Geophysical surveys performed on the Island of Molokai, Hawaii : final report

Abstract

The TDEM geophysical technique was effective at mapping the fresh/saline groundwater interface in the Kawela Plantation area of Molokai. Soundings were made along two survey lines which extended from an elevation of approximately 1,300 ft (400 m) ASL to 2,700 ft (820 m) ASL. Below approximately 2,100 ft (640 m) ASL, the fresh/saline water interface was mapped in both lines. In Line 2, a significant thickening of the basal groundwater lens occurs between soundings made at elevations of 1,470 ft (450 m) ASL and 2,100 ft (650 m) ASL. It is likely that a change in horizontal hydrologic permeability, possibly caused by a leaky volcanic dike, is present in the area between the lines. Above approximately 2,100 ft (640 m) ASL, the TDEM soundings do not map the saline/fresh groundwater interface. The- depth of exploration for these soundings indicates that the interface has to be below an elevation of approximately 750 ft below sea level. Although it is probable that volcanic dike damming structures occur either at the higher elevation soundings or immediately down slope from them, it is unknown if high-level groundwater is present at these sites.