Technical Report No. 24
COASTAL EVIDENCES OF GROUND WATER CONDITIONS IN THE VICINITY OF ANAEHOOMALU AND LALAMILO, SOUTH KOHALA, HAWAII
Doak C. Cox, Frank L. Peterson, William M. Adams, Chester Lao, John F. Campbell, Richie D. Huber
The rocks at the surface in Anaehoomalu and Lalamilo primarily are lavas from Mauna Loa which flowed through the saddle between Mauna Kea and Hualalai. These basalt lava flows, partly pahoehoe, but mainly aa, are generally quite permeable. No perched water or dike-impounded water is known in the area, certainly not close to the shore, and the fresh ground water is believed to occur in a thin layer at about sea level. Ground-water flux through the Anaehoomalu and Lalamilo districts is thought to be quite low, owing to the small recharge which results from the generally low rainfall and the high evapotranspiration. Computations of recharge from annual rainfalls and losses from evapotranspiration suggest that the discharge along the shoreline averages only a few million gallons per day per mile.
Onshore reconnaissance, which included sampling and electrical resistivity, and an offshore conductivity survey confirm that fresh and brackish water appears to be discharging along the coastline from Hapuna Bay to Keawaiki Bay in quantities too small to be easily detected. No anomalously high ground-water levels which might indicate the presence of large ground-water flows were detected.
A preliminary survey of salinity of coastal wells, springs, and water holes, and a visual search for ground water discharging at the shoreline did not show any evidence of such discharges on the order of millions of gallons per day. These data prompted an offshore salinity survey and electrical resistivity profiling onshore.
The offshore reconnaissance did not detect distinctly fresh water. However, ten areas of discharge of brackish or fresh water were detected. Although it is extremely difficult to determine the exact volume of discharge, a reasonable estimate of the volume of flow of brackish and fresh-water discharges ranges from a few tens of thousands of gallons per day at eight discharge points to perhaps one million gallons per day from each of the two larger discharge zones. No evidence indicates that the flow of brackish and fresh water from the ten discharge points detected by this survey exceeds a few million gallons per day.
All of the brackish and fresh-water discharges were detected in bays. It is not known whether this represents the true pattern of discharge or whether water being discharged along headland areas is very rapidly dissipated because of the high concentration of surf energy.
Electrical resistivity profiles using a Wenner spread were taken at 185 locations and electrical resistivity soundness, using a Schlumberger spread, were made at three of these, location. The soundings showed ground water to be very conductive up to sea level, and also indicated that the 70-foot spacing between sounding sites was appropriate for the profiling. Neither a depression in the fresh-saline interface nor a higher permeability region having fresher water was indicated in the profile. 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, but no detectable fresh-water body of any thickness was found.