Alongshore freshwater circulation in offshore aquifers

Abstract

Flow within offshore aquifers is important for the management of fresh groundwater in coastal zones. Alongshore head gradients have been observed in many coastal aquifers; however, this process is typically neglected, and offshore aquifers are primarily assessed using cross-sectional representations. This study explores offshore implications of alongshore head gradients (onshore), including the alongshore circulation of submarine groundwater, using 3D synthetic numerical models. Model parameter ranges were guided by those observed in seven real-world cases. Each 3D simulation is compared against three 2D cross-sectional numerical models to identify the potential mismatch between 2D and 3D approaches. Results show that alongshore freshwater circulation (AFWC) is greater where alongshore head gradients are steeper, onshore heads are lower (on average), or for low hydraulic conductivity of the offshore aquitard. We show that AFWC can be significant, with a maximum AFWC rate of 70% observed across the simulated cases. The 3D models show that even though onshore heads indicate active seawater intrusion, alongshore head gradients can protect regions of low head such that recirculated freshwater, instead of seawater, is drawn into onshore wells. We conclude that in many situations, freshwater flowing from offshore to onshore aquifers may be recirculated groundwater from the onshore aquifers from where the onshore head is higher, rather than relic freshwater, as has been concluded from cross-sectional analysis in previous studies. These observations have implications for coastal groundwater management, and importantly, suggest that long-lived contaminants that discharge offshore may return onshore.