Abstract
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.
Highlights
In view of the increasing use of groundwater resources worldwide, there is a need to develop efficient methods to quantify the recharge rate and the time of migration of contaminants from the ground surface to the groundwater table
Several computer codes can be used for this purpose, some of them freely available, for example, HYDRUS-1D [13], SWAP [14], UNSAT-H [15], HELP [16]
A constant value of a = 6cm was used in all simulations. This value corresponds to 0.01 of the length scale of profiles A-F described below. We chose it in line with our previous simulations described in Reference [21], in order to minimize the influence of dispersion on the solute travel time and facilitate comparison with simplified methods, which take into account only advection
Summary
In view of the increasing use of groundwater resources worldwide, there is a need to develop efficient methods to quantify the recharge rate (i.e., the amount of water from precipitation reaching the groundwater table) and the time of migration of contaminants from the ground surface to the groundwater table. Szymkiewicz et al [21] carried out a comparison between transient and steady-state based methods for estimating time lag for soil profiles with and without root zone They showed that even if the average recharge rate obtained from transient simulations is used as the input in the steady state methods, the resulting travel times vary considerably. In the current study we extended our previous analysis by taking into account: (i) layered structure of soil profiles and (ii) a more detailed model of vegetation cover, including variable-in-time split between evaporation and transpiration For this purpose, we used SWAP numerical code, which contains a detailed module for grass growth and transpiration [14]. The methods based on steady flow assumption gave results relatively close to the transient simulations for coarse textured soils but tended to overestimate the travel time in fine textured soils, in agreement with our previous findings [21]
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