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Sent By: REGIONAL BOARD ANNEX; 916 2553164; Feb-2,' '5 10:48; Page 5/6 Chapin Broth rs Investment, Inc. - 3 - 16 February 2005 1766 Monte bialblo., Stockton, San Joaquin County stand ard values for inputs,when site-specific values are available, may have affected the sensitivity of the Qrou dwater model. A revised srrrninrlwAtar mnrlPI chn"Id ;"I! it' �14,;"11v�i^" OF Sensitivity testing. • The j roundwater 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 mc.nitoring data,while groundwater flow direction is predominantly to the south. The model lumps all sedimentary units into one laver. The revised groundwater model should include additional layers to show the effect of preferential pathways. • Whil` the groundwater model supports a finding that "...a state of equilibrium between the reehargo flux rate 6nd the biodegradation rate... ",actual monitoring well data for MW-3 in 2003-04 for Total Petroleum Hydrocarbons as gasoline(TPH-g, 1,600-2,600µg/L)shows an order of magnitude diffe ncc between groundwater monitoring data over the 2004 TPH-g equilibrium concentration,which was n odeled at 270µg/L due to biodegradation alone(Report,page 19). The groundwater model does not r resent actual site-specific field conditions or show agreement with groundwater modeling data_ A rev sed 2004 groundwater model equilibrium concentration should show reasonable aLueement with site-s ecific data from the same period of time,under actual.conditions(degradation modeled with pump ng influence from the groundwater pump and treatment system). • The Reporl on page 22 quotes the Regional Rnard's Tri-Regional Guidance Appendix A Criterion 6 for No 1?irther Action as: "The plume is stable and suficiendy limited in lateral and vertical extent and cont minant concentrations detected in groundwater show a decreasing trend with time. One hydr ilogic cycle (four quarters) of monitoring after active remediation measures have ceased is usually cons eyed to be the minimum necessary to determine site groundwater and plume conditions". The Report then states groundwater modeling using the MT313 module for contaminant "...transport shows that i he hydrocarbon contaminant concentrations will drop significantly.for the first three years fulluived by constant levels as rite plume stabilizes...Based on the results of the model, it is ATC's (con, ltant)opinion that one hydrologic cycle is not necessary and the site should be grantedNFA(no furt r Action closure)... " The Report states on page 19 that by 2015,TPH-g concentrations will reach I µg/L in MW-3, which is bolo` Water Quality Goals(50 lig/L). The Report's Fiburc 17 (attached)shows Run#2 of the MT3D groui dwater module in the year 2013. Figure 17 shows that the plume is over 400 feet long by 120 feet wide, with T?H-g concentrations of2,360µg/L in MW-1, 708 µg/L in MW-3, and 3,929µg/L in MW-4. If Fij ure 17 is a correct interpretation for M"1731D Run #2,then the results of groundwater modeling showa in Figure 17 do not show a decline in core concentrations nor predict that water quality objectives (Tas and Odor, 50}tg/L for TPH-g)will be met in a rcasonable timeframe. Figure 17 shows that there is ge' orally no decline in"rPH-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 port_ Therefore the closure request is incomplete without post remediation groundwater moni oring. 3)Additionally there are several inaccuracies listed in the Petition: • On page five of the section labeled History of Site,the Petition states: 15 Oeloher 2003 a meeting was held at the S.J County Environmental Health office to diwuss Iw. . ._..�.,.:ue:i�ti. Ilio jGll(!Yl''i}t lia]t1CijJUieQ Mt. 11C11'Itn hnod, lbir. Noel California Environmental Protection Agency Recycled Paper