SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Response to Russell Chapir I etter - 2 - 6 April 2009 ' 1766 Monte Diablo Ave., SL :ton, San Joaquin Co. 2) The latest quarterly monitoring report (4Q2008) in GeoTracker provides the sample results from 15 December 2008. Maximum onsite groundwater concentrations (MW-4) were TPHg, 30,000 ug/L; benzene, 1 ,700 ug/L; toluene, 24 ug/L; ethylbenzene, 180 ug/L; total xylenes, 160 ug/L; TAME, 200 ug/L; and 1,2-DCA, 9.2 ug/L. Of note are the MW-4 TPHg concentrations that peaked at 45,000 ug/L in September 2008, at a level not exceeded since December 1997. The latest quarterly remediation report (4Q2008) states GWP&T system was in operation during the fourth quarter, and removed 763.6 lbs. of TPHg between November 2007 and December 2008. For comparison, between April 2002 and May 2005, 193.4 lbs of TPHg were removed. The GWP&T system is currently removing an average of 58.7 lbs. of TPHg per month, compared to 5.2 pounds per month average between April 2002 and May 2005. This is an order of magnitude (10X) increase in efficiency of the treatment system during the last year of operation, due to increased groundwater concentrations. The GWP&T system is currently providing some hydraulic control of the groundwater plume, as offsite concentrations have declined since 2Q2008. However, the TPHg plume extends under the street at least 75 feet offsite to the west (4Q2008, MW-3, TPHg, 800 ug/L) of the site, which is two orders of magnitude less than 29,000 ug/L reported in February 2008, an all time (order of magnitude above historical) high for the offsite well. Prior to startup (pre-2002) of the GWP&T system, monitoring from January 1994 to present shows the groundwater plume had migrated to the west offsite, under the street and a vacant lot (see attached air photo) that Mr. Chapin stated he purchased to provide access for the investigation monitoring wells. In 2004, concentrations rebounded after the GWP&T system was shut down in 2003 and have continued to increase with the GWP&T system running through 2008, which is a major concern since these levels (one to two orders of magnitude greater in 2008) have not been seen for at least 10 years, and do not support Mr. Chapin's consultant estimate of a 75% reduction in mass.. There are no active USTs on the site, and no explanation was given why TPHg and benzene concentrations spiked up significantly in 2007-08 and continue to show elevated concentrations while under active remediation. According to GeoTracker, there are no public supply wells within '/z mile of the site. There is a school on the northwest corner of the intersection of Monte Diablo and Buena Vista (diagonally across the street from the site). Groundwater flow direction has recently been measured towards the east and southeast, but according to monitoring data and contamination concentration contours, the groundwater plume moves towards the west. 3) In the latest two reports (4Q2008 quarterly monitoring and 4Q2008 remediation), the consultant recommends limited quarterly monitoring, continued operation of the GWP&T system, and a batch extraction event for onsite MW-4. I would consider these recommendations as cost effective and technically with merit at this time (as does the County). I would also collect a minimum of two more quarters of data before considering a rebound test during the fourth quarter of this year, but only if concentrations in monitoring wells and the GWP&T system decreases significantly over the next two quarters. Until then. I would not recommend additional pilot studies (a 2008 pilot study for dual phase extraction did not detect any vapor contamination from the vadose zone) as the GWP&T system is currently removing sufficient mass to justify its operation (as does the County). Finally I would recommend AGE to conserve Cleanup funds by combining the quarterly monitoring report and the remediation report into one document, since there is a great deal of redundancy (mostly the same data