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WASTE DISCHARGE REQUIREMENTS PACIFIC GAS AND ELECTRIC McDONALD TRACT SAN JOAQUIN COUNTY 6. A Closure Plan was submitted by Discharger on 30 April 1991. Discharger also submitted a Preliminary Hydrogeological Assessment Report(PHAR) evaluating surface and ground water in the vicinity of the surface impoundments. The results of the PHAR indicated that chloride and barium in the waste material had leached into the shallow ground water and migrated southwest, to the adjacent surface water body (Whiskey Slough). Results of soil and waste sampling indicated that only Pond 3 contained a significant quantity of waste and was required to be closed under Chapter 15. Surface impoundments 1 and 2 did not contain any appreciable waste and were not closed under Chapter 15 requirements. The Closure Plan was conditionally approved by the Board on 9 April 1992. Description of Site 7. The surface of McDonald Island is mapped as Holocene peats and muds of tidal wetland. Near surface deposits at the perimeters of McDonald Island consist of sands, silts, and clays deposited by dredging activities or by periodic flooding of the island. The soil types on McDonald Island are highly organic. The first two soil types are thought to constitute the bottom of the surface impoundments and the berms. The remaining two soil types in the vicinity of the disposal surface impoundments are also characterized as poorly drained with low-to-rapid permeabilities. The berm fill, which mantles most of the pond site, is up to 14 feet thick and consists of intermixed silts, sandy silts, clayey silts, silty sands, and sands. The fill appears to be of local origin. The contact between native soils and fill in the vicinity of the surface impoundments is estimated to be approximately at the elevation of the pond bottom and the surrounding ground surface. 8. All or portions of McDonald Island may be subject to inundation by a flood of 100-year return period. This is due to the fact that 3,000 feet of island levees have a maximum crest elevation of+8.00 feet including 600 feet which are between +6.5 and+7.0 feet whereas the 100-year flood at McDonald Island is projected to have an elevation between 7.9 to 8.2 feet above mean sea level. 9. Depth to groundwater in the vicinity of the surface impoundments ranges from 4 to 15 feet, depending on location, surface elevation, and season. Boring logs indicate that the thick clay unit underlying the surface impoundments, and the interbedded silts and clays are also saturated. Ground water in sediments underlying the site appears to occur under unconfined conditions. 10. The ground water level is more than 5 feet below the bottom of all three surface impoundments. Ground water generally flows in the northwesterly direction below the disposal surface impoundments with a maximum gradient of 0.002 feet per foot as measured between January 1990 and March 1991. 11. Land within 1,000 feet of the site is used for irrigated agriculture. Water supply is obtained from Delta channels by means of siphon. The nearest residence and domestic water well is about 3/4 mile southeast of the site. 12. Beneficial use of ground water is domestic water supply. Whiskey Slough provides a habitat for local wildlife and supports aquatic organisms. Beneficial uses of Delta waters are municipal, agricultural, industrial water supply, recreation, esthetic enjoyment, and preservation and enhancement of fish, wildlife, and other aquatic organisms.