Post-pumping seawater intrusion at the field scale: Implications for coastal aquifer management

Post-pumping seawater intrusion (PP-SWI) is the phenomenon of seawater intruding further inland than the location of a well, after pumping has ceased. Recent work by Stoeckl et al. (2019) assessed PP-SWI at the laboratory scale using physical and numerical modeling, confirming it as a physical phenomenon and linking it to disequilibrium in the flow field following the cessation of pumping. This work extends that of Stoeckl a al. (2019) by systematically evaluating sensitivity of the PP-SWI phenomenon to two aquifer management parameters, i.e. pump distance from coast and pump rate. The analysis is carried out at the field-scale using variable density numerical modeling. The numerical simulations show that PP-SWI and derived aquifer usage measures (e.g. cumulative water abstraction, time for the saltwater to reach the pump i.e., active pumping time) have a non-linear relation to pump rate and pump distance from coast. PP-SWI does not occur below a minimum pump rate (hinting at sustainable pumping rates under certain conditions) but may reach hundreds of years in some cases (i.e., SWI continues for hundreds of years after pumping has ceased). Most importantly, the results did not reveal a clear relation between pump distance from coast and the occurrence nor extent of PP-SWI. For real-world aquifer management and monitoring, this implies that groundwater salinity measurements at different wells (with different distances from the coast) may not provide sufficient information for managing PP-SWI. Our work indicates that a better measure to estimate PP-SWI is the ratio of pump rate versus the inflow from upstream recharge, where inflow estimates could be drawn from lysimeter measurements, a regional water balance, or different types of hydraulic models.