Geophysical surveys for assisting in determining the groundwater resources, Puu O Hoku Ranch site, Island of Molokai, Hawaii

Abstract

This report contains the procedures and results of surface geophysical surveys conducted to assist in determining the groundwater resources on property owned by Puu o Hoku Ranch, Ltd., Island of Molokai, Hawaii. The geophysical method employed during this survey was Time-Domain Electromagnetic (TDEM) soundings. The surveys were performed by Blackhawk Geometries (Blackhawk) for Puu o Hoku Ranch, Ltd. during December 5 to December 8, 1998. Survey oversite was provided by Tom Nance of Tom Nance Water Resource Engineering (TNWRE). The TDEM soundings for this survey were positioned on the ranch property above and below the main water tank, and on pasture land located below the ranch buildings. The Puu o Hoku Ranch of Molokai is located on the eastern portion of the island. The ranch property lies on the eastern dipping flank of the East Molokai Volcano. The main geologic feature on this portion of the Island of Molokai is the eastward trending rift zone, which is suggested by Fiske and Jackson (1972) to extend away from the caldera complex of the East Molokai Volcano toward the coastline to the mouth of Halawa Valley. A mapped volcanic cone near Koalii (Stearns and Macdonald, 1947) is also located in the study area. The main objective of the geophysical survey was to assist in characterizing the hydrologic regime at the Puu o Hoku Ranch site for a proposed groundwater well. Groundwater resources can occur on the Island of Molokai basically in two modes: • In a basal mode, where a lens-shaped body of fresh water floats on saline water, and • In a high-level mode, where the groundwater occurrence is controlled by subsurface damming structures. The surficial volcanic rocks in an island setting are generally highly permeable, and this allows rainwater to infiltrate directly downward through the island mass. The basal groundwater lens extends from the outer edges of subsurface structures (i.e., impervious dikes) to a discharge area near the shoreline. The subsurface structures can impede the flow of groundwater from the interior of the island toward the sea and result in fresh water-filled compartments that can extend above and below sea level. These types of occurrences are referred to as high-level ground water on the Hawaiian Islands. At the Puu o Hoku Ranch study area, groundwater was expected to occur mainly as a basal fresh-brackish water lens. Previous TDEM surveys on the Hawaiian Islands have reliably mapped the boundary between fresh water in the basal mode and high-level water occurrences. Geophysical surveys, combined with other hydrogeologic information, are used to provide optimum locations for well placement and completion depths.