Time domain electromagnetic surveys for assisting in determining the groundwater resources, Phase II study, Island of Lanai, Hawaii

Nance, Tom
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Blackhawk Geosciences, Inc.
Tom Nance Water Resource Engineering
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"Time domain electromagnetic (TDEM) surveys were performed by Coleman Energy & Enviromnental Systems- Blackhawk Geosciences Division (CEES-BGD) under contracts with the Lanai Water Company, to assist in determining the ground water resources of the Island of Lanai, Hawaii. This type of geophysical survey has proven to be a cost effective means to aid in providing an understanding of potential ground water resources on Lanai and other Hawaiian Islands. By comparing TDEM data with known well information and with other surveys in Hawaii, the geophysical data is used to provide knowledge on the geologic/hydrologic regime on the island. A number of wells on Lanai were available for comparison, and they were used to compare interpretations of the ground water resources. Previous TDEM surveys on Lanai and other Hawaiian Islands have reliably mapped the boundary between fresh water in the basal mode and high-level occurrences. Previous surveys were confmed to small segments of islands. This is the first investigation covering an entire island, and therefore, provides an opportunity to address the potential water resources of an entire island. A total of 99 TDEM soundings were collected during three separate periods, February 1993, August 1994, and October-November 1994, and information from twelve wells are available. This report incorporates the findings of all three surveys, and from the twelve wells. TDEM soundings were collected at locations accessible by vehicle, and on the rugged windward side of the island soundings were acquired using helicopter support. The objective in station location selection was to provide adequate data density around the island for delineating areas of basal ground water from areas where ground water damming structures exist, and where high-level water can occur. Additional soundings were placed in areas not previously surveyed so that a map of the entire island in terms of the geologic/hydrologic regime could be constructed. The results of the comprehensive evaluation and interpretation of the TDEM geophysical data indicate that: • Basal ground water is present around the perimeter of the island, but the fresh/brackish water lens is limited. Thicknesses of fresh/brackish water above sea level have been calculated to be a minimal resource. • A geologic/hydrologic discontinuity interpreted as a ground water damming structure is present around the entire island. • The potential for high-level ground water is present throughout the central portion of the entire island. Well data was available for comparison to substantiate these findings in twelve areas near Lanai City and in the Palawai Basin area."
groundwater, TDEM, Lanai, Hawaii, Geology--Hawaii, Groundwater--Hawaii, Water-supply--Hawaii, Geology, Groundwater, Water-supply
383 pages
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