ARCHIVED REPORTS_2010_2011 ANNUAL MONITORING REPORT

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ARCADIS Mr. Eric Rapport April 26, 2011 RWQCB Comment:EtOH and Benzene, and static water levels over time (same plot, with two dependant variables), monitoring wells MW-01, MW-03A, MW-03B, MW-04B, MW-05A, MW-06, MW-07A, MW-07B, and MW-08 through MW-13. Response: Plots of ethanol, benzene, and static water levels for monitoring wells MW-01, MW-03A, MW-03B, MW-04B, MW-05A, MW-06, MW-07A, MW-07B, MW-9, MW-11, MW-12, and MW-13 are presented in Figures 1 through 12. The plotted data include results from the most recent groundwater sampling event(January 2011). Concentrations of ethanol and benzene in MW-08 and MW-10 have been below detection limits for their entire monitoring history, so plots were not prepared for these two wells. RWQCB Comment: Predicted EtOH and Benzene, in 2, 5, and 10 years from now, same wells, based on predicted half-lives. Goodness of fit with either all, or de- seasonalized data, as appropriate. Response: As background information, please note USEPA guidance (USEPA 2002) outlines three main types of first-order attenuation rates for MNA evaluations: concentration versus time-rate constants, concentration versus distance bulk- attenuation rate constants, and biodegradation rate constants (which are used in solute transport models). Site-specific conditions determine the applicability of each type of constant (USEPA 2002). Each constant can be alternatively expressed as a half-life. Concentration-versus-time rate constants provide an estimate of the source attenuation rate. That method can't be applied at this site due to the extreme seasonal and annual variations in ethanol and benzene concentrations (i.e. no constant time-rate degradation). As discussed in the Hydrogeologic Evaluation Summary and Proposed Remedial Alternative, prepared by ARCADIS and dated December 21, 2010 (ARCADIS 2010), groundwater concentrations at the Site show significant seasonal variation, with a peak in ethanol and benzene concentrations typically observed during February through April. The peak concentration observed in any given year is likely related to the amount of rainwater infiltrating into the source area during the wet season. These variations in ethanol and benzene concentration are evident in Figures 1 through 12. Concentration-versus-distance rate constants provide an estimate of the attenuation that occurs downgradient from the source zone, along the dissolved-phase plume flow path. This method is applicable for the Site using the observed annual concentration maxima of ethanol and benzene. These first-order rate constants were calculated for ethanol and benzene using data from April 2009, the sampling event of \\Arcadis-us\officedata\Folsom-CA\Aproject\UPRR\RC000738-UPRR-Storrie\Deliverables\MNA-Implementation Plan RTC\Storrie_MNA- Page' IP_RTC(4-26-11).docx 215