Saltwater Circulation Driven by Shoreline Curvature in Coastal Aquifers

Abstract

In coastal lowland and plains, where dense populations and extensive agriculture thrive, fresh water is a necessity. Management of coastal groundwater requires quantitative models of groundwater flow and transport to reveal salinity distributions and submarine groundwater discharge (SGD). Historically, computational burdens in modelling coastal groundwater necessitated the use of 2-D models along cross-sections. However, a variety of different features add fully three-dimensional complexity to near-shore groundwater flow and complex patterns of saline-fresh groundwater mixing. Shoreline geometry is one such feature which has not been systematically assessed. We modelled mixing processes between freshwater and saltwater in three dimensions using the coupled surface/subsurface flow and transport code HydroGeoSphere. Various concave and convex coastlines were constructed with geomorphological features characteristic of those in China. Our simulation suggested that the lateral groundwater flow can cause significant velocity perpendicular to the plane of the cross-sections, accounting for from 2% to 12% of longitudinal velocity. The lateral velocity component of groundwater increases with shoreline curvature, causing nonlinear responses of lateral saltwater circulation. The mixing zone and SGD change as a function of coastline curvature, which depends mainly on the magnitudes of convergent/divergent flow. In addition, shoreline curvature increases the mean travel time of fresh SGD, while a limited impact on the travel time of saline SGD is found. Our results highlight the nonnegligible influence of coastline geometry on lateral groundwater flow, freshwater-saltwater mixing, and SGD characteristics in commonly concave and convex shorelines. This study has important implications for management of groundwater resources, comprehension of biogeochemical processes along the coastal lowlands and plains.