HR: 17:30h
AN: H44A-07    [Abstracts]
TI: Use of gravity and drawdown information to estimate hydraulic properties during unconfined aquifer testing
AU: * Blainey, J B
EM: jblainey@email.arizona.edu
AF: Department of Hydrology and Water Resources, University of Arizona, 1133 E. North Campus Dr., Tucson, AZ 85721, United States
AU: Ferre, T P
EM: ty@hwr.arizona.edu
AF: Department of Hydrology and Water Resources, University of Arizona, 1133 E. North Campus Dr., Tucson, AZ 85721, United States
AB: High-resolution gravimetry measured at the ground surface may offer a cost-effective method to augment aquifer tests in unconfined aquifers without the expense of installing additional monitoring wells or piezometers. However, evaluating the usefulness of gravity, used either alone or together with piezometric measurements, to help constrain hydraulic parameters through aquifer testing is difficult because observations of drawdown in a monitoring well and gravity measurements respond to different physical states of the system at different spatial scales. Measurements of drawdown provide a direct measure of the energy potential local to the observation location, while measurements of gravity change indirectly measure changes in water storage and depend on the spatially-weighted average of the subsurface density change integrated over a large measurement volume. Hydrogeophysical numerical experiments are conducted to examine the value of drawdown and gravity responses for constraining aquifer test analyses based on synthetic measurements at nine ground-surface locations after seven days of pumping. We confirm that the conditions conducive to this use of gravity measurements are those that lead to a sufficiently large change in gravity signal: high drainable porosity of the aquifer, low depth to the water table, and small distance from the pumping well to a gravity measurement location. However, we further demonstrate that the ability of a gravimeter to detect a signal above the noise in the data is not a sufficient condition to guarantee utility of the measurement method to constrain hydraulic parameter estimation. Specifically, the inference of hydraulic properties requires consideration of parameter interaction and parameter sensitivity to instrument responses in addition to instrument resolution and signal detectability. For conditions amenable to gravity monitoring, combined use of drawdown and gravity data resulted in unbiased and precise estimates of both hydraulic conductivity and specific yield. This offers improvement over monitoring drawdown alone, which often provides poor estimates of specific yield.
DE: 1829 Groundwater hydrology
DE: 1846 Model calibration (3333)
DE: 1894 Instruments and techniques: modeling
SC: Hydrology [H]
MN: 2007 Fall Meeting