Quantification Of Groundwater Discharge In Huixian Wetland Using Radon Isotopes

As an important part of the global hydrological cycle,groundwater discharge is a research hotspot in the alternation between surface water and groundwater.Taking Baxian Lake in Huixian Wetland as the research object,this paper sampled the water of Baxian Lake,groundwater in the study area and other surface water bodies during the dry and abundant periods during 2021～2022,analyzed their water chemistry,natural radon isotope temporal distribution and metal temporal and spatial distribution characteristics,established a ²²²Rn mass balance model,and conducted quantitative assessment of groundwater discharge at the bottom of the lake and groundwater environmental health risk assessment.The main conclusions of the paper are as follows:（1）The pH values of the water during the dry-wet period were 8.09 and 7.80,respectively,and the total dissolved solids（TDS）ranged from 147.60 to 566.00 mg·L^-1 and 93.20 to 502.05mg·L^-1,respectively.This indicates that the water is weakly alkaline and has a low mineralization level.When compared to the standards for drinking water,only NO₃^-exceeded the limits in terms of physicochemical properties and conventional hydrated ions.According to the Schuler classification method,Piper ternary diagram,and Schoeller diagram,the hydrochemical type of the study area is of the HCO₃-Ca type.Correlation analysis,Gibbs diagram,and ion composition ratio analysis suggest that the hydrochemical composition is mainly affected by the chemical weathering and dissolution of carbonate minerals,as well as weak cation exchange adsorption.The water saturation index results show that the carbonate rocks in the study area are in an unsaturated state,indicating good dissolution ability.（2）In terms of spatial distribution,the activity of ²²²Rn in water follows the order of groundwater>other surface water>lake water.In terms of temporal distribution,the overall activity of ²²²Rn during the wet period is higher than during the dry period.Through analysis of influencing factors,it was found that ²²²Rn has no significant correlation with most conventional ions and metal elements in water chemistry,but only shows a significant negative correlation with K and NO₃^-during the wet period.The results of the steady-state ²²²Rn mass balance model showed that the groundwater discharge flux accounted for 99.728%and 98.543%of the input during the dry and wet period,respectively,and is the main source of replenishment for Baixian Lake water.The discharge rates during the dry and wet period were 83.31±16.30L·d^-1 and 38.71±3.67 L·d^-1,respectively.（3）On a spatial scale,metal concentrations were highest in groundwater,followed by other surface waters and lake water.On a temporal scale,metal concentrations were higher during the wet period than during the dry period,with only groundwater showing Mn exceeding the standard dry-wet periods.The metal flux carried by groundwater seepage at the lake bottom was highest for Sr during the dry period and highest for Mn during the wet period,with Li being the lowest in both periods.The health risk assessment indicated that none of the metals exceeded the risk limit,but Mn exceeded the standard for effective reference dose,which suggests a potential health risk.