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Numerical analysis of passive capillary wick samplers prior to field installation

Abstract:

Accurately measuring water fluxes and associated nutrient or contaminant concentrations through the vadose zone is difficult because an appropriate suction needs to be exerted on the soil to sample water under unsaturated conditions. Passive capillary wick sampling systems are cheap and reliable instruments resulting in acceptable measurements of water fluxes in the vadose zone; however, their success in measuring realistic fluxes depends on their compatibility with the soil and climatic conditions in which they are installed. This study was developed in the preplanning phase of a field experiment with its main objective the monitoring of dissolved organic matter and the associated transfer of Cu and pesticides through the vadose zone. We studied a combination of two-dimensional and axisymmetrical three-dimensional numerical analyses using the HYDRUS-2D software to identify what sampler geometry, wick type, wick length, and number of wicks are most suitable for the soil conditions at the experimental site. An AM3/8HI wick with seasonally varying wick length (40 cm in winter and 100 cm in summer) was found to be most appropriate for the soil and climatic conditions of the experimental field. The numerical analysis indicated that well-designed wick samplers had a negligible effect on the soil moisture content close to the sampler. A double-ring wick sampler is proposed to minimize the effect of the area between the installation pit or trench and the sampler. This approach is easily applicable and transferable to other soil and wick types and climatic conditions. The study emphasizes the suitability of numerical modeling to optimize experimental design before installation. © Soil Science Society of America.