Monitoring surface and subsurface water storage using confined aquifer water levels at the Savannah River Site, USA

Surface and subsurface water storage is an important component of hydrologic models, needed to account for groundwater recharge, lateral water movement, and evapotranspiration. Yet methods for estimating above- and belowground water storage include substantial uncertainties. This paper demonstrates that water levels in the Gordon aquifer (a confined aquifer at the USDOE Savannah River Site, near Aiken, SC) fl uctuate in response to changes in total water storage. An increase in surface loading is known to cause a measurable increase in aquifer fluid pressure whenever the aquifer skeletal compressibility is sufficiently large. Water levels in the Gordon aquifer respond rapidly during precipitation events, which is consistent with increased loading that compresses the aquifer at depth. Instantaneous barometric and loading efficiencies of approximately 6 and 91%, respectively, are consistent with a poorly consolidated aquifer. Because the aquifer has a high loading efficiency, it behaves like a geological weighing lysimeter that appears to estimate water storage. Following precipitation events, aquifer water levels decline over time, presumably due to unloading by evapotranspiration plus net lateral water export. The water-storage signal is improved by incorporating (i) the lagged response between aquifer load and borehole water-level changes and (ii) the removal of periodic Earth tides.