Modeling fluid pressure propagation into basement rock during managed aquifer recharge in the Virginia Coastal Plain Aquifer, USA

Astract Coastal aquifers are under increasing threat due to excessive groundwater withdrawal, resulting in freshwater depletion, seawater intrusion, and land subsidence. To address these issues, the Sustainable Water Initiative for Tomorrow (SWIFT) Research Center in Suffolk, Virginia (USA), has been testing managed aquifer recharge (MAR) in the Potomac Aquifer of southeast Virginia since 2018. The Potomac Aquifer is hydraulically connected to the underlying crystalline basement, which may allow injection-induced pressure transients from MAR operations to propagate into the underlying basement rock, potentially inducing a seismic response. To begin understanding the magnitude and extent of basement pressure propagation from MAR operations in Virginia, this study implements ensemble simulation methods to reproduce the pilot-scale SWIFT aquifer recharge program from 2018 to 2022, where the a priori spatial distribution of basement permeability is unknown. The simulation ensemble quantifies basement fluid pressure propagation and corresponding uncertainty for five injection scenarios that are based on the measured injection volume during the pilot-scale recharge program, as well as the permitted rate of 3.79 × 10 3 m 3 /day, and the anticipated injection rate (7.57 × 10 3 m 3 /day) for a single injector when the SWIFT project becomes fully operational. Results show that pressure change of ~ $$\Delta$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Δ</mml:mi> </mml:math> 40 kPa may reach the basement at injection rates of 7.57 × 10 3 m 3 /day, which is the anticipated injection rate for full-scale SWIFT injection wells. Moreover, uncertainty analysis reveals that this basement pressure build-up is localized to a 2-km radius of the injector within the upper 200 m of the underlying basement.