A field scale study of pumping-induced drainage and recovery in an unconfined aquifer

Abstract

An aquifer test involving seven days of pumping and five days of recovery was performed in the unconfined aquifer at Canadian Forces Base Borden in Ontario, Canada. Hydraulic head levels were intensively monitored in eleven observation wells, and moisture content profiles were frequently logged using a neutron moisture probe at six different radial distances from the pumping well. Capillary fringe thickness variations were inferred by comparing these hydraulic and moisture content measurements. Allowing for the effects of lithologic variations, the response of the moisture content profile is predominately characterized by a downward translation during pumping-induced drainage. Our observations indicate that there is minimal excess storage above the capillary fringe at late times. The capillary fringe extended significantly with pumping in comparison to its static thickness; this extension progressively increased during the entire test. The capillary fringe extension decreased with increasing radial distance and was inferred to have eventually extended out to between 20 and 25 m. This extension represented a significant volume of excess capillary fringe storage that persisted into late times. A direct relationship between the vertical gradients below the water table and the excess capillary fringe storage is not apparent from these experimental data. Analogous to the pumping test observations, the response of the moisture content profile during recovery is essentially an upward translation. While the hydraulic head levels recovered nearly to their pre-pumping elevations, the moisture content profiles stabilized approximately 0.20 m below their pre-pumping positions. This recovery differential leads to a capillary fringe compression that converted excess storage during pumping into storage deficit during recovery. In contrast to the significant radial variation of the capillary fringe extension during pumping, the capillary fringe compression is relatively uniform out to at least 15 m during the late recovery stage. The recovery deficit and entrapped residual air observed below the upward translating transition zone are strong indications that significant hysteretic processes are occurring at the field scale.