Estimation of the position of the sharp interface in littoral aquifers with hydraulic head differences within the fresh-water zone

Abstract

In littoral zones where fresh groundwater discharges into the sea, a vertical difference in hydraulic head always exists within the fresh-water zone, i.e., the Dupuit assumption is not satisfied. The hydraulic head difference may affect the sharp interface and needs to be taken into account when position of the interface is determined. Examination of the equilibrium status of a point on the interface reveals that the location of the interface in a homogeneous and isotropic coastal unconfined aquifer can be determined based on the fresh-water head and the salt-water head at one point on the sharp interface. Considering a linear increase or decrease in the hydraulic head within the fresh-water zone, a method for estimating the position of the sharp interface is described on the basis of the fresh hydraulic heads at two arbitrary points in the fresh-water zone and the salt hydraulic head at one arbitrary point in the salt-water zone in the same vertical straight line in the littoral zone. The method using corresponding pressures at the points is also given. The Ghyben-Herzberg model is a specific case of the method. This method requires three nearest observation wells, two revealing 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. The Observations of piezometric heads in the northwest Beihai coast in China are used to illustrate the application of the method to determine the position of the sharp interface, which ranges from 32.56 to 49.15 m under the mean sea level during the specific observation period.