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Mr. Jeff Baker • - 2 - 8 September 2006 <br /> r <br /> contamination are known as trigger wells. In groundwater Zone B midway between these two <br /> types of observation wells is one well used as a mid-point observation well. <br /> The ozone injection systems were brought online in August 2004. STTC anticipated <br /> groundwater contaminant concentrations to increase for up to one year during re-equilibration <br /> of the groundwater system. Benzene, toluene, ethylbenzene, and xylene (BTEX), and methyl <br /> tertiary butyl ether (MTBE) concentrations were then to be evaluated at the observation wells <br /> for three consecutive quarters from the fourth quarter 2005 to second quarter 2006. If an <br /> increase of one order of magnitude over historical concentrations were observed at the <br /> remedial observation wells or mid-point compliance well, then a contingency plan to install <br /> ozone injection wells east of those observation wells would be implemented. Also, if <br /> concentrations exceed Water Quality Objectives (WQOs) at the trigger wells, a contingency <br /> plan would be implemented to install ozone injection wells east of the trigger wells. The <br /> contingency plan is described in STTC's 27 July 2003 Response to RWQCB Letter Dated May <br /> 9, 2003 And Addendum to Corrective Action Plan Dated April 30, 2003. <br /> As of the second quarter 2006, contaminant concentrations at the observation wells have not <br /> exceeded the threshold criteria as described above. However, benzene concentrations at <br /> Zone B remedial observation well PS/WC-1M have increased by an order of magnitude each <br /> of the last three quarters from 14 micrograms per liter (Ng/L) in the fourth quarter 2005 to <br /> 1,600 pg/L in the second quarter 2006. MTBE concentrations at the well have increased from <br /> 4.1 pg/L to 74 pg/L during the same period. The historical average benzene and MTBE <br /> concentrations at this well are listed in the Second QMR as 3,167.8 pg/L and 47.69 pg/L, <br /> respectively. There have been no exceedances of WQOs at the trigger wells as of the second <br /> quarter 2006. <br /> Regional Water Board staff is concerned about the benzene concentration increase at <br /> observation well PS/WC-1 M. In the Annual Report and First QMR, STTC states that the <br /> contingency plan will be implemented when the threshold criteria have been exceeded. <br /> However, in the Second QMR STTC no longer commits to implementing the contingency plan <br /> and states that the contingency plan will be considered once the threshold criteria are <br /> exceeded. Increasing trends show that the ozone injection system is not effectively reducing <br /> benzene or MTBE concentrations near observation well PS/WC-1 M. The threshold criterion <br /> for benzene at observation well PS/WC-1 M has been exceeded as of the second quarter <br /> 2006. STTC needs to develop a work plan to remediate dissolved benzene that has migrated <br /> past the ozone injection wells as described for the contingency plan in their 27 July 2003 letter. <br /> The work plan is due to this office by 13 October 2006. <br /> If you have any question regarding this letter, you may contact me at (916) 464-4719 or by <br /> email at mclardy@waterboards.ca.gov. <br /> f' x v C76 <br /> Mark W. Clardy, P.G. <br /> Engineering Geologist <br /> Private Sites Cleanup Unit <br /> cc: see attached page <br />