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WASTE DISCHARGE REQUIREMENTS -4- JOHNS-MANVILLE MANUFACTURING COMPANY, INCORPORATED FORMOSA PLASTICS, USA SAN JOAQUIN COUNTY GEOLOGY 16. The landfill is underlain by unconsolidated sediments of the Victor, Laguna, Mehrten, and Valley Springs Formations. The Victor Formation is approximately 120 feet thick, consisting of clays and silty sands. The predominant soil type is the Stockton Adobe Clay, with a depth of approximately 30 inches. Under the surface soils are interbedded silty clays and silty sands to a depth of at least 40 feet. 17. The closest known potentially active fault in the vicinity of the site is the Tracy-Stockton fault, which passes approximately 2 miles north-northwest of the landfill. The last known activity associated with this fault occurred in 1881, producing a magnitude 5+ earth- quake (Richter scale). This is considered to be the maximum probable earthquake for this site. SITE DESCRIPTION 18. Land within 1000 feet of the facility is used for industrial and agricultural purposes. 19. The facility is within the 100-year flood plain where flood depths are between one and three feet. 20. Groundwater levels are encountered at 18 to 47 feet below ground surface. The variances in elevations are possibly due to the presence of more than one water-bearing zone. The flow is to the east-northeast, with a gradient of 0.1 - 0.2%. A perched zone might exist in the area south of the surface impoundments. 21. The beneficial uses of ground water are domestic, municipal, agricultural and industrial supply. 22. The facility receives an average of 14.17 inches of precipitation per year, as measured at the Stockton Airport (data US Weather Service, 1949 - 1980). The annual evaporation is approximately 67.4 inches per year. 23. The 100 year, 24 hour precipitation is approximately 2.8 inches. 24. Surface drainage is west-southwest to North Littlejohns Creek, tributary to French Camp Slough which flows into the San'joaquin River.