Abstract
Estimation of contaminant travel time through the vadose zone is needed for assessing groundwater vulnerability to pollution, planning monitoring and remediation activities or predicting the effect of land use change or climate change on groundwater quality. The travel time can be obtained from numerical simulations of transient flow and transport in the unsaturated soil profile, which typically require a large amount of data and considerable computational effort. Alternatively, one can use simpler analytical methods based on the assumptions of steady water flow and purely advective transport. In this study, we compared travel times obtained with transient and steady-state approaches for several scenarios. Transient simulations were carried out using the HYDRUS-1D computer program for two types of homogeneous soil profiles (sand and clay loam), two types of land cover (bare soil and grass) and two values of dispersion constant. It was shown that the presence of root zone and the dispersion constant significantly affect the results. We also computed the travel times using six simplified methods proposed in the literature. None of these methods was in good agreement with transient simulations for all scenarios and the discrepancies were particularly large for the case of clay loam with grass cover.
Highlights
Reliable determination of contaminant travel time in an unsaturated zone is required to assess groundwater vulnerability to pollution, to plan monitoring and remediation activities and to predict the effect of changes in climate or land use practices on groundwater quality [1,2,3]
While the susceptibility of aquifers to pollution can be quantified using different approaches, like the index methods (e.g., DRASTIC [4,5]) or the grey water footprint method (GWF) [6,7], methods which explicitly take into account the time scale of contaminant transport are often preferable [3]
There is a need for more detailed comparison of the results of simple methods with numerical simulations based on the Richards’ equation. We focus on such a comparison for two soil profiles, which can be considered typical for post-glacial areas in Northern Poland
Summary
Reliable determination of contaminant travel time in an unsaturated zone ( known as an unsaturated time lag) is required to assess groundwater vulnerability to pollution, to plan monitoring and remediation activities and to predict the effect of changes in climate or land use practices on groundwater quality [1,2,3]. While the susceptibility of aquifers to pollution can be quantified using different approaches, like the index methods (e.g., DRASTIC [4,5]) or the grey water footprint method (GWF) [6,7], methods which explicitly take into account the time scale of contaminant transport are often preferable [3]. The resulting travel times are considered as safer (shorter) estimates, compared to the case of non-conservative contaminants, which are subject to retardation or attenuation. A number of approaches have been developed to estimate the time lag in the vadose zone, ranging from simple analytical formulas to comprehensive numerical models. Analytical equations were developed for the case of steady downward water flow, purely advective transport and constant water content in the soil profile, or at least in each soil layer [1,8,9]
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