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COASTAL GROUNDWATER SALINITY AND THE WATER TABLE
Two approaches are used to simulate variable density coastal groundwater. The fundamental approach uses the pressure gradient and density while the second computes flows on the basis of equivalent freshwater head and relative density. The estimation of gradients and flows from field data is considerably more complex (and uncertain) using freshwater heads. We review this background.
There appears to be confusion about freshwater heads that may arise in part from the different ways the reference datum might be selected. In some cases, a freshwater head correction appears to have been made assuming that Z (elevation of measuring point relative to a datum) is the height of the water column in a well or the depth of water in a surface water body:
In groundwater models such as SEAWAT and the SWI2 MODFLOW package, Z can be the elevation of the computational grid cell center or an adjusted elevation if the water table lies inside a cell. We illustrate how such grid-cell-based point measurements of head and equivalent freshwater head vary as the water table is approached.
In horizontal 1-dimensional, single-row models, Z is necessarily at the mid-elevation of the row of cells. While simulated freshwater heads change uniformly across a fresh/salt water interface under these circumstances, actual heads show a break in slope due to the interface. So the question arises as to whether such a break in slope might be seen in the water table. The answer appears to be no. The physics and the conversions to or from freshwater head ensure that if hf or h = Z, that is, if the water table head is measured at the water table, then hf = h.
We explore these issues by looking at simple models and our recent update of USGS’s Urban Miami-Dade model runs.