| Wetlands have unique biogeochemical cycles with many chemical transformations and chemical transport processes that are not shared by other ecosystems.Although the wetland covers only 6% of the Earth's surface,it provides a living environment for 20% of the known species on Earth,and has an irreplaceable ecological function.Compared with other terrestrial ecosystems,wetland plants are the dominant species,and most of aquatic plants have the ability to utilize HCO3-for photosynthesis,while the main anions in surface water and groundwater are HCO3-in karst region.In addition,the hydrological conditions are the determinants of wetland formation and development,and the water conditions control the biogeochemical processes and functions of wetland ecosystems.Therefore,the study of water chemistry compositions and stable isotopes of wetland water can explore the sources of major ion,inorganic carbon and water,and the geochemical processes that controlling the hydrochemical composition and stable isotope in the wetland can be investigated by the difference between the chemical composition of the surface water and groundwater flowing into the wetland.Based on this,We chose a karst-dominated wetland,the Caohai wetland in Weining County,Guizhou Province,Southwest China.We analysis the composition of water chemistry and stable isotope in river,groundwater and wetland water in the watershed during different seasons,and systematically studied the ion sources of Caohai watershed,revealed the geochemical process controlling the water chemistry of the wetland.The conclusions are as follows:?1?Ca-HCO3 is the chemical type of surface water and groundwater around Caohai,and the chemical composition of inflowing waters is affected by carbonate erosion and anthropogenic activities.In the high-flow season,the main ions Ca2+ and HCO3-decreased gradually from east to west in the water of Caohai wetland,and the chemical type is consist of Mg-HCO3,Mg-SO4,Ca-SO4 and Ca-HCO3,The p H of wetland water in the west side was weakly acidic,the content of DO was low,EC was high,and Eh was negative,indicating that the water in this area showed a reduced state.On the contrary,the p H of the west side was weakly alkaline,DO was high,EC was low,indicating that the side of the water in the oxidation state.In the low-flow season,the water chemistry type is Ca-HCO3 because of the aquatic vegetation was IV dead.Although the spatial variability of the water chemistry and physicochemical parameters are similar to that of the high-flow season,the variation range of the variable parameter is small.Based on the factor analysis of the chemical composition of wetland wetlands,it was found that the hydrochemical composition of the wetland was mainly affected by biogeochemical processes,while the low-flow season was mainly affected by evaporation and condensation,the next is biogeochemical processes.?2??13CDIC changes in the groundwater and river water from-13.66‰ to-8.64‰ in the high-flow season,and from-12.61‰ to-7.80‰ in the low-flow season.The DIC in groundwater mainly comes from the dissolution of carbonate rock and soil CO2,Seasonal variations in DIC concentrations and the ?13CDIC indicate the influence of local rainfall events in the high-flow season and the longer residence time of water in the low-flow season.The DIC of river water mainly comes from groundwater in high-flow season,and the DIC isotopic composition of river water is relatively higher than that of groundwater,due to the photosynthesis and CO2 emission of phytoplankton in river water.During the high-flow season,the p CO2 in the emergent plant zone was higher than that of the atmosphere because of respiration of roots and sediments.Because of photosynthesis,the p CO2 in the submerged plant zone was lower than that of the atmosphere.Furthermore,in this zone,the ?13CDIC ranged from-15.13‰ to-6.68‰?mean value of-10.79‰?,and may have been influenced by chemically enhanced fractionation and photosynthesis.In the low-flow season,the p CO2 was mostly higher than atmospheric p CO2 because of organic matter decomposition in sediments.Additionally,during this season,the ?13CDIC ranged from-6.23‰ to-0.06‰?mean value of-2.52‰?,and may have been influenced by CO2 evasion,CO2 reduction to methane,and phytoplankton photosynthesis.?3?The surface water and groundwater are of meteoric origin result from the hydrogen and oxygen isotopes composition.The isotopes in wetland water were well mixed vertically in both seasons.However,marked seasonal and spatial variations were observed in the stable isotopes of wetland water.During the high-flow season,the isotopic composition in surface wetland water varied from-97.13‰ to-41.73‰ for the ?D and from-13.17‰ to-4.70‰ for the ?18O.The composition of stable isotopes in the eastern region of this wetland was lower than in the western region.These may have been influenced by uneven evaporation caused by the distribution of aquatic vegetation.During the low-flow season,?D and ?18O in the more open water with dead aquatic vegetation ranged from-37.11‰ to-11.77‰,and from-4.25‰ to-0.08‰,respectively.This may result from high evaporation rates in this area with the lowest atmospheric humidity and source of water.Groundwater fluxes were calculated by mass transfer and isotope mass balance approaches,suggesting that the water sources of the Caohai Wetland were mainly from groundwater in the high-flow season,while the groundwater has a smaller contribution to wetland water during the low-flow season.?4?The mean value of sulfur isotope in the groundwater in high-flow season and low-flow are-14.40‰ and-15.00‰,respectively,The mean value of sulfur isotope in the river water are-13.91‰ and-13.44‰.The mean value of sulfur isotopes in the high-flow season and low-flow season are-6.68‰ and 1.18‰,respectively.In the high-flow season,the sulfate mainly comes from groundwater,precipitation,while in the low-flow season,the sulfate mainly comes from the domestic wastewater and precipitation.The change of sulphate source is related to the change of water source of wetland. |
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