ARCHIVED REPORTS_ANNUAL 2010 GROUNDWATER MONITORING AND SAMPLING REPORT

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' Mr.Lee Higgins—CEMC Annual 2010 Groundwater Monitoring and Sampling Report April 19, 2010 Alden Park Page 3 of 7 ' PREVIOUS ENVIRONMENTAL ACTIVITIES ' Several environmental investigations have been conducted at the site since 1971, and are briefly discussed as follows: • Site assessments were conducted to determine the extent of hydrocarbon-affected ' soil and groundwater in May 1971, October 1986, September 1987, and April 1996. An area of hydrocarbon-affected soil and groundwater centered at the former reservoir location was identified. The former oil-storage reservoir concrete lining was found to be relatively intact at approximately 6 feet bgs. Monitoring wells were installed during the investigations in 1986 (MW-I/KJC), ' 1987 (MW-1 through MW-17), and 1996 (MW-18 through MW-21). MW-19 was properly destroyed in March 2000, and MW-1/KJC, MW-2, MW-13, and MW-17 were properly destroyed in April 2006. ' • A municipal water supply-well survey was conducted in 1986. It was concluded that local municipal wells do not withdraw water from the upper water-bearing ' zone. • Soil venting and oil-trap collection were initiated in 1989 and terminated in 1995. In 1996, it was reported that approximately 3,000 gallons of oil had been ' remediated through a combination of biodegradation, volatilization, and extraction. The bio-vent building was demolished, and an associated vacuum well was properly destroyed in 2006. • During an alley reconstruction project in April/May 2007, approximately 173 tons (119 cubic yards) of hydrocarbon-affected soil were excavated and removed. ' Five oil-seep vaults that collected SPO and associated petroleum hydrocarbon- affected soil were removed in May 2007. • The results of a health-risk assessment reported that residual petroleum ' hydrocarbons in soil should not result in health risks greater than those considered acceptable by regulatory agencies. Acceptable excess cancer risk levels used in the evaluation were 1x10-6 for residential exposure and 1x10-5 for non-residential ' exposure (for park maintenance workers). A reference list of environmental investigation reports for the site is included as ' Attachment A. GROUNDWATER MONITORING AND SAMPLING ' MONITORING Monitoring and sampling has been conducted quarterly and/or semi-annually at the site ' since 1995 under MRP-R5-2003-0801. In December 2006 the Central Valley RWQCB issued MRP Order No. R5-2006-0812 to replace the previous MRP2. The new MRP stipulated that groundwater sampling would be performed annually, and SPO recovery twould continue to be performed quarterly. ' 2. Central Valley RWQCS,2006. Monitoring and Reporting Program Order No.R5-2006-0812,Alden Park,Tracy, San Joaquin County. December 15.