SITE INFORMATION AND CORRESPONDENCE_FILE 2

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r Chapin Brothers Investment, Inc. - 3 - 16 February 2005 1766 Monte Diablo, Stockton, San Joaquin County standard values for inputs,when site-specific values are available,may have affected the sensitivity of the groundwater model. A revised groundwater model should include a discussion of sensitivity testing. • The groundwater model does not address the issue of groundwater flow direction and preferential pathways through sand bodies. Contaminant transport direction appears to be towards the west as.shown in monitoring data,while groundwater flow direction is predominantly to the south. The model lumps all sedimentary units into one layer. The revised groundwater model should include additional layers to show the effect of preferential pathways. • While the groundwater model supports a finding that "...a state of equilibrium between the recharge flux rate and the biodegradation rate... ", actual monitoring well data for MW-3 in 2003-04 for Total Petroleum Hydrocarbons as gasoline (TPH-g, 1,600-2,600 µg/L)shows ariorder of magnitude difference between groundwater monitoring data over the 2004 TPH-g equilibrium concentration,which was modeled at 270 pg/l,due to biodegradation alone(Report,page 19). The groundwater model does not represent actual site-specific field conditions or show agreement with groundwater modeling data. A revised 2004 groundwater model equilibrium concentration should show reasonable agreement with site-specific data from the same period of time,under actual conditions (degradation modeled with pumping influence from the groundwater pump and treatment system). • The Report on page 22 quotes the Regional Board's Tri-Regional Guidance Appendix A Criterion 6 for No Further Action as: "The plume is stable and sufficiently limited in lateral and vertical extent and contaminant concentrations detected in groundwater show a decreasing trend with time. One hydrologic cycle (four quarters) of monitoring after active remediation measures have ceased is usually considered to be the minimum necessary to determine site groundwater and plume conditions". The Report then states groundwater modeling using the MT3D module for contaminant "...transport shows that the hydrocarbon contaminant concentrations will drop significantly for the first three years followed by constant levels as the plume stabilizes...Based on the results of the model, it is ATC's (consultant)opinion that one hydrologic cycle is not necessary and the site should be granted NFA(no further action closure)... " The Report states on page 19 that by 2015,TPH-g concentrations will reach 1 µg/L in MW-3, which is below Water Quality Goals (50 µg/L). The Report's Figure 17 (attached)shows Run#2 of the MT31) groundwater module in the year 2013. Figure 17 shows that the plume is over 400 feet long by 120 feet wide,with TPH-g concentrations of 2,380 gg/L in MW-1, 708 µg/L in MW-3, and 3,929 µg/L in MW-4. If Figure 17 is a correct interpretation for MT31)Run#2,then the results of groundwater modeling shown in Figure 17 do not show a decline in core concentrations nor predict that water quality objectives (Taste and Odor, 50 gg/L for TPH-g)will be met in a reasonable timeframe. Figure 17 shows that there is generally no decline in TPH-g concentrations after nine years,and that an NFA cannot be granted based on the groundwater modeling showing plume stability with declining concentrations, as stated in the Report. Therefore the closure request is incomplete without post remediation groundwater monitoring. 3)Additionally,there are several inaccuracies listed in the Petition: • On page five of the section labeled History of Site,the Petition states: 15 October 2003 a meeting was held at the S.J. County Environmental Health office to discuss necessity for groundwater remediation. The following participated Mr. Harlin Knoll,Mr. Noel California Environmental Protection Agency ea Recycled Paper