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SECOND QUARTER 2003 GROUNDWATER MONITORING REPORT AND ADDITIONAL ASSESSMENT REPORT JAMAR OIL DISTRIBUTION JUNE 2003 Both the Modesto and Riverbank formations are part of the Great Valley sedimentary sequence of rocks which fill the large Great Valley sedimentary basin which was originally formed as a foreare basin between the Sierran arc to the east and the subduction zone to the west. Stockton is located within the San Joaquin Valley which consists of the southern two thirds of the Great Valley province. The northwesterly flowing San Joaquin River and its tributaries are the main drainage feature of the San Joaquin Valley. According to the USGS Stockton East, 7.5 Minute Quadrangle,which covers the subject site, the closest surface water body is Mormon Slough, which lies approximately 1400-feet south-southeast of the subject site and is crossed by Oro Avenue. Past quarterly monitoring at the subject site has shown the direction of groundwater flow to be south-southeast. The subject site lies at an elevation of approximately 29 feet above Mean Sea Level (MSL). Summary of Previous Investigations In 1995, a 300 gallon waste oil UST and a 500 gallon kerosene UST were excavated and removed from the site. No environmental issues were documented with the removal of these tanks. i In December 1998, two 6,000-gallon and one 10,000-gallon gasoline USTs were excavated and removed from the site and replaced with a two-compartment 20,000-gallon UST meeting EPA and State of California upgrade requirements. Soil samples taken in conjunction with the tank removal contained detectable concentrations of petroleum hydrocarbons and on January 27, 1999, an Unauthorized Release form was filed with the County. An additional Unauthorized Release was filed with the County on June 11, 1999. In response to a directive from the County, John P. Cummings & Associates (JPCA) conducted a subsurface investigation on September 8-9, 1999, by installing and sampling three monitoring wells (MW-1, MW-2, and MW-3). MW-1, MW-2, and MW-3 were drilled to approximately 78 feet bgs, 74 feet bgs, and 74 feet bgs, respectively and sampled every 5 feet. Groundwater samples were collected from the three wells on September 23, 1999. On August 21, 2000, JPCA did a further subsurface investigation by drilling and sampling one soil boring(B1) and installing and sampling two additional monitoring wells (MW-4 and MW-5). The soil boring was drilled to approximately 115 feet bgs and sampled every 5 feet. MW-4 and MW-5 were drilled to approximately 75 feet bgs with soil samples collected every 5 feet. Groundwater samples were collected from MW-1, MW-2, MW-3, MW-4 and MW-5 on August 30, 2000. The results of this investigation are documented in JPCA's 2"d Phase II Environmental Assessment Report dated October 25, 2000. • GHH assumed environmental consulting responsibilities for the site in July 2001. 2