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U 0 I Ms.Margaret Lagorio, SJCEHD October 21,2003 Joe's Truck Plaza Project No.2033-15600-01 ' Work Plan for Additional Site Assessment Page 4 SUBSURFACE CONDITIONS A predominantly silty sand stratum underlies the study area to a depth of approximately 13 to 18 bgs. Within this silty sand stratum, a clean, coarse sand layer, approximately 2 to 5 feet thick, is encountered at a depth of 6 to 10 feet bgs. This coarse sand appears laterally extensive beneath the site, but exhibits some discontinuity in the vicinity of the former northern UST's. . i Underlying the silty sand stratum is a stratum of hard clay, first encountered between 13 and 18 feet bgs and extending to at least 22 feet bgs. Laterally, the clay layer appears be continuous beneath the site, except in the vicinity of GP 41where it was not encountered to the depth of 20 feet bgs. The vertical extent`of the hard clay layer has not been adequately evaluated. Further subsurface exploration is warranted to characterize the vertical and horizontal extent of the hard clay. In the deepest boring advanced at the site (MWO-1X), the hard clay layer overlies another strata of silty sand (referred to as the lower sandy zone) that extends vertically ; from approximately 22 to 28 bgs. A second layer of clay was encountered at the base of this boring, from approximately 28 to 30 feet bgs. The lateral and vertical extents of the lower sandy and clayey strata have not been adequately evaluated. Quarterly Monitoring Quarterly groundwater monitoring and sampling began in February 2000. Through the third quarter 2003, groundwater flow in the shallow-screened monitoring wells has been generally northeast to east. Sporadic northerly and northwesterly flow has also been observed. Groundwater gradients have been relatively flat, between 0.00083 and 0.0015 ft/ft. Groundwater beneath the site has been measured at depths between 6 and 10 feet bgs. Groundwater elevations have fluctuated from 1 to 2 feet between monitoring events, but these fluctuations do not appear to be seasonal. PROPOSED SCOPE OF WORK Based on the data gaps discussed in the SCM and outlined above, the primary objectives of the scope of work proposed in this Work Plan are: • Further characterize the lateral extent of the dissolved MTBE plume within the upper sandy zone by installing wells MW-14 through MW-18, • Further characterize the distribution of dissolved MTBE mass within the plume by ; installing wells MW-7 through MW-13, P:\Joe's Truck Plaza\Work Plans\Joes workplan 102103.aoc i