SITE INFORMATION AND CORRESPONDENCE

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Page 1 of 1 i Nuel Henderson [EHJ k From: Nuel Henderson [EH] Sent: Wednesday, September 03, 2008 3:04 PM To: ' Donna Heran [EH]; Laurie Cotulla [EH] Cc: Margaret Lagorio [EH]; Lori Duncan [EH] Subject: Agricultural Commissioner's Office Donna and Laurie, have reviewed Soil Vapor Sampling and Human Health Risk Assessment Report,just received, and have had a preliminary discussion of the site with Jim Barton, who has also briefly reviewed the report. My main concern with the site is a lack of both a true down-gradient monitoring well and a monitoring well in the former tank pit, but overall the site doesn't appear to present a high risk to groundwater. The vertical extent of impacted soil in the former UST pit was established with soil boring SB-1 at 15 and 20 feet, the former ND, the latter had just a bit of xylenes. Most of the contaminant mass is at or near 10 feet below surface grade, coincident with the water table which ranges from —7.2 to 12.6 feet blow the top of the well casing, most commonly at—8.5 feet bsg. + f Silty sand was encountered in all borings and monitoring well borings at 15 and/or 20 feet bsg, so the monitoring a wells are completed in the likely lateral migration pathway, giving me a bit more confidence that a significant groundwater plume would be detected, if present, by the obliquely down-gradient monitoring wells. I think thtthe [ natural fluctuation of the flow direction, dispersion and diffusion of a dissolved plume should cause such a plume to be picked up by the wells periodically. Out of 12 monitoring ewvents, low'concentrations of diesel (68.8 to 436 .i ppb)were detected three times in MW-2 and once in MW-3, gasoline once in MW-2 (159 ppb), and benzene once / in MW-2 (71.87 ppb). As noted by JB, the benzene is suspiciously high for a normally ND well and with the u �l coincident low gasoline concentration. Although soil is impacted in shallow soil beneath the former UST pit, it appears to be of limited lateral extent and very limited vertical extent. Contaminants were not detected in the soil gas samples collected to evaluate for potential vapor intrusion problems. Neither Jim nor I see the soil or groundwater data as justifying requiring installation of a more true down-gradient well at this time, especially in view of the limited sensitive receptors in the area: Jim's initial feeling'is that his management will support a site closure, but he wants to review the site in Amore detail tomorrow, and I should do a bit more work on it. I anticipate Jim and I will discuss it further 1 tomorrow. J€ Nuel # { l i i II i1 I 9/3/2008