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' SECOR <br /> i <br /> i I <br /> estimated for TPHg, it is reasonable to assume that TPHg,has also biodegraded to a negligible level: i <br /> This is confirmed by the fact that TPHg was no longer detected in onsite groundwater since 2001. i <br /> 7.2 GROUNDWATER ! <br /> The purpose of Tier 2 RBCA for groundwater is to address the benzene exceedance in groundwater,in II <br /> the Tier 1 RBCA. # <br /> f <br /> Biodegradation Rate of Benzene in Groundwater <br /> Although groundwater is not a potential direct contact exposure route, some projections were performed <br /> r <br /> in order to satisfy the second reporting criterion for an NFAR`for groundwater that exceeds.water quality <br /> objectives, as in the case of benzene at this Site. This includes estimating when the water quality <br /> objective for benzene.is predicted to be achieved (RWQCB, 2004a). By rearranging Equation 1 and' i <br /> solving for t,the following equation is obtained. <br /> ln(S I <br /> t = _ (Eq. 2) <br /> x <br /> i <br /> Where: <br /> t = Time required to reach SL, which is MCL of.1 :pg/L for benzene; <br /> SL = Screening level,which is MCL of.1 pg/L for benzene; <br /> C = Current RME EPC of benzene(pg/L), which is 2.12 pg/L; <br /> A - First-order decay constant(years'); equals 0.693/t„2, where t112 is the half-life in years <br /> Equation 2, along with published groundwater half-life of benzene from Howard et at. (199 1)were used to <br /> calculate the time required for benzene to reach its drinking water SL of 1 pg/L. <br /> Table 5 shows the modeling results, which indicate that based on the low half-life value (fast <br /> biodegradation, least conservative scenario), benzene is expected to reach its drinking water SL of 1 pg/L <br /> within 0.2 year, respectively. Based on the high half-life value (slow biodegradation, more conservative <br /> scenario), benzene is expected to reach its drinking water SL of 1 pg/L within 1.1 year. In order to <br /> evaluate the biodegradation rate specific to this Site, these modeling results based on literature <br /> biodegradation rates can be verified at a later date using the actual monitoring data. This was done by <br /> i <br /> rearranging Equation 1 and solving for A. <br /> i <br /> i <br /> In(Ciz ) <br /> C� (Eq. 3) i <br /> . t <br /> 16 <br /> i <br />