Influence of layered soil structure on the groundwater recharge rates and contaminant travel time: A numerical study using SWAP model and comparison with simplified methods

In view of the increasing use of groundwater resources worldwide, there is a need to develop efficient methods to quantify the natural recharge rate and the time of migration of contaminants from the ground surface to the groundwater table. These two tasks are closely related, since they both require knowledge of water velocity in the vadose zone, which is generally variable in space and time. In order to achieve this goal, numerical models of unsaturated flow and transport are often used. Nevertheless, the widespread use of numerical modeling is still hampered by the limited availability of realistic data for specific scenarios, need for the user expertise, relatively long times of computation and possible convergence problems in transient flow simulations. For this reason, simplified approaches to quantify recharge and contaminant travel time remain a useful tool in hydrogeological practice. In this work we focus on the comparison of transient flow and transport simulations with the simplified methods for soils showing heterogeneous (layered) structure. We performed numerical simulations of transient water flow and solute transport using the SWAP computer program for 10 layered soil profiles, composed of materials ranging from gravel to clay. In 3particular, 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 modeling approach for coarse textured soils, but tended to overestimate the travel time, if a layer of fine textured soil was present near the surface. The work is partly funded by National Centre for Research and Development (NCBR), Poland, (BIOSTRATEG3/343927/3/NCBR/2017 "Modelling of the impact of the agricultural holdings and land-use structure on the quality of inland and coastal waters of the Baltic Sea set up on the example of the Municipality of Puck region – Integrated info-prediction Web Service WaterPUCK") and by National Science Centre (NCN), Poland (2015/17/B/ST10/03233 "Groundwater recharge on outwash plain").