Technical Report No. 129
UNDERGROUND RESIDENCE TIMES AND CHEMICAL QUALITY OF BASAL GROUNDWATER IN PEARL HARBOR AND HONOLULU AQUIFERS, OAHU, HAWAII
Theodorus H. Hufen, Paul Eyre, and William McConachie
Information on the large-scale movement and origin of groundwater in southern Oahu was obtained by examining the natural isotopic and chemical composition of water samples. Six basal groundwater systems (Pearl Harbor, Moanalua, Kalihi, Beretania, Moiliili, and Waialae) were studied and are discussed in this report. The experimental data base comprises the analyses of 84 groundwater samples, 2 surface water samples, and 49 rain-water samples for the following parameters, radiocarbon (14C), carbon (13C), carbon 13 tritium (3H) major ions, nitrate, silica, and pH. These data were supplemented with isotope and chemical data obtained in previous studies and selected data from the literature. Carbon isotope data, radiocarbon and carbon 13, on basal groundwater samples were evaluated with respect to “recharge reference” values. The latter were established on the basis of isotope analyses of water samples obtained from inland sources, such as tunnels tapping high-level dike compartments or wells located close to the mountains. Tritium data on basal groundwater samples were evaluated with respect to values for local rainfall and the recharge reference. Radiocarbon dating was used as an index of age to identify those groundwaters whose underground transit or residence times are in excess of about 200 years. Radiocarbon and tritium were used to identify young water, less than 20 yr old, through the detection of artificially high radiocarbon and tritium activities resulting from atmospheric testing of nuclear weapons during the past two decades. Underground residence times of basal waters in the Kalihi, Beretania, and Moiliili systems are quite long compared to the residence times of the other systems. Rain water infiltration in regions of high elevations located far inland is a definite source for most of the recharge to these systems. Basal water in the Waialae system has a short residence time compared to the other Honolulu systems. Moanalua basal waters were not uniform in their isotopic and chemical characteristics and indicated very long as well as very short residence times. Basal water in the western, inland portion of the Pearl Harbor system is similar in underground residence time to that in the Kalihi, Beretania, and Moiliili systems but water in the eastern and seaward portions is characterized by short residence times and, in many cases, the presence of young water. Many regions in this basal water system and, in particular, the eastern part, show the presence of transition-zone water, return irrigation water, and caprock-type water, either singularly or in some combination. Simple mixing models utilizing several suitable chemical parameters made it possible to determine the possible origins of water present in the discharge of a selected source or the presence of a particular type of water in the discharge of several sources located within a few miles of each-other.