A combination of geologic mapping, seepage runs, water-budget computation, analysis of stream-gaging records, study of stream sediment, and measurement of polychlorinated biphenyls (PCBs) was used to investigate the geohydrology and possible routes of PCB transport from areas of possible contamination in the U.S. Coast Guard's Omega Station in Haiku Valley, Oahu, Hawaii. The PCBs were a contaminant in fuel that was sprayed on vegetation and ignited during defoliation efforts at the Omega Station. Haiku Valley is a stream valley eroded into a thick section of dike-intruded, highly permeable lava flows. The valley is partly filled with alluvium, pyroclastics and massive lava flows. A shallow aquifer system is formed by a permeable unit of pyroclastics which is bounded below by less permeable alluvium, massive lava flows, and weathered basalt. A deeper aquifer system is present in the dike-intruded lava flows. Much of the area of suspected PCB-contaminated fuel application in Haiku Valley is situated on the geologic unit that forms the shallow aquifer. A water budget calculated for the drainage area of a stream-gaging station (16275000) at the downgradient boundary of the Omega Station indicates that the shallow and deep aquifers receive recharge of 5.1 cubic feet per second from the 0.98 square-mile drainage area; approximately 10 percent of the drainage area is suspected to have been contaminated by PCBs. Approximately 4 cubic feet per second of water is withdrawn from the aquifers by a well and a water tunnel in the valley, but the geology of the area indicates that some of the water withdrawn by the tunnel also comes from recharge beyond the surface-water divides of Haiku Valley. Base flow to the stream is about 1.2 cubic feet per second. A water-balance calculation between recharge, well and tunnel withdrawals, and stream base flow, indicates that 1.0 cubic feet per second or more of the water recharging the drainage area may travel through the subsurface and discharge downstream beyond the limits of the Omega Station. Mass-balance calculations indicate that the concentration of PCBs that dissolves in the ground-water as it infiltrates contaminated areas will be on the order of 10-7 micrograms per liter, but these calculations are based on a number of assumptions where data are lacking. Analysis of sediment from Heeia Stream indicates that the sediment is transporting PCBs past station 16275000. Although PCB concentrations in unfiltered stream water samples collected at the station were below detection limits 0.1 micrograms per liter, PCB concentrations ranged from 64 to 230 micrograms per kilogram in suspended sediment extracted from stream water. Because most of the suspended sediment in Heeia Stream is deposited in a wetland near the coast, the potential for PCB accumulation is greatest in the wetland.