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A geohydrologic investigation of North Halawa Valley, Oahu, Hawaii, and its stream was undertaken in response to concern that runoff from the H-3 highway draining into the stream might seep into the ground and seriously contaminate potable water pumped at a nearby skimming well. North Halawa Stream flows over highly weathered alluvium or highly weathered basaltic basement for almost its entire course. Measurements of discharge at selected points along the stream indicate that infiltration occurs along some reaches. The infiltration of water from North Halawa Stream varies with season and stage. Water lost by the stream probably passes into and out of a perched aquifer in the alluvium of North Halawa Valley. Some of the water could pass from the alluvium to the basal aquifer from which the skimming well draws its water. INTRODUCTION North Halawa Valley overlies the Pearl Harbor aquifer (fig. 1), an important source of potable water for the island of Oahu. Freshwater in the Pearl Harbor aquifer is part of a large, lens-shaped body of ground water that is thickest in the central part of Oahu and thins toward the coastline. This lens of freshwater, known as the "basal lens," floats on saltwater that penetrates from the ocean into the basalt flows of which the island is composed. The basal lens is divided into multiple aquifers, including the Pearl Harbor aquifer, which are separated by structures of low permeability (Eyre and others, 1986). The Pearl Harbor aquifer is bounded on the northeast by the dike zone of the Koolau Range, on the southeast by valley-filling sediments, on the south by the sediments of the coastal plain, and on the west by the dike zone of the Waianae Range (Eyre, 1987). The nature of the aquifer's northern boundary is uncertain, but it separates the Pearl Harbor aquifer from the Schofield aquifer to the north (Dale and Takasaki, 1976). A large part of the water developed from the Pearl Harbor aquifer is pumped from the Halawa shaft (plate 1), which is operated by the Honolulu Board of Water Supply. Approximately 8 to 12 Mgal/d are pumped from the shaft's development tunnels, which skim the top of the aquifer. The shaft is the only producing well in the study area (plate 1), but accounts for approximately 10 percent of Honolulu's water supply (Chester Lao, Board of Water Supply, City and County of Honolulu, oral commun., 1989). This investigation, undertaken in cooperation with the State of Hawaii, Department of Transportation, stems from concern that runoff entering North Halawa Stream might seep into the ground and contaminate the ground water. 10 K IL O M E T E R S N or th H al aw a V al le y F i g u r e 1. L o c a t i o n o f N o r t h H a l a w a V a l l e y a n d P e a r l H a r b o r aq ui fe r. The objective of this investigation is to determine the general geohydrologic setting of North Halawa Valley through which a new interstate highway (H-3) will be constructed. This report includes (1) a description of the geology of North Halawa Valley from the crest of the Koolau Range to the Halawa shaft, (2) a description of the Pearl Harbor aquifer in this region, including the depth of the aquifer below the stream channel and an estimate of the zone of capture of the Halawa shaft, (3) a survey of stream seepage by measurements of discharge at selected points along the stream, and (4) a study of stream discharge using records from two U.S. Geological Survey (USGS) stream-gaging stations (16226000 and 16226200) on North Halawa Stream (plate 1). REGIONAL SETTING AND GEOMORPHIC FEATURES North Halawa Valley is a narrow, V-shaped valley on the leeward, southwestern flank of the Koolau Range, the younger of two large shield volcanoes that make up the bulk of the island of Oahu. The axis of the valley trends roughly northeast-southwest and extends 5 mi from the crest of the Koolau Range at the 2,400 ft elevation, to where the Koolau Basalt dips under the sediments and volcaniclastic deposits of the coastal plain at about 50 ft elevation. Rainfall ranges from an annual average of 150 in. in the upper valley to 40 in. in the lower valley (Blumenstock and Price, 1967). The main shield-building stage of the Koolau Range ended approximately 2 million years ago. The original form of the shield volcano has been carved by erosion (Macdonald and others, 1983). A thick sequence of thin lava flows constitute the basement rock in the study area. Material eroded from the shield has been deposited on the lower slopes and has partially filled the valley. The resulting topography consists of steep valley walls composed of basalt flows of the Koolau Range, a moderately sloping colluvial apron at the base of the valley walls, and nearly flat-lying sediments on the valley floor (plate 1).

Geology and Stream Infiltration of North Halawa Valley, Oahu, Hawaii