Geophysical surveys for groundwater evaluation near Waikoloa Village, Hawaii

Abstract

"This report contains the results of a time domain electromagnetic (TDEM) geophysical survey conducted to aid in the evaluation of fresh water resources near Waikoloa Village on the Island of Hawaii. The work was performed for Waikoloa Development Company (WDC) between January 15 and January 18, 1990. The objective of the survey was to add to the data base (35 soundings) obtained in December 1988. In this survey eight additional soundings were acquired in the area of interest south of the existing Waikoloa well field. The concepts in the evaluation of the geophysical study can be understood from the hydrogeologic cross-section, typical of a volcanic island. The volcanic rocks are generally highly permeable and rainfall rapidly infiltrates into the ground and migrates downward to the water table, and eventually discharges into the ocean. Fresh water in these settings is found in two environments: 1. Dike-confined waters -- Typically, above the rift zone, intrusive dikes originating from a magma source below can form ground water dams, and behind these natural dams significant quantities of ground water can be stored. 2. Basal fresh water -- The high permeability of the volcanic rocks allows sea water to enter freely under the island, and a delicate balance is reached where a lens of fresh water floats on sea water. In cases of hydrostatic equilibrium, the Ghyben-Herzberg relation states that for every foot of fresh water head above sea level there will be about 40 ft of fresh water below sea level."