Geophysical surveys for assisting in determining the ground water resources near the Pookela Water Tank, Island of Maui, Hawaii

Abstract

This report contains the results of geophysical surveys conducted to assist in determining the ground water resources near the Pookela Water Tank located in Makawao, Maui, Hawaii. The surveys were performed by Blackhawk Geometries (Blackhawk) for the Malama Group (Malama) during May 4 to May 14, 1998. The geophysical method employed during this survey was Time Domain Electromagnetic (TDEM) soundings. The TDEM soundings were positioned in the vicinity of Makawao. The main objective of the geophysical survey was to assist in characterizing the hydrologic regime in the study area for a proposed ground water well. Ground water resources can occur on the Island of Maui basically in two modes: • In a basal mode where a lens of fresh water floats on saline water, and • In a high-level mode where the ground water occurrence is controlled by subsurface damming structures. The volcanic rocks are generally highly permeable and this allows rainwater to infiltrate directly downward through the island mass. In the Makawao area, ground water was expected to occur as both a deep fresh/brackish water interface and possible high-level water at locations above subsurface damming structures. From extensive TDEM studies in the North Kana area on Hawaii, a main ground water boundary has been located that generally parallels the coast line. From well production data near the Keauhou area, it is now postulated that other secondary damming structures exist both upslope and perpendicular to the main damming structure. A major ring fracture developed from a caldera is outlined, which is upslope and near parallel to the coast line with radial fractures which form near perpendicular to the major ring fracture. It is thought that the fractures are zones of weakness where intrusive dikes preferentially form, and these dikes can act as vertical ground water barriers. The radial fractures could divide the high-level water behind the ring fractures- into different compartments. The water production from a single compartment will be determined by several factors; including permeability, porosity, size of and recharge to the compartment.