HYDROGEOLOGY OF THE HAWAII SCIENTIFIC DRILLING PROJECT BOREHOLE KP-1 .2. GROUNDWATER GEOCHEMISTRY AND REGIONAL FLOW PATTERNS

A series of downhole and surface water samples were taken from the I-k m-deep KP-1 borehole located on the eastern flank of the island of Haw aii. Early samples from depths of more than 700 m showed salinities ne arly equivalent to seawater but having anomalous cation concentrations that are attributed to ion exchange between formation fluids and resi dual drilling mud clays. Later deep samples found only minor variation s from seawater cation chemistry that are consistent with low-temperat ure weathering of basalts; delta(18)O values are equivalent to seawate r values and are consistent with this interpretation. Carbon 14 activi ties of dissolved inorganic carbonate indicate a water age ranging fro m 5890 to 7170 years B.P. and fluid transport rates of 1.8 to 2.2 m/yr . Fluid samples from perforations at 310 m in the borehole demonstrate that a freshwater aquifer is present at the Mauna Kea/Mauna Loa inter face; borehole resistivity logs indicate that it is similar to 200 m t hick. Although it has not yet been possible to obtain samples of the f reshwater zone without contamination from the deep saline fluids, the chloride concentrations of the low-salinity zone are estimated using a mixing enthalpy calculation to be less than 100 mg/L. Light stable is otope data indicate that the fresh water at 320 m is derived from rech arge entering the island at an average elevation of 2000 m. Inferred C -14 activities of the dissolved bicarbonate in the freshwater zone ind icate an average calibrated age of 2200 years B.P. and an average flui d velocity of at least 14 m/yr. A regional water flow model is propose d that suggests that the fresh water found at the 320-m depth is deriv ed from rainfall recharge from the middle elevations of Mauna Kea volc ano. This rainfall is channeled beneath the Mauna Loa lavas by the thi ck soil layer separating the two volcanoes. A second shallow fresh-to- brackish water zone, derived from Mauna Loa recharge, is also inferred to exist below the carbonate formation that underlies the shallow bas al lens. The results of our preliminary study of the groundwater syste m below the KP-1 drill site demonstrate that intervolcano and interflo w aquicludes can have a substantial impact on water circulation and di scharge from young island volcanoes.