不同地下水端元选取对^(222)Rn质量平衡模型量化湖底地下水排泄的影响

Influence of Different Groundwater End-members on ^(222)Rn Mass Balance Model Used to Quantify the Lacustrine Groundwater Discharge

222Rn质量平衡模型是量化湖底地下水排泄最为有效的方法之一,其中地下水的222Rn活度是222Rn质量平衡模型中最重要的端元之一。为研究不同地下水端元选取对222Rn质量平衡模型的影响,以长江中游典型牛轭湖——天鹅洲湿地为研究区,利用野外采样数据,基于222Rn质量平衡模型,分别以湖区周边井水、湖岸孔隙水、沉积物孔隙水这3个端元或端元组合的222Rn活度作为地下水端元值,估算了地下水向天鹅洲湿地的排泄通量,评估了不同地下水端元或端元组合的选取对222Rn质量平衡模型量化湖底地下水排泄的影响。结果表明:(1)湖水中222Rn活度仅比不同类型地下水低1个数量级,指示了地下水强烈地向湖泊排泄的过程;(2)在湖泊222Rn通量的源项中,地下水排泄的222Rn通量占99%,而沉积物扩散的222Rn通量仅占1%;(3)选取不同地下水端元所得湖底地下水排泄通量依次为:湖区周边井水(2.00×10^(6)m^(3)/d)>湖岸孔隙水(1.44×10^(6)m^(3)/d)>沉积物孔隙水(0.96×10^(6)m^(3)/d)。根据天鹅洲湿地区孔隙承压含水层与湖泊在大尺度上较为强烈的水力联系和湖泊内部小尺度上浅层地下径流向湖泊的排泄,以及十分有限的沉积物扩散渗透,综合考虑将井水和湖岸孔隙水中222Rn平均活度作为地下水端元值来进行湖底地下水排泄通量的估算最为合适。

The 222Rn mass balance model is one of the most effective methods in quantifying the Lacustrine Groundwater Discharge(LGD).The 222Rn activity of groundwater is one of the most important end members in the 222Rn mass balance model.However,the influence of different groundwater end-members on the 222Rn model has been poorly understood.In order to fill this gap,the study area is a typical oxbow lake in the middle reach of Yangtze River,i.e.,Tian-E-Zhou watershed.Based on the 222Rn mass balance model,the research selects the well water around the lake,piezometer water in the lake shore,and pore water from the sediment equilibrium experiment to calculate the LGD flux,and evaluates the influence of different groundwater end-members on the estimation of LGD flux.The results are as follows:(1)The 222Rn activity of lake water is only one order of magnitude lower than that of groundwater,indicating the intense groundwater discharge to the lake;(2)In the source term of 222Rn flux in the lake,groundwater discharge accounts for 99%,while sediment diffusion accounts for only 1%;(3)The LGD flux estimated from different groundwater end members are as follows:well water around the lake(2.00×10^(6)m^(3)/d)>lake shore pore water(1.44×10^(6)m^(3)/d)and>sediment pore water(0.96×10^(6)m^(3)/d).Given the close hydraulic link between the porous confined aquifer and the lake at a large scale and the discharge of shallow underground flow into the lake at a small scale,and the quite limited sediment diffusion and penetration,it is most appropriate to take the average 222Rn activity of well water and piezometer water as the groundwater end-member value to estimate the LGD flux.