Evaluation of the vulnerability of a leaky aquifer considering the retardation effect of an aquitard for specific pollutants: case study in the Tongzhou Plain, China

The effect of an aquitard above a leaky aquifer on retarding characteristic pollutants is very important for evaluating groundwater vulnerability. A case study is presented of the leaky aquifer in the Tongzhou Plain of China. On the basis of analyses of the environmental and hydrogeological characteristics of the study area, an indicator system to evaluate the leaky aquifer’s vulnerability was constructed, comprising eight indicators: the depth to groundwater (D), the net recharge (R), the impact of the vadose zone (I), the hydraulic conductivity of the phreatic aquifer (C), the pollution load (P), the phreatic water quality (Q), the ability of the aquitard to retard NH₄⁺ (A), and the ability of the aquitard to retard NO₃⁻ (N). The retardation capacity of the aquitard for the characteristic pollutants in the study area (ammonia nitrogen and nitrate nitrogen) was obtained by numerical modelling with a HYDRUS simulator. The results show that the retarding effect of the aquitard on nitrogen pollutants and the hydraulic conductivity of the phreatic aquifer have significant effects on the vulnerability of leaky aquifers, and that other factors such as groundwater depth and pollutant load, have relatively limited effects. The research can provide a reference methodology for evaluating the vulnerability of porous leaky aquifers in alluvial plain areas.