Technical Memorandum Report No. 13
Geophysical Evidences for Ground Water Conditions in the Vicinity of Anaehoomalu and Lalamilo
W. M. Adams, F. Peterson, C. Lao, and J. F. Campbell
May 1968
SUMMARY
An offshore reconnaissance did not detect distinctly fresh water. However, eleven areas of discharge of brackish water, all with probable chloride concentrations greater than 1000 parts per million were detected. Although it is extremely difficult to determine the exact volume of discharge, a reasonable estimate of the volume of flow of brackish water discharges ranges from a few tens of thousands of gallons per day at nine discharge points to perhaps one million gallons per day from the two largest zones of shoreline discharge. No evidence indicates that the flow of brackish water from the eleven discharge points detected by this survey exceeds a few million gallons per day. All of the brackish water discharges were detected in bays. It is not known whether this represents the true pattern of discharge or whether brackish water being discharged along headland areas is very rapidly dissipated owing to the high concentration of surf energy associated with these features. Electrical resistivity profiles using a Wenner spread were taken at 185 locations and electrical resistivity soundings were made at three of these locations, using a Schlumberger spread. The soundings showed the ground water to be very conductive up to sea level, and also indicated that the interval of 70 feet used was appropriate for the profiling. The profile did not observe any anomaly that could be attributed either to a depression in the fresh-saline interface or to a higher permeability region having fresher water. Since profiling data are relative, interpretation is rather simple. By the combined use of sounding and profiling data, a layer of brackish water eight feet thick, equivalent to a head of two-tenths of a foot, should have been discernible. No fresh water was present in any detectable thickness.