Onshore entrapment of sea water in coastal aquifers by rapid coastline
progradation
Abstract
We hypothesize that onshore saline groundwater in delta systems may have
resulted from rapid shoreline progradation during the Holocene. To
explore this hypothesis, we develop a model for the transport of saline
groundwater in a shore-normal longitudinal cross-section of an evolving
ocean margin. The transport model uses a control volume finite element
model (CVFEM), where the mesh of node points evolves with the changing
aquifer geometry while enforcing local mass balance around each node.
The progradation of the shoreline and evolution of the aquifer geometry
is represented by assuming the shoreline advances at a prescribed speed
with fixed top and foreset slopes. The combined model of transport and
progradation, is used to predict the transient trapping of saline water
under an advancing shore-line across a range of realistic settings for
shoreline velocity and aquifer hydraulic properties. For homogeneous
aquifers, results indicate that the distance behind the shoreline, over
which saline water can be detected, is controlled by the ratio of the
shoreline prorogation rate to the aquifer velocity and the Peclet
number. The presence of confining units probably had the greatest impact
in sequestering onshore seawater behind an advancing shoreline. Further
support for the validity of the proposed model is provided by fitting
model predictions to data from two field sites (Mississippi River and
Bengal Deltas); the results illustrate consistent agreement between
predicted and observed locations of fossil seawater.