Estimation of Unconfined Aquifer Hydrologic Properties Using Gravity and Drawdown Data

An unconfined aquifer test using temporal gravity measurements was conducted in shallow alluvium near Fort Collins, Colorado on September 26-27, 2009. Drawdown was recorded in four monitoring wells at distances of 6.34, 15.4, 30.7, and 60.2 m from the pumping well. Continuous gravity measurements were recorded with a Scintrex CG-5 gravimeter near the closest well, at 6.3 m, over several multi-hour intervals during the 27 hour pumping test. Type-curve matching of the drawdown data performed assuming Neuman’s solution yields transmissivity T, specific yield Sy, and elastic component of storativity S estimates of 0.018 m2s-1, 0.041, and 0.0093. The gravitational response to dewatering was modeled assuming drawdown cone geometries consistent with Neuman’s drawdown solution for 4200 realizations of T, Sy, and S. Minimization of the root mean square misfit between the modeled and observed gravity change during drawdown results in T=0.0033 m2s-1, Sy=0.45, and S =0.0052. Drawdown conforming to Neuman’s solution was forward modeled using 1400 realizations of T, Sy, and S. Minimization of the root mean square misfit between these forward models and observed drawdown in the monitoring wells results in T=0.0080 m2s-1, Sy=0.26, and S=0.000004. Discrepancy between type-curve matching results, gravity analysis results, and drawdown modeling is attributed to heterogeneity and anisotropy within the aquifer, and possibly a relatively large drawdown in the aquifer, conditions which fail to satisfy the assumptions made for Neuman’s solution. In this aquifer test, gravity was most sensitive to transmissivity, less sensitive to specific yield, and insensitive to the specific storage-saturated thickness quotient. Simultaneous deployment of multiple gravity stations and consideration of a drawdown solution that includes anisotropy and the possibility that drawdown is large compared to aquifer thickness is recommended to better constrain gravity-derived aquifer property estimates of transmissivity and specific yield during similar tests in the future.