Laserfiche WebLink
ARCADIS Mr. Eric Rapport <br /> April 26, 2011 <br /> the most recent observed peak in ethanol and benzene concentrations. Half-lives of <br /> 2 days (ethanol) and 5.4 days (benzene)were calculated (ARCADIS 2010). Peak <br /> concentrations similar to previous seasonal high concentrations for ethanol and <br /> benzene were not observed in April 2010, likely due to an early rainy season that <br /> caused an earlier concentration peak. <br /> USEPA guidance (USEPA 2002) also clarifies appropriate uses of the different types <br /> of rate constants for evaluation of MNA.While bulk attenuation rates derived from <br /> concentration versus distance plots can be used to evaluate attenuation of the <br /> overall groundwater plume, they cannot be used to predict how long contamination <br /> will persist into the future (USEPA 2002). The half-lives calculated for the Site (based <br /> on concentration versus distance)therefore are not appropriate for predicting ethanol <br /> and benzene concentrations at the monitoring wells in 2, 5, and 10 years. An <br /> alternative means for evaluating degradation of ethanol and benzene at the site is <br /> proposed in the following response. <br /> RWQCB Comment: Concentrations of EtOH and Benzene, in 2, 5, and 10 years <br /> from now, same wells, that would indicate an anomaly relative to predicted half-lives. <br /> Response: As described above, half-lives appropriate for the Site (based on analysis <br /> of concentration versus distance) cannot be used to predict concentrations at a <br /> specific point time in the future. <br /> To evaluate ongoing attenuation of ethanol and benzene at the Site, a program for <br /> periodically evaluating site-specific half-lives for ethanol and benzene is proposed. In <br /> 2, 5, and 10 years, concentration versus distance half-lives for ethanol and benzene <br /> is proposed to be calculated based on the measured annual peak concentrations for <br /> the previous years. For clarity, the proposed evaluation years will be 2013, 2016, and <br /> 2021. <br /> The year 2 evaluation is proposed to consider annual peak ethanol and benzene <br /> concentrations for years 1 and 2 (years 2012 and 2013); the year 5 evaluation is <br /> proposed to consider annual peak concentrations for years 3, 4, and 5 (years 2014 <br /> through 2016); and the year 10 evaluation is proposed to consider annual peak <br /> concentrations for years 6 through 10 (years 2017 through 2021). The results of the <br /> evaluation shall be included with the results of MNA monitoring in the semiannual <br /> groundwater monitoring report following the observance of peak concentrations. <br /> As part of the half-life evaluation, the calculated half-lives will be compared to <br /> previously calculated values for the site, including those calculated based on the <br /> April 2009 data. If the half-life for a given year is more than twice as large as the <br /> \\Arcadis-us\officedata\Folsom-CA\Aproject\UPRR\RC000738-UPRR-Storrie\Deliverables\MNA-Implementation Plan RTC\Storrie_MNA- Page' <br /> IP_RTC(4-26-11).docx 3/5 <br />