Determination of the Location of the Fresh-Salt Water Interface in Coastal Unconfined Aquifers

Mathematical description of the relationship between the fresh-salt water is of important significance in the investigation of groundwater hydraulics in costal zones and evolution of the coastal environment. Examination of the equilibrium status of a point in the salt water zone and the description of piezometric heads at two points in both the salt water and fresh water zones reveals that the location of the fresh water/salt water interface in a coastal homogeneous, isotropic unconfined aquifer can be estimated based on piezometric heads at two points in the same vertical line tapping respectively the salt water zone (including the interface) and the fresh water zone (from the water table to the interface) when the groundwater flow system is in a steady state and satisfies the Dupuit assumption. The Ghyben-Herzberg relation is a special case of these methods and the Hubbert relation is also a special case of these methods. They require two nearest piezometric wells which respectively tap the fresh water and salt water zones. In addition, considering a linear increase or decrease in the hydraulic head in the fresh water zone where the Dupuit assumption is not met, another method for estimating the location of the interface is described based on fresh water heads at two points in the fresh water zone and salt water head at one point in the salt water zone in the same vertical line in the coastal zone. This method using corresponding pressures at the points is also given. The Ghyben-Herzberg relation is a special case of this method. It requires three nearest piezometric wells, two tapping the fresh water zone and one tapping the deeper salt water zone, or three transducers installed in one well, two in the fresh water zone and one in the deeper salt water zone.