Capture Zones for Passive Wells in Heterogeneous Unconfined Aquifers

Many piezometers/wells produce at such low rates and frequency that their impact on local hydraulics is negligible, e.g. monitoring piezometers or homeowner wells installed in highly permeable soils. Defining a deterministic capture zone for such wells is often of limited utility as the capture zone is effectively a single flow line directed upgradient. In order to provide more insight into the capture zone of such wells, a statistically based capture zone, termed a “percentile capture contour” (PCC), is introduced and analyzed. The capture zone is defined by quantifying, for a given travel time, the variation of the length and orientation of the flow line emanating upgradient from the well. Capture zone variation herein depends on second-order stationary random hydraulic conductivity fields and is calculated using Monte Carlo analysis. Monte Carlo analysis yielded increases in mean travel distances as the variance and/or integral scale in log K increased, but decreases as the angle between the principal direction of the correlation structure and the regional flow increased. The average travel distance exceeded the travel distance estimated by a homogeneous solution using the geometric mean hydraulic conductivity. Transverse variation depended both on the variance and integral scale of log K, but was insensitive to the orientation of the principal correlation direction. The mean orientation of the flow path varied with the principal direction of the correlation structure, deviating up to 20° from the orientation of the hydraulic gradient. These observations are consistent with flow following preferential pathways and indicate that significant uncertainty exists for source prediction of water feeding passive wells.