SITE INFORMATION AND CORRESPONDENCE

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Mr. Vaughn 2 12 May 1995 The next portion of this memo briefly summarizes the geologic and hydrogeologic characteristics of the STTC terminals area in order to provide basic information with which to assess the pilot testing proposal. SITE CONDITIONS IN THE STTC TERMINALS AREA: The STTC terminals are underlain by a sequence of fluvio-deltaic sedimentary deposits that consist of a heterogeneous assemblage of channel and overbank deposits. On a large scale, the strata are laterally discontinuous, although the extent to which a given depositional unit can be traced horizontally varies across the terminals area. Based on boring logs, the subsurface soils may be subdivided into 4 sequences, or zones, the A,B, C, and D zones. The zones differ from one another in dominant soil type and in degree of lithologic variability. Within the vertical section, boundaries between the zones are defined on the basis of marker beds, usually clays or clayey silts. The A zone comprises an "upper" and a "lower"portion which differ from each other in the amount, stratigraphic thickness, and horizontal continuity of the sand layers. In the lower zone sand units are traceable in subsurface logs over a greater distance than in the upper zone. Lower- portion sand units are up to ten feet thick, but the upper-portion sands are thinner and more discontinuous. Compared to the lower portion, the upper portion of the A-zone contains relatively more silt and clay. The thickest A-zone sands are in the southern and central part of the site. The sands thin to the east. The STTC terminals are underlain by a relatively thin vadose zone, which varies from approximately 3 to 9 feet below ground surface (bgs) according to season and location. Cross- sections presented with the workplan indicate that the vadose zone in the vicinity of the proposed test areas is composed dominantly of fine-grained, low-permeability soils (clays and silts). Hydrocarbon (HC) contaminants in the area include free product at a maximum thickness (in December 1994) of 0.02 feet in well PS/P-11, and in lesser thicknesses (0.01 foot to sheen ) in seven other wells. This is a decrease in thickness from historical maxima. Dissolved hydrocarbons (gasoline, benzene, toluene, ethylbenzene, xylenes, and diesel) are found in the A- zone in concentrations up to 75,000 ug/l in well AR/MW-5A in December 1994 and benzene up to 23,000 ug/1 in well TO/MW-I in December 1994. PROPOSED PILOT-TESTING STRATEGY: The two proposed test locations are designated the Southern Test Area (STA) and the Northern Test Area(NTA). The STA is approximately southeast of the Santa Fe Pacific Pipe Line and ST Services locations, near the proposed pilot-scale ground water interceptor trench. The NTA is in the northern portion of the terminals site, in the general area of the ARCO, Time, and Tesoro facilities. STTC proposes a phased approach to the pilot testing. First,pilot-test wells will be installed in the STA, after the trench is constructed. When site conditions allow, pilot-scale SVE, air sparging and ground water pumping tests will be conducted. Depending on the results of the pilot tests, they will evaluate the need for additional testing at the NTA. Testing at the second area will occur