Objectives
The most easily applied approaches to estimating offshore freshwater limits are based on sharp-interface assumptions, which neglect dispersive mechanisms and offshore circulation of seawater. The difference between sharp-interface and dispersive models has been investigated extensively for onshore coastal aquifers; however, the role of dispersion in controlling offshore freshwater-seawater interactions is not well understood. The main purpose of this study is to explore how dispersion affects the key features of offshore freshwater-seawater interactions, which includes the interface location and width, freshwater and seawater circulation rates.
Design and methodology
We conduct a series of dispersive numerical experiments in SEAWAT using cross-sectional models of uniform characteristics, i.e., isotropic and homogeneous, to represent simplified offshore aquifer conditions that allow us to compare it with the Werner and Robinson (2018) solution.
Original data and results
Results show that dispersion affects the tip (i.e., where the interface intercepts the top of the aquifer) and toe (i.e., where the interface intercepts the bottom of the aquifer) differently. Enhanced dispersion causes the toe to advance seaward, as expected; whereas the tip shows a non-monotonic relationship with dispersion that depends on the contrast between aquifer and aquitard hydraulic conductivities. The mixing zone at the toe widens as dispersion increases, similar to onshore cases, whereas the mixing zone at the tip has a surprisingly non-monotonic relationship with dispersion. The freshwater and seawater circulation rates increase with dispersion, as opposed to the non-monotonic relationship found in onshore aquifers.
Conclusions
Counteractions between dispersion, refraction, density and advective forces explain the different behaviour caused by dispersive processes in offshore aquifers to that observed in onshore settings.
References:
Werner, A. D., Robinson, N. I. (2018). Revisiting analytical solutions for steady interface flow in subsea aquifers: Aquitard salinity effects. Advances in Water Resources,116, 117-126, doi:10.1016/j.advwatres.2018.01.002.