Date of Award
Master of Science in Geology
Reinhard K. Frohlick
The withdrawal of fresh groundwater from a coastal aquifer may induce the flow of salt water from the sea towards the well. This migration of salt water into a fresh water aquifer is known as salt water intrusion
The single most important design factor of a fresh water supply well in a coastal area is the depth to the salt/fresh water interface. In many cases salt water intrusions could probably be minimized if the position of the salt/fresh water interface could be determined before a domestic or municipal well is installed. In some highly developed coastal areas the position of the salt/fresh water interface has been determined in numerous boreholes. The extensive field data collected was then used to predict how various pumping schemes would affect the position of the interface with the use of computer modeling. Pinder and Page (1976) used this type of approach for a study on Long Island. However, on a smaller scale, the drilling of boreholes may be prohibitively expensive and the use of computer modeling out of the question.
In this study the depth to the salt/fresh water interface was determined adjacent to a deep borehole by performing a Vertical Electrical Sounding (VES). The depth to the interface as determined by the VES was in agreement with the observed depth in the borehole. Vertical Electrical Soundings were also made in other areas in order to determine if the method could be successfully applied to determine the depth to the salt/fresh water interface in locations without deep boreholes. In order to make this evaluation the depths to the salt/fresh water interface as determined from the VES curve interpretations in these areas were compared with the water quality data obtained in shallow wells and with the results of applying three theoretical methods. The three methods, described by Kashef (1983), Todd (1980) and Glover (1959), all provide solutions for the depth to a salt/fresh water under natural (nonpumping) conditions.
The depth to the salt/fresh water interface as determined by the Vertical Electrical Soundings was in agreement with the theoretical calculations in the areas where the position of the interface was not greatly affected by the pumping of fresh water from the aquifer. However, in the areas where the position of the salt/fresh water interface was known to have been affected by pumping, the depth to the interface interpreted from the VES curves was always less than predicted by the three theoretical methods.
Kowalski, Richard, "A GEOPHYSICAL INVESTIGATION OF SALT WATER INTRUSION INTO A COASTAL AQUIFER SOUTH DARTMOUTH, MASSACHUSETTS" (1985). Open Access Master's Theses. Paper 2011.