USING CYCLIC PUMPING PATTERNS AND LAND-SURFACE DEFORMATIONS TO EVALUATE AQUIFER PROPERTIES

A 62-day controlled aquifer test represented by high volume cyclic pumping from a thick unconsolidated aquifer in Mesquite, NV was conducted to monitor vertical and horizontal surface deformations. High-precision global positioning system receivers and antennas with choke rings were positioned at various radial distances from a municipal pumping well that pumped water at a rate of 18,000 m3/d in approximately 8-hour cycles. An 83m thick brittle unsaturated zone resides above the dynamic aquifer system. Nonetheless, highly consistent horizontal deformations of up to 8 mm were observed with a precision of 0.2 mm. Vertical deformations of up to 10 mm with a precision of 2 mm were also observed. Hydraulic-head fluctuations and long term declines were monitored adjacent to the pumped well. The cyclic patterns of hydraulic head along with the measured surface deformations can be shown through numerical simulation to be highly diagnostic of aquifer properties including horizontal and vertical hydraulic conductivity, elastic and inelastic specific storage, aquifer thickness, and Poisson’s ratio. Simulation results indicate that the aquifer properties required to match the drawdown and recovery associated with individual cycles are significantly different than those that most closely match the observed long-term average. The observed response to individual pumping and recovery cycles are most dependent upon horizontal and vertical hydraulic conductivity and elastic specific storage, while the long-term average is more highly dependent upon confining unit properties, inelastic specific storage, Poisson’s ratio, and aquifer thickness. These results indicate that the collection of relatively inexpensive deformation data can greatly aid our understanding of the aquifer system and the subsequent calibration of numerical models.