Long-term effects of subsurface structures on groundwater level in a typical urban area of Shanghai, China

The effect of subsurface structures in blocking groundwater seepage has a long-term influence on groundwater level (GWL). A finite difference method (FDM) model considering the actual distribution of subsurface structures in an urban area of Shanghai (China) was established to predict GWL in the phreatic aquifer (Aq0) and the first confined aquifer (AqI). The equivalent hydraulic conductivity (K_eq) of model elements containing subsurface structures, calculated by the effective medium theory, was applied to the model. The predicted GWL fitted the monitored value in Aq0 well. Additional subsurface structures were added to the model to analyze the influence of the distribution type and the proportion (%) of the volume of subsurface structures that occupy the aquifer (V_u). Four scenarios with different distribution types (concentrated, subconcentrated, subscattered, and scattered) and ten scenarios with V_u varying from 5 to 50%, were analyzed. In all scenarios, the regional average GWL in AqI increased compared to the actual conditions because of the decrease in K_eq and the blockage effect on groundwater flow. The influence of scattered distribution on the regional GWL distribution was the smallest, and the subscattered distribution resulted in the most nonuniform GWL redistribution. The blockage effect of the subsurface structures gradually increased with increasing V_u. The increasing rate of ΔL_av (difference in regional average GWL between the predicted and actual scenarios) becomes considerable when V_u is ~29%. Hence, the projected increase in volume of subsurface structures in AqI under the assumed subscattered distribution is suggested to be <29%.