A geohydrologic investigation of Honolulu's basal waters based on isotopic and chemical analyses of water samples

Abstract

Basal groundwaters in the Honolulu area were studied by a combination of three methods: radiocarbon age dating, tritium tracing, and examination of chemical data. Basic data used in this effort consisted of isotopic and chemical data on water samples. Isotope data included radiocarbon, carbon 13, and tritium; chemical data were primarily chloride, bicarbonate and silica. Water samples were mostly from groundwater sources in the Honolulu area, Pearl Harbor area, and Windward Oahu. Method of collection as well as procedures and equipment used for the analyses were described. The main objective of the study was the determination of basal water radiocarbon ages; how they were obtained and how they related to other parameters such as tritium, chloride, location or depth of sampling. A general objective was to show that studies such as this one, based largely on isotopic analysis of water samples, can yield hydrologic and geohydrologic information that enhances the understanding of the dynamics of groundwater systems in general and that of basal groundwater systems in particular. Radiocarbon ages obtained refer to the average time basal water samples resided in the basal aquifers, i.e. the time elapsed between recharge and sampling. Age calculations were based on differences in radiocarbon activity between basal water samples on the one hand and the activity of a recharge reference on the other, this difference being due to loss of radiocarbon through radioactive decay. Radiocarbon activity of the recharge reference was obtained from radiocarbon data on dike waters, high level perched waters and others. Processes such as carbonate dissolution, carbonate exchange and salt water intermixing were examined in some detail because, if they occur, they alter the relative or absolute content of aqueous radiocarbon, thus resulting in false ages. On the basis of theoretical considerations and experimental data it was shown that the effects of such nonradiogenic processes in principle can be detected and adjusted for with the aid of carbon 13 data. No such adjustment was applied in calculating the majority of basal water radiocarbon ages because respective carbon 13 data and recharge reference carbon 13 data had distributions that largely overlapped each other. Averages of radiocarbon ages for Isopiestic Areas 1 ~o 3, called geochemical mean residence times, ranged between 270 and 550 years, whereas displacement mean residence times calculated from geologic and hydrologic data ranged between 30 and 40 years. On the basis of this and other observations, a hypothesis was advanced that old water from pre-development bottom storage is still being flushed out of some systems. In accordance with this hypothesis, it was concluded that during the period of study old stored water was discharged by several sources in Isopiestic Areas 1, 2, 3, and 4 and, in particular, by sources in Areas 3 and 4. The amount of old stored water in the discharge from Kalihi Shaft varied as a function of time, a feature attributed to variations in pumping rate.