SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Mr. Jeff Baker -4 - ' 15 December 2008 Stockton Terminals Technica*mmittee On 3 December,.Stantec responded via email saying that their interpretation of the Regional Water Boards strategy for developing threshold concentrations for TBA for RO wells PS/MW-18 and PS/WC-1M was to calculate historical means from data dating back to February 2001. This explained the discrepancy between the Regional Water Board staff's and Stantec's threshold concentrations. Regional Water Board staff wrote back to Stantec on 3 December and explained that our 30 September 2008 email had determined that, since TBA is not presently included in the list of trigger constituents, the TBA threshold concentrations would be calculated using a median concentration of all historical data. Regional Water Board staff met with STTC representatives on 10 December. During the meeting the parties mutually agreed to a strategy for determining an historical mean for PS/MW-18 and PS/WC-1 M. The monitoring data between 2001 and 2008 would be randomly replicated to simulate a much larger normally distributed data set and evaluated for a mean concentration using a frequency histogram. The historical average concentration from this algorithm would serve as the threshold concentration for the two wells. Using this procedure, Regional Water Board staff calculated that the mean concentrations for PS/MW-18 and PS/WC-1 M were 14 pg/L and 134 pg/L, respectively. The calculations were sent via email to Stantec on 12 December 2008 for incorporation into the revised contingency plan. During the 10 December meeting, STTC also presented preliminary data and results from the PS/MW-20A through PS/MW-20D, OW-3CR, OW-3DR, and TS/MW-3D well installations. However, STTC requested to extend the 15 December 2008 due date for the TS/MW-3D pilot study work plan for remedial testing of the C zone and D zones to 27 January 2009. They said the well installation report for the installed wells would be submitted by 15 December, as planned. Our comments are presented below. 1. Regional Water Board staff concurs with the STTC's request that TBA be excluded from the list of trigger well constituents. The existing plan provides for installation and operation of a second ozone sparge array in the event that this constituent migrates beyond the ozone sparge system in excess of threshold concentrations. The threshold concentrations will be determined using the methodology discussed during the 12 December meeting. The resolution of these final issues will allow STTC to prepare the revised Contingency Plan. Please submit the revised plan by 27 January 2009. 2. We concur with STTC's request to submit the TS/MW-3D pilot study work plan by 27 January 2009 and the Well Installation Report by 15 December 20008. 3. As seen in the groundwater elevation maps in the Third QMR, the site-wide resurvey of the well casing elevations has eliminated the apparent groundwater mounds and sinks historically present in the vicinity of PS/WC-1 M, PS/WM-20, and OW-46. This result correctly shows that the anomalous elevations calculated in previous reports resulted from erroneous elevation data instead of actual groundwater conditions.