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iI Mr. Harlin Knoll February 15, 2000 Page 2 of 4 j had occurred. Soil boring locations are shown on Figu e 2. Analytical results are summarized in Table 1. One 10,000-gallon UST for storage of diesel fuel and t o 10,000 gallon USTs for the storage of gasoline and associated piping were removed under permit from PHS/EHD on January 21, 1999. Analytical results of sidewall soil samples and groundwater grab sample collected from beneath the USTs indicated that elevated levels of petroleum hydrocarbons were present in the soil and groundwater. Sample locations are shown on Figure 2. Analytical results are summarized in Table 2. 2.2 Soil Boring Investigation In July 1999, five direct push soil borings were advanced along the perimeter of the former UST location to investigate the vertical and lateral extent of soil and groundwater contamination, and four hand augered soil borings were advanced to investigate the lateral extent of shallow soil contamination. Soil boring locations are shown on Figure 2. Analytical results are summarized in Tables 3 and 4. The results of this investigation were submitted to PHS HD in a report dated October 19, 1999. Based upon the localized contamination and its current accessibility, Ground Zero recommends the implementation of remedial actions consisting of excavation and limited groundwater extraction, as well as the installation of groundwater monitoring wells to further define the extent of groundwater contamination. This workplan addendum addresses proposed interim soil and groundwater remedial actions. The installation of groundwater monitoring wells will be scheduled upon completion of excavation and backfilling activities and have been addressed under separate cover. h 2.3 Hydrogeological Setting The site is located in Thornton, California, approximately 5,100 feet southwest of the Mokelumne River, the nearest body of surface water. Surface elevation at the site is approximately eight feet above sea level. Based on historical information provided by San Jo quip County Flood Control and Water Conservation District and groundwater information fro rn a nearby site, groundwater beneath the l site is expected to flow in a southwesterly direction. Groundwater was first encountered at a depth of 6 to 7 feet in the borings drilled on the property. 3.0 NATURE AND EXTENT OF CONTAMINATION 3.1 Soil Contamination The data from the soil boring investigation combined with data previously obtained have largely :I defined the extent of soil contamination. Soil contarnination appears to be limited to depths between approximately 6 and 13 feet bgs and is generally found in the area immediately adjacent to G:1Data1GROLTNDZEIMANSER`Manser 2linterimwh.doc