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The Pearl Harbor area is underlain by an extensive basal aquifer that contains large supplies of fresh water. Because of the presence of a cap rock composed of sedimentary material that is less permeable than the basaltic lava of the basal aquifer, seaward movement of ground water is retarded. The cap rock causes the basal water to stand at a high level; thus, the lens of fresh water that floats on sea water is thick. Discharge from the basal ground-water body, which includes pumoage from wells and shafts, averaged 250 million gallons per day during 1931-65. Because the water level in the basal aquifer did not decline progressively, recharge to the ground-water body must have been approximately equal to discharge. Although pumping for agricultural use has decreased since 1931, net ground-water discharge has increased because of a large increase in pumping for urban use. Substitution of ground water for surface water in the irrigation of sugarcane has also contributed to a net increase in ground-water discharge. The development of Mililani Town will further increase discharge. The increase in ground-water discharge may cause an increase in chloride content of the water pumped from wells near the shore of Pearl Hrrbor unless the increased discharge is balanced by increased recharge to the 1 isal aquifer. The aquifer is recharged by direct infiltration and deep percolation of rain, principally in the high forested area, by infiltration and percolation of irrigation water applied in excess of plant requirements, by seepage of water through streambeds, and possibly by ground-water inflow from outside the area. Recharge is greatest in the uplands, where rainfall is heavy and where much infiltration takes place before rainwater collects in the middle and lower reaches of stream channels. Once water collects in and saturates the allu"ium of stream channels, additional inflow to the streams will flow out to sea, only slightly decreased by seepage. Average annual direct runoff from the 90-square-mile Pearl Harbor area is 47.27 million gallons per day, or 11.1 inches; this is 13.3 percent of the average annual rainfall (83.3 in.) over the area. Average annual direct runoff in streams at the 800and 400-foot altitudes is 29 and 38 million gallons' per day, respectively. Kipapa Stream has the largest average annual direct mnoff at those altitudes 6 and 9 million gallons per day, respectively. Bl B2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES Because streams are flashy and have a wide range in discharge, only 60 percent of the average annual runoff can be economically diverted through ditches to recharge areas. The diversion may be increased slightly if reservoirs are used in conjunction with ditches to temporarily detain flows in excess of ditch capacity. The planned irrigation use of some of the perennial flow available in Waikele Stream near sea level will decrease pumping fron and increase recharge to the basal aquifer. Suspended-sediment load is mainly silt and clay, and it increases rapidly with increased discharge. Thus, the use of streamflow for artificial recharge poses problems. High flows must be used if recharge is to fee effective, but flows must not be so high as to cause clogging of recharge facilities with sediment or woodland debris. Practical tests are needed to determine the advantages and disadvantages of different types of recharge structures, such as a reservoir or basin, largediameter deep shafts, deep wells, or combinations of all these structures. INTRODUCTION Planned developments in the Pearl Harbor basin indicate a need to increase pumping from the basal aquifer, a basaltic lava, to meet the growing demand. Mililani Town alone will require pumpage greater than 10 mgd (million gallons per day) to supply a projected population of 60,000. The first of about 11 required water-supply wells for this development was drilled in January 1968. The increased draft may cause an increase in chloride content of the water pumped from wells near the shore of Pearl Harbor because of sea-water encroachment. The increare in chloride content can be minimized if recharge to the basal ground-water body is increased. The purpose of this study was to determine the amount of streamflow that might be available for recharging the basal aquifer in the Pearl Harbor area. The investigation was limited to those streams entering Pearl Harbor. This report was prepared by the U.S. Geological Survey under the direction of Mearle M. Miller, district chief, Water Resources Division, as part of a water-resources investigation program under cooperative agreement with the City and County of Honolulu, Board of Water Supply, George L. Yuen, manager and chief engineer. Acknowledgment is given the Waiahole Water Co., Waialua Sugar Co., U.S. Navy, and U.S. Army for their cooperation and assistance. The Pearl Harbor area is defined in this report as the part of the island of Oahu bounded by the southern drainage divide of South Halawa Stream, the crest of the Koolau Range, the northern drainage divide of Waikakalaua and Waikele Streams, and the southwest drainage divide of Waikele Stream (fig. 1).

Availability of streamflow for recharge of the basal aquifer in the Pearl Harbor area, Hawaii