 2019 to analyze the groundwater circulation process in the tunnel affected zone and the tunnel unaffected zone under the same climatic conditions and geological backgrounds and the distribution,source and migration process of C and S in the karst critical zone,then to quantify the effects of H₂SO₄ weathering carbonate rocks,and to reveal the process and mechanism of tunnel construction on the C-S coupling cycle process of the karst critical zone.Main conclusions are shown as following:?1?The slopes of the atmospheric precipitation?²H and?¹⁸O trend lines were the highest,followed by soil water,and then the groundwater,the tunnel drainage was the lowest.The average d value of the study area from epikarst springs to underground rivers has decreased gradually,and the mean values of?²H and?¹⁸O of soil water and groundwater in the tunnels affected areas were more positive and has smaller changes than the tunnels unaffected areas.The difference in the d value of the groundwater system also showed that the tunnels affected areas is lower than the tunnels unaffected areas,which indicated that the construction of the tunnels has affected the connection between surface water,groundwater and soil water,the soil water and surface water were more likely to evaporate.At the same time,the groundwater flow field was changed and its circulation process was accelerated by the tunnels,the process of water-rock interaction in the groundwater system became more and more intense.?2?The rain hydrochemistry type was Ca-SO₄,the hydrochemistry of soil water,epikarst springs,shallow groundwater and underground river were all Ca-HCO₃ type,but tunnel drainage water was Ca-HCO₃-SO₄ type.The hydrochemical type has changed significantly,which was influenced by many factors,such as geological background,human activities,atmospheric rainfall and land use types.The tunnels construction has changed the physical and chemical properties of the soil and has caused a large loss of nutrients in the soil.it also accelerated the dissolution and oxidation of substances in the formation,and changed the groundwater hydrochemical properties.?3?The tunnels construction not only changed the physical and chemical properties of the soil in the tunnels affected areas,accelerated the groundwater circulation process,enhanced surface-underground air circulation,and affected the soil water and groundwater system S cycle process.The?³⁴S-SO₄^2-average values of soil water,epikarst springs,shallow groundwater,underground river and tunnel drainage in the tunnels affected areas were 2.11‰,3.78‰,4.16‰,4.71‰and 15.49‰,respectively.The mean values of?³⁴S-SO₄^2-of soil water,lakes and underground rivers in the tunnels unaffected areas were 2.26‰,5.97‰and 5.97‰,respectively.In the tunnels affected areas,?³⁴S-SO₄^2-has a gradually positive process from soil water to tunnel drainage.The?³⁴S-SO₄^2-and?¹⁸O-SO₄^2-values indicated that the source of SO₄^2-in the water bodies were affected by atmospheric precipitation,sulfide oxidation,gypsum dissolution,and human activity inputs?fertilizer and domestic sewage?.The construction of the tunnels made the underground aquifer more broken,and the effect of water and rock became stronger,which strengthened the surface and underground air circulation,resulted in the enhancement of the process of gypsum dissolution and sulfide oxidation in the stratum.According to the IsoSource model interpretation,the main sources of SO₄^2-in epikarst springs showed:sulfide oxidation?43%?>fertilizers,domestic sewage?37%?>gypsum dissolution?15%?>precipitation?14%?;surface water:fertilizer,domestic sewage?49%?>sulfide oxidation?23%?>precipitation?19%?>gypsum dissolution?11%?;underground river water:fertilizer,domestic sewage?55%?>sulfide oxidation?21%?>precipitation?14%?>gypsum dissolved?10%?.The tunnel drainage:gypsum dissolution?66%?>sulfide oxidation?23%?>precipitation?5%?>fertilizer,domestic sewage?3%?,and the spatial difference of SO₄^2-sources of TGW1-TGW3 was higher extremely.In the tunnels affected areas,the vertical variation of epikarst springs to tunnel drainage shows that the contribution of atmospheric precipitation was decreased,but the contribution of gypsum dissolution was increased,the epikarst springs,shallow groundwater and underground river water was about 6%,while tunnels drainage was 66%.?4?The construction of the tunnels accelerated the process of gypsum dissolution and sulfide oxidation,not only increased the SO₄^2-concentration in the groundwater,but also formed H₂SO₄ dissolved carbonate rocks and affected the carbon cycle process.The DIC concentrations of soil water,epikarst springs,shallow groundwater,underground rivers and tunnels drainage showed that the rainy season was lower than the dry season,mainly due to the dilution of rainfall.Soil water has the lowest DIC concentration,and underground river water has the highest DIC concentration.In addition,the average?¹³C_DIC value of the tunnel affected area was gradually deviated from soil water to the tunnel drainage.The average?¹³C_DIC value of the tunnel drainage was approximately 2.82‰ more than the underground river water.The[Ca²⁺+Mg²⁺]/HCO₃^-molar ratio of groundwater was between 0.5?1 and[Ca²⁺+Mg²⁺]/[HCO₃^-+HNO₃^-+H₂SO₄^2-]?1,indicated that its water quality was affected by the common of H₂CO₃ and H₂SO₄ and enriched?¹³C_DIC.H₂CO₃ dissolved carbonate rocks were the main water-rock process,but the role of H₂SO₄ in the weathering of carbonate rocks cannot be ignored.In all water bodies,the average proportions of(Ca²⁺+Mg²⁺)and DIC formed by H₂SO₄ dissolved carbonate rocks were 25%and 19%.However,the effect of H₂SO₄ on the weathering of carbonate rocks in the tunnels affected areas was significantly higher than that in the tunnels unaffected areas.