Hydrogeophysical investigation of unconfined aquifer drainage behavior using temporal microgravity and water level data

2020 Fall.Includes bibliographical references.Unconfined aquifers are commonly characterized by an analysis of water level changes in response to groundwater pumping during an aquifer test. Traditional analytical models predict the rate and extent of water level changes based on transmissive and storage properties of the aquifer. These models commonly assume instantaneous and complete dewatering of the pore space above a falling water table, which neglects time-dependent storage changes in the unsaturated zone. By sensing pumping-induced water mass changes in both the saturated and unsaturated zones, gravity surveys provide an opportunity for improved characterization of unconfined aquifers. In this study, a time-lapse microgravimetric survey was performed during pumping from a shallow unconfined aquifer in northern Colorado. Water level data were collected at four monitoring wells located along a radial transect at 6.34, 15.4, 30.7. and 61.2 meters from the pumping well. Gravity measurements were collected adjacent to the second well at 15.4 meters. Pumping from the aquifer resulted in a water level decline ranging from 0.35 meters at the distant well to 1.5 meters at the closest well. A total of 3.89 ∙ 106 kg of water mass was pumped during the test, resulting in a decline in gravitational acceleration of 27.2 microGals at the fixed measurement location. The gravity data are not adequately explained by traditional analytical models that predict negligible mass changes as the water table beings to stabilize. This highlights potential inaccuracies in drawdown model assumptions that are not readily discernible with water level data alone