The 3.9-to 2.9-Ma Waianae Volcano is the older of two volcanoes making up the island of Oahu, Hawaii. Exposed on the volcanic edifice are tholeiitic shield lavas overlain by transitional and alkalic postshield lavas. The postshield "alkalic cap" consists of aphyric hawaiite of the Palehua Member of the Waianae Volcanics, overlain unconformably by a small volume of alkalic basalt of the Kolekole Volcanics. Kolekole Volcanics mantle erosional topography, including the uppermost slopes of the great Lualualei Valley on the lee side of the Waianae Range. Twenty new K-Ar dates, combined with magnetic polarity data and geologic relationships, constrain the ages of lavas of the Palehua member to 3.06-2.98 Ma and lavas of the Kolekole Volcanics to 2.97-2.90 Ma. The geochemical data and the nearly contemporaneous ages suggest that the Kolekole Volcanics do not represent a completely independent or separate volcanic event from earlier postshield activity; thus, the Kolekole Volcanics are reduced in rank, becoming the Kolekole Member of the Waianae Volcanics. Magmas of the Palehua and Kolekole Members have similar incompatible element ratios, and both suites show evidence for early crystallization of clinopyroxene consistent with evolution at high pressures below the edifice. However, lavas of the Kolekole Member are less fractionated and appear to have evolved at greater depths than the earlier Palehua hawaiites. Postshield primary magma compositions of the Palehua and Kolekole Members are consistent with formation by partial melting of mantle material of less than 5-10% relative to Waianae shield lavas. Within the section of Palehua Member lavas, an increase with respect to time of highly incompatible to moderately incompatible element ratios is consistent with a further decrease in partial melting by approximately 1-2%. This trend is reversed with the onset of eruption of Kolekole Member lavas, where an increase in extent of partial melting is indicated. The relatively short time interval between the eruption of Palehua and Kolekole Member lavas appears to date the initial formation of Lualualei Valley, which was accompanied by a marked change in magmatic conditions. We speculate that the mass-wasting event separating lavas of the Palehua and Kolekole Members may be related to the formation of a large submarine landslide west and southwest of Waianae Volcano. Enhanced decompression melting associated with removal of the equivalent volume of this landslide deposit from the edifice is more than sufficient to produce the modeled increase of 1-2% in extent of melting between the youngest Palehua magmas and the posterosional magmas of the Kolekole Member. The association between magmatic change and a giant landsliding event suggests that there may be a general relationship between large mass-wasting events and subsequent magmatism in Hawaiian volcano evolution.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8. Specific conductance is given in microsiemens per centimeter (mS/cm) at 25° Celsius. Microsiemens per centimeter is numerically equal to micromhos per centimeter. Concentrations of chemical constituents in water are given in milligrams per liter (mg/L). Datums Vertical coordinate information was not available at the time of the study. Horizontal coordinate information: base map used was a navigational chart prepared by the Defense Mapping Agency, surveyed in 1944, 2 nd edition printed 1985. Navigational chart was based on a "local datum," established at the time of survey, and was not updated using WGS 72 for the 2 nd edition. vi This page is intentionally blank.

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The State of Hawaii Department of Transportation Stormwater Monitoring Program was implemented on January 1, 2001. The program includes the collection of rainfall, streamflow, and waterquality data at selected sites in the Halawa Stream drainage basin. Rainfall data were collected at two sites, and streamflow data were collected at 3 sites for the year July 1, 2001 to June 30, 2002. Waterquality data were collected at five sites, which include the three streamflow sites. Six storms were sampled during the year July 1, 2001 to June 30, 2002, for a total of 44 samples. For each storm event, grab samples were collected nearly simultaneously at all five sites, and flowweighted, time-composite samples were collected at the three sites equipped with automatic samplers. Samples were analyzed for nutrients, trace metals, oil and grease, total petroleum hydrocarbons, fecal coliform, biological oxygen demand, chemical oxygen demand, total suspended solids, and total dissolved solids. Quality assurance samples were also collected to verify analytical procedures and insure proper cleaning of equipment.

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