SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected EnvironmentSBMR is in the Schofield groundwater area of the central Oahu groundwater flow system, the largestand most productive flow system on Oahu (Oki 1998). The central flow system is bounded on thenorth and south by coastal sedimentary deposits, known as caprock, because they overlie rocks thatare more permeable and can confine the groundwater contained in those rocks within the coastal zone(USAG-HI 2004).The Schofield hydrologic sub-area lies on the divide between the northern and southern parts of thecentral Oahu flow system. The northern portion includes the Mokuleia, Waialua, and Kawailoa hydrologicunits, while the southern portion includes the Ewa, Pearl Harbor, Moanalua, Kalihi, Beretania,and Kaimuki hydrologic units.The Schofield sub-area is bounded on the north and south by vertical low permeability features thatreduce or prevent groundwater flow. These features might be dike intrusions or possibly depositionalfeatures (Oki 1998). Because the groundwater elevation inside the “dams” is higher than outside, thegroundwater in the Schofield Plateau is called high-level groundwater. Rift zones associated with theWaianae and Koolau volcanoes contain clusters of vertical or nearly vertical dikes that bar groundwaterflow. The eastern and western sides of the Schofield sub-area are bounded by dike zones of theKoolau and Waianae volcanoes, respectively.Beneath the Schofield Plateau, groundwater occurs in the Schofield High-Level Groundwater Body,where depth to groundwater is approximately 600 feet or more, depending on the ground surface elevation.Additionally, groundwater occurs in the basal aquifer and dike-impounded groundwater systemassociated with the dike intrusions within the Waianae volcanics. Groundwater also occurs locallyin perched aquifers above the High-Level Groundwater Body or the basal aquifer.Recharge over most of the SBMR ranges between about 10 and 25 inches per year. Recharge ishigher along the eastern slope of the Waianae Range and in the southeast margin of the reservation(Shade and Nichols 1996 as cited in USAG-HI 2004). Recharge near the southeast margin of therange is greater because of contributions from irrigation. Most of the recharge to the central sector(Wahiawa aquifer system) is from the Koolau Range (USAG-HI 2004).Annual groundwater pumpage in the Schofield groundwater area (Wahiawa aquifer system) is estimatedto be less than 10 mgd and has decreased since 1979, when total pumping was about 20.6 mgd(Oki 1998 as cited in USAG-HI 2004). While this is less than half the estimated sustainable yield ofthe aquifer, any consumptive use of groundwater in the Central Sector decreases the underflow to theadjacent Pearl Harbor Sector or North Sector.WAAF lies over the southern boundary of the Schofield high-level water body, as described above.The water table declines from about 275 feet amsl on the north side of WAAF (high-level groundwater,or transitional) to about 30 feet amsl on the south side (basal aquifer; USGS 1996). Groundwaterflows south, toward the Pearl Harbor aquifer.SBER occupies a portion of the Waipahu/Waiawa groundwater hydrologic unit in the Pearl Harborhydrologic sector, just south of the hydrologic divide that separates it from the central hydrologic sector.Groundwater in the eastern part of SBER includes high-level volcanic dike-impounded groundwaterthat overlies and is probably connected to the basal aquifer hydraulically. This area is part of a135-square-mile area in the Northwest Rift Zone of the Koolau Range, which is the most importantand productive of the dike-impounded groundwater reservoirs on the island. The USGS has estimatedthat approximately 560 billion gallons of water are stored above sea level in this natural groundwaterreservoir (Takasaki and Mink 1985).February 2008 3–21 2/25th SBCT Final EIS