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Numerical Analysis of Recharge Rates and Contaminant Travel Time in Layered Unsaturated Soils

This study focused on the estimation of groundwater recharge rates and travel time of conservative contaminants between ground surface and aquifer. Numerical simulations of transient water flow and solute transport were performed using the SWAP computer program for 10 layered soil profiles, composed of materials ranging from gravel to clay. In particular, sensitivity of the results to the thickness and position of weakly permeable soil layers was carried out. Daily weather data set from Gdańsk (northern Poland) was used as the boundary condition. Two types of cover were considered, bare soil and grass, simulated with dynamic growth model. The results obtained with unsteady flow and transport model were compared with simpler methods for travel time estimation, based on the assumptions of steady flow and purely advective transport. The simplified methods were in reasonably good agreement with the transient modelling approach for coarse textured soils but tended to overestimate the travel time if a layer of fine textured soil was present near the surface. Thus, care should be taken when using the simplified methods to estimate vadose zone travel time and vulnerability of the underlying aquifers.