卫河流域河流-地下水流系统氮素运移的数值模拟

Numerical simulation of nitrogen transport in river-groundwater system in the Weihe River Basin

卫河是海河流域污染最严重的河流之一,该河如何影响附近浅层地下水的水质是长期受到重视但缺乏定量研究的关键问题。为探讨这一问题,利用Hydrus-2d模型模拟河流-非饱和带氮素的迁移转化,以GMS软件中的RT3D模块模拟氮素在饱和含水层中的运移,将包气带底部淋滤出的污染物浓度定为饱和带溶质运移模型的上边界条件,首次实现了河流-非饱和带-饱和含水层氮素运移的联合模拟,得到河流线状污染源对浅层地下水的影响程度及范围。研究结果表明:由于吸附作用、硝化-反硝化作用的存在,从河流上游到下游,包气带厚度加大,运移至含水层中的NH_4^-N、NO_2^-N浓度呈下降趋势,而NO_3^-N浓度则呈上升趋势。随着入渗时间的增长,进入饱和含水层中的NH_4^-N、NO_2^-N、NO_3^-N的浓度逐渐升高并最终保持稳定。污染的河流对浅层地下水的影响呈带状分布,污染物随入渗水流在包气带中垂直入渗;在饱和含水层中以水平运移为主,污染羽偏向地下水流动的方向,其影响距离不超过500m。

The Weihe River is one of the most polluted rivers in Haihe River Basin,and the effect of pollution on nearby shallow groundwater has been a great concern for many years.However this key issue has not been studied quantitatively.To explore this issue,Hydrus-2 dnumerical models were constructed to simulate transport and transformation of nitrogen in the river-unsaturated zone,while RT3 D module in GMS software was used to simulate nitrogen transport in saturated aquifers.Concentrations of pollutants leached at the bottom of vadose zone were regarded as the upper boundary condition of solute transport model in saturated zone.A joint simulation of nitrogen transport in river-unsaturated zone-saturated aquifer network was performed for the first time,which successfully delineated the influence of the polluted river on shallow groundwater.The simulation results show that,due to adsorption,nitrification and denitrification,the thickness of the vadose zone increases from the upper reaches of the river to the lower reaches,causing decrease of NH4^-N and NO_2^-N and increase of NO_3^-N concentration in the aquifer.As the infiltration time increases,NH_4^-N,NO_2^-N and NO_3^-N concentrations in the saturated aquifers gradually increase and stabilize eventually.The pollutant enters the vadose zone vertically with the infiltration water,while horizontal migration is the main form of transport in the saturated aquifer.The pollution plume disperses in the direction of groundwater flow,extending its influence as far as 500 maway from the river.