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c <br /> ST Services Stockton Termin, - 4 - 26 March 2008 <br /> Mr. Joe Aldridge <br /> sulfide in the hydrocarbon plume emanating from the Site. Therefore, we cannot concur <br /> that the decreasing petroleum hydrocarbon concentrations observed in downgradient wells <br /> is occurring as the result of the biodegradation by the sulfate reduction metabolic pathway. <br /> 5. NuStar's regression model for PS/MW-15, which estimates that TPHg can be cleaned up <br /> within 80 years, indicates that MNA is not an appropriate cleanup choice for this Site. A <br /> more effective option which deserves further consideration was simulated using <br /> MODFLOW in NuStar's 5 July 2007 Addendum to Cleanup Plan. The MODFLOW <br /> simulation showed that cleanup of the Site can be accomplished in 30 to 50 years by <br /> extracting and treating groundwater from 16 wells at a sustainable flow rate of 0.013 <br /> gallons per minute (gpm). Alternatively, the model also showed that site cleanup would <br /> take as much as 180 years if the extraction rate was increased to 3 (gpm) because most <br /> the groundwater would be extracted from water bearing strata underlying the most highly <br /> impacted strata. The model assumed that the source area contains about 1,000,000 <br /> gallons of polluted groundwater. The data presented in the analyses continue to show that <br /> MNA is not an effective cleanup option for this Site and the additional year of analysis <br /> requested by NuStar is unwarranted. In addition, since the Report states that a <br /> contingency plan for active remediation could be implemented in two to three months, it is <br /> unclear why this cannot be implemented now. <br /> 6. SPH has not been observed in ST/MW-1 since August 2005. However, the latest benzene <br /> concentration of 22,000 pg/L exceeds 1% of its pure phase solubility of 1,750 mg/L <br /> indicating SPH persists even though it is not observable in the well. This confirms the <br /> absence of degradation in this source area well. <br /> 7. The Annual Report documents that UP/MW-1 has not been sampled since January 2007, <br /> although MRP No. R5-2007-0818 requires quarterly monitoring. NuStar needs to provide <br /> an explanation for excluding this well from the sampling schedule and must perform the <br /> required sampling in this well in the future. <br /> 8. We do not concur that an additional B-zone well is necessary However, we were unable to <br /> verify that the vertical extent of the plume has been delineated, particularly in the source <br /> area. Additional groundwater monitoring well installation or hydropunch sampling is <br /> needed to define the plume before groundwater remediation can be accomplished <br /> effectively. Due to high A- and B- Zone hydrocarbon concentrations, this investigation <br /> must be conducted using drilling methods that would eliminate cross contamination of the <br /> lower zones. <br /> By 28 April 2008, please submit a groundwater investigation work plan to delineate the vertical <br /> extent of the plume in the source area. If you have any questions regarding this letter, you may <br /> contact me at (916) 464-4811 or betaylor(a)waterboards.ca.gov. <br /> IAN TAYLOR, P.G. <br /> Engineering Geologist <br /> cc list on next page <br />