A field demonstration of groundwater vulnerability assessment using transport modeling and groundwater age modeling, Beijing Plain, China

Groundwater vulnerability is often regarded as the resistance of aquifers to pollutions, which is in fact the groundwater environment vulnerability. Similarly the renewable capacity of groundwater system can be regarded as the indicator for groundwater resources vulnerability. In this paper, the concept of groundwater vulnerability is extended to include both the groundwater environment vulnerability and groundwater resources vulnerability, and the extended overall groundwater vulnerability is assessed by examining the solute transport process from ground surface to the water table (for groundwater environment vulnerability), and evaluating the renewable capacity of groundwater system (for groundwater resources vulnerability). For this case study in Beijing Plain, a 1D-modeling, 2D-mapping model is constructed in HYDRUS to simulate the solute transport process through vadose zone. The transit times and solute concentrations at the water table are selected as the indicators for groundwater environment vulnerability. Meanwhile, a 3D groundwater age model in MODFLOW/MT3DMS is employed to model the age distribution in saturated zone, and the modeled ages are taken as the indicators for groundwater resources vulnerability. These two models are coupled together by exchanging their vertical infiltrations and water table elevations. Overall vulnerability is finally assessed by taking all the indicators into account. The result suggests the overall vulnerability can finely represent the synthetic impact of solute transport characteristics (groundwater environment vulnerability in vadose zone) and groundwater system renewability (groundwater resources vulnerability in saturated zone). The most vulnerable regions of Beijing Plain are mostly in agricultural areas, the medium vulnerable areas mostly appear at pumping sites, and the least vulnerable areas are mostly distributed in recharge regions.