Water quality of HGP-A well waters

Abstract

A water quality monitoring program of the HGP-A well was conducted for downhole depth samples and continuous discharge samples in 1976-77. The well water was slightly saline (about 5% ocean water) and nearly deplete of magnesium. But the water contained high concentration of silica and sulfide. The chemical composition of the well water did not vary much with depth even though the sample depth reached the well bottom located approximately 5800 feet below sea level. The well fluid temperature was higher than any Hawaii groundwater and recorded a maximum of 358°C (676°F) at the bottom of the well. The water was contrastingly low in tritium compared with the surrounding groundwater as close as 1 mile away, thus suggesting existence of possible geologic boundaries. During continuous flow tests, the total well yield was 76,000 lb per hour or equivalent to 150 gpm with 70% steam and the rest water. For most water quality parameters measured, there was a 1-3 day build up period before approaching a steady-state concentration. Coupling water quality data with other measurements, the well is judged either poorly developed or is located in a formation of low transmissivity. The moderately high rainfall and high surficial permeability allow substantial rain water infiltration. Actual subsurface water circulation is open to interpretation because of uncertain geologic formations. There are inferred existence of deep dikes or intrusive bodies located on the ocean side of the well and acting as barrier against sea water encroachment.