All sorts of boundaries could be involved with a subject like this, geology boundaries, soil boundaries, surface water elevation boundaries, groundwater boundaries, tide range boundaries, plant transpiration boundaries as water users, temperature and humidity boundaries with respect to evaporation, deep seepage or losses of heavy contaminants that defy normal flow pathways.  Complexity is added since salt water is heavier than freshwater.  I added a paper by Toth that suggests in relatively flat tidal lands, the area of groundwater circulation is much more confine than mountainous areas.  In this analysis, there may be instantaneous times that solute transport is landward, seaward, laterally to or from streams and channels, downward by gravity and upward by plant or capillary flow.  But in the measuring and modelling, there should be some general trends or net flow transport directions that relate to the basic models such as Darcy's Law.  I am not up to date on models that may help you, but assume there is interest and may be some on this complex subject area.  The Reynolds paper describes some of the testing procedures of an interesting glacial outwash groundwater hydrology study, that perhaps does not directly apply to your question, but may help with some of the processes and boundaries involved that he evaluated.  If you want his plate figures, let me know.

For water flow ; you can specify watertable level as a lower boundary condition and evapotranspiration in case of being present of vegetation as atmospheric boundary condition at the top of the ground . 

For solute transport ; you need to determine the type of the solute. There are three main order of boundary condition for solutes .

Interface boundary between groundwater and tidal water may be delineated by the parameter like Chloride and Bicarbonate, along with various other chemical parameters. Tidal water has very low HCO3  and very very high concentration of chloride. In case of groundwater, in general HCO3 is high, and Cluoride is comparatively very less. Good published work available online.