SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected EnvironmentAdditional groundwater is believed to be present below sea level but has not been estimated. Althoughthe dikes impede the flow of groundwater, they do not prevent it, and groundwater leaks fromthe dike complex at an estimated rate of 280 mgd. This is more than half of the total estimated yieldof water from all sources from the Koolau Range of 450 to 580 mgd (Takasaki and Mink 1985).The SRAA lies in the upper portion of the Ewa-Kunia sub-unit of the Peal Harbor hydrologic unit.The Ewa-Kunia sub-unit lies along the southern edge of the subsurface basalt groundwater dam thatunderlies the Schofield Plateau. This sub-unit is recharged in part by groundwater that overflows thisdam and flows southward from the Central or Wahiawa hydrologic unit. It is unlikely that groundwatercontributes significantly to flows in Waikele Stream because perched groundwater is deeper thanthe stream channel (Golder Associates 1998).Several wells have been drilled south of SBMR near Kunia Village. One, called the Navy Well, isabout 1 mile north of Kunia Village and provides most of the drinking water for Kunia Village. Thewell is believed to be completed in the high-level aquifer rather than in the basal aquifer at this location(Golder Associates 1998). The direction of groundwater flow beneath the SRAA is thought tofollow the regional trend and likely flows south.Groundwater QualityThe Southern Oahu Basal Aquifer, which underlies SBMR and part of the East Range, was designatedby the USEPA as a Sole Source Aquifer in 1987 under Section 1424(e) of the Safe DrinkingWater Act (USEPA 2003b). A sole source aquifer supplies at least 50 percent of the drinking waterconsumed in the area overlying the aquifer and represents a water supply source for which there is noalternative that could “physically, legally, and economically supply all those who depend on it fordrinking water.” Under the program, all federally funded projects in the area overlying a sole sourceaquifer are subject to review by USEPA to ensure that they do not endanger the water source.The quality of groundwater in the Schofield groundwater area is generally good. However, the regionalgroundwater system has been affected by pesticides and fertilizers related to agricultural practices,and groundwater quality in the SBMR has been affected by contaminants from industrial activities.The two major groundwater contaminants at SBMR are chlorinated solvents (trichloroethylene[TCE]) and carbon tetrachloride.For the purpose of cleaning up the contaminated groundwater, the groundwater beneath SBMR hasbeen identified as an “operable unit” (OU2), requiring remediation under the Comprehensive EnvironmentalResponse, Compensation, and Liability Act (CERCLA), as amended by the SuperfundAmendments and Reauthorization Act (SARA). The source of the carbon tetrachloride contaminationwas identified as a former landfill located on SBMR. The source of the TCE contamination was notdetermined. The distribution and extent of groundwater contamination is discussed further in detail in2004 Transformation FEIS (USAG-HI 2004). The ROD for OU2 identified the groundwater remedyas continued pumping of contaminated groundwater by SBMR supply wells and treatment of the extractedwater at the wellhead by air-stripping (Harding Lawson Associates, Inc. 1996). In August2000, the USEPA delisted SBMR from the USEPA’s National Priorities List (NPL) because it determinedthat the site remediation, including continued wellhead treatment of groundwater and longtermmonitoring, was adequate to protect human health and the environment (USAG-HI 2004).Groundwater quality in the dike-impounded groundwater system of SBER is generally excellent, withchloride concentrations lower than 20 milligrams/liter (mg/L). Dike-impounded groundwater is notknown to be contaminated with organic chemicals within the central Oahu flow system (Oki 1998 ascited in USAG-HI 2004). As described above, high-level groundwater in portions of SBER is con-February 2008 3–22 2/25th SBCT Final EIS