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Treadwell&Wb MEMORANDUM TO : Vicki McCartney, REHS FROM: Dtxstyne Sutherland and David Dixon DATE: 20 January 2005 JAPl 2 1 2005 PROJECT: Former County Motor Pool Site ENVIRONMENT HEALTH 130 North Hunter Street, Stockton PERMIT/5EWCES SUBJECT : Addendum to 4 November 2004 Workplan This addendum to our 4 November 2004 soil and groundwater investigation workplan for the Former County Motor Pool Site at 130 North Hunter Street in Stockton, presents modifications to the soil boring program on the basis of discussions with you and the results of our 20 and 21 December 2004 sampling at the site. We are proposing to drill two additional borings to help delineate the northern and 0 leff&extent of groundwater contamination in the shallow and deeper zones. This work will be conducted on 25 "a"A 24 Tµr=_,�, 2nnc in conjunction with the well abandonment activities that you have already approved. - Rationale for Additional Work The object of this additional work is to help complete the site characterization, and to assist with evaluating remedial options for the Site. The recent deep groundwater sample collected at boring S13-3 (60-feet deep) has high TPH as gasoline (TPHg) concentration, 91 ,000 micrograms per liter (µg/1), and the highest benzene concentration yet detected at the Site, 9,400 pg/1. Shallow groundwater concentrations (30-feet deep) were significantly lower in the shallow zone (820 p g/1 and 13 1Ag/1 for TPHg and benzene respectively), however, the northern extent of the deep and shallow zone impacts are not yet characterized. We propose to advance S13-7 (Figure XJ to help delineate the northern extent of the deep and shallow zone impacts. a The shallow groundwater sample collected at boring S13-4 had relatively high TPHg concentration, 4, 100 µg/1, and a low benzene concentration (4.0 µg/1). A deep groundwater sample was not obtained at SB-4, but TPHg and benzene concentrations in soil at 59 .5 feet deep in borings SB-4 and SB -3 are similar, indicating that the deep groundwater zone at S13-4 may have similar groundwater impacts to SB-3 . This data indicate that the eastern extent of the deep and shallow zone impacts are not yet characterized, and we propose to advance SB' (Figure 1 ) to help delineate the eastern extent of these impacts. 2 Proposed Soil Boring and Sampling 1 . Soil borings, S13 -6 and SB-7 will be advanced to 70 feet below ground surface (bgs). n This is 10-feet deeper than previous borings because the vertical extent of soil impacts has not yet been identified. Grab groundwater samples will be collected Treadwell & Rollo, Inc. Environmental & Geotechnical Consultants 555 Montgomery Street, Suite 1300, San Francisco, CA 94111 Telephone (415) 955-9040 Facsimile (415) 955-9041