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

Abstract

Time Domain Electromagnetic (TDEM) surveys were performed by Blackhawk GeoSciences for the Lanai Water Company in August 2001. The objective of the TDEM surveys is to add additional detail to previous TDEM interpretations, and to explore for anomalous basal groundwater occurrences. The previous TDEM surveys conducted by Blackhawk are summarized in Blackhawk project report number 9081-000, dated December 15, 1994. In this (present) TDEM survey, seven areas were surveyed. The specific objectives of the present surveys were: • To refine the location of the interpreted groundwater damming structure at selected locations (Areas 1 and 2), and • To explore for anomalous basal groundwater occurrences (Areas 3-7). The TDEM surveys in Area 1 resulted in repositioning of the interpreted groundwater structure approximately 800 to 1,000 feet further west (seaward) than previously interpreted. This new position reduces the available basal groundwater resource in this Area. The TDEM surveys conducted in Area 2 show that the position of the main groundwater damming structure is unchanged; however a geologic structure (interpreted as a rift zone) was interpreted in Maunalei Gulch. Groundwater wells in the vicinity of these structures have historically been poor. The results of the exploration for anomalous basal groundwater occurrences in Areas 3-7 indicate that Areas 4 and 6 may have better than normal basal groundwater resources compared to other basal mode occurrences on Lanai. To date, most wells drilled for basal mode groundwater occurrences on Lanai have been poor. This interpretation is based on the relative depth of the saltwater intrusion over a limited area at the sites. At both sites the depth to saltwater intruded formations is deeper than expected, inferring a thicker brackish/fresh water resource. Other factors, such as a decrease of permeability of the formation, or existence of altered volcanics may contribute to this increase in depth of the saltwater intruded formations. Both of these conditions would result in low production wells. The results from this survey do not indicate that the existence of anomalous basal groundwater resources. The present survey supports the conclusions derived from previous surveys, i.e.: • Basal groundwater is present around the perimeter of the Island, but the fresh/brackish water lens is limited. The thickness of fresh/brackish water above sea level is expected 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 significant high-level ground water is present throughout the central portion of the entire Island. Well data has substantiated this interpretation.