USE OF PRECIPITATION AND GROUNDWATER ISOTOPES TO INTERPRET REGIONAL HYDROLOGY ON A TROPICAL VOLCANIC ISLAND - KILAUEA VOLCANO AREA, HAWAII

Isotope tracer methods were used to determine flow paths, recharge are as, and relative age for groundwater in the Kilauea volcano area of th e Island of Hawaii. A network of up to 66 precipitation collectors was emplaced in the study area and sampled twice yearly for a 3-year peri od. Stable isotopes in rainfall show three distinct isotopic gradients with elevation, which are correlated with trade wind, rain shadow, an d high: elevation climatological patterns. Temporal variations in prec ipitation isotopes are controlled more by the frequency of storms than by seasonal temperature fluctuations. Results from this study suggest that (1) sampling network design must take into account areal variati ons in rainfall patterns on islands and in continental coastal areas a nd (2) isotope/elevation gradients on other tropical islands may be pr edictable on the basis of similar climatology. Groundwater was sampled yearly in coastal springs, wells, and a few high-elevation springs. A real contrasts in groundwater stable isotopes and tritium indicate tha t the volcanic rift zones compartmentalize the regional groundwater sy stem, isolating the groundwater south of Kilauea's summit and rift zon es. Part of the Southwest Rift Zone appears to act as a conduit for wa ter from higher elevation, but there is no evidence for downrift flow in the springs and shallow wells sampled in the lower East Rift Zone.