Effects of hydraulic conductivity heterogeneity on vadose zone response to pumping in an unconfined aquifer

Abstract

Summary It is of great importance to be able to properly simulate flow mechanisms within the vadose zone as they are controlling factors for solute transport to groundwater bodies. The highly detailed pumping test data set collected by Bevan et al. (2005) at the Borden aquifer provides a unique opportunity to investigate the controlling factors for fluid flow and moisture distribution in the vadose zone in response to pumping. The field results show capillary fringe thicknesses that increased with duration of pumping, and decreased with distance from the well. This increased thickness was persistent for the duration of the 7 day test. Previous numerical analysis of the pumping test has been able to replicate the observed moisture contents at discrete time intervals ( Moench, 2008 ), but the physical mechanisms controlling the moisture content observations are poorly understood. Hydraulic conductivity heterogeneity has been proposed as a potential mechanism for the capillary fringe extension observed in the field. Using stochastic hydraulic conductivity fields, moisture content distributions observed during the Bevan et al. (2005) pumping test were simulated using a variably saturated numerical model. A Monte Carlo analysis was performed and ensemble results are presented. The ensemble mean hydraulic head drawdown was an adequate match to the field data, although there was a small but consistent over prediction of drawdown. The ensemble mean thickness of the capillary fringe was not significantly different from the result of a homogeneous simulation, and did not replicate the increasing thickness observed in the field. The ensemble mean capillary fringe extension was not found to be dependent upon distance from the pumping well or duration of pumping. Zones of perched water were formed in many of the heterogeneous realizations; however, neutron probe data collected during the field experiment does not support the presence of these perched zones.