SITE INFORMATION AND CORRESPONDENCE

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Mr. W. T. Nickerson - 2 - 12 December 2000 The graphs depicting four-point moving averages for various constituents clearly show that most constituents are present in greatest concentrations in MW-6 and MW-8 which are the proximate downgradient wells from the northern former evaporation pond (northern pond), and present in lesser concentrations in MW-2 which is at the northern pond. These constituents include ethylene dibromide .(EDB), chloroform, 1,2-DCP, and nitrate. This suggests that the center of mass of the plume has migrated from the northern pond to the vicinity of MW-6 and MW-8. The graphical averages also show that after the October 1998 installation of caps over the former ponds (northern and southern ponds), concentrations of 1,2-DCP, 1,2,3-trichloropropane, nitrogen, chloroform, and EDB decreased significantly in MW-2, MW-6, and MW-8. To date, there is not a corresponding reduction in constituent concentrations in MW-5, which is downgradient of the southern pond. Table 3 of the first and third QMRs list MW-12 as containing a nitrate concentration of 223.5 mg/l in March 1999. The correct value should be 23.5 mg/l, as presented in the 1999 QMRs. DRAFT REVISED MONITORING AND REPORTING PROGRAM On behalf of UNOCAL, Sierra-Pacific proposes to reduce the frequency of groundwater elevation monitoring from quarterly to semi-annually, remove MW-6 from the monitoring schedule, and reduce monitoring in the deep monitoring well MW-7 and in upgradient wells MW-1 and MW-3 from semi- .annually to annually. It also proposes that analyses for chlorinated herbicides be removed from the schedule for the furthest downgradient wells MW-9 and MW-12 until they are detected in the mid- gradient well MW-5, and that the total Kjeldahl nitrogen analyses be replaced with ammonium analyses. Table 1 summarizes UNOCAL's proposal and Board staff's concurrence or counter-proposal as discussed in more detail below. Table 1. Proposed Changes to MRP No. 96-816 , Western Farm Service, Stockton Element of Change UNOCAL's Proposal Board Staff's Proposal TKN analyses Change to ammonium Concur Remove from schedule MW-6 Exchange MW-5 for MW-6 Reduce monitoring frequency from Groundwater elevation monitoring Concur quarterly to semi-annually Reduce monitoring frequency from MW-1, MW-3, MW-7 Concur. Add MW-6 semi-annually to annually Reduce EPA 8150 (chlorinated Analyze in MW-9 and MW-12, Analyze annually in MW-9 until herbicides) analyses contingent on detection in MW-5 detected. Then analyze semi- annually in MW-9 and MW-12. As Mr. Chase stated in his 28 July letter, Board staff provided verbal approval in 1997 to UNOCAL to substitute ammonium analyses for TKN. This change is formalized in the draft revised MRP. We concur with the frequency reduction for quarterly groundwater monitoring, and reducing monitoring frequency for the deep well MW-7 and upgradient wells MW-1 and MW-3 to annually in the fall. The monitoring history for MW-1 and MW-3 shows small seasonal variations in nitrate concentrations with the maximum occurring in the fall. MW-10 is further upgradient than MW-1 and MW-3, shows greater seasonal variations than the other two wells, and will continue to be monitored semi-annually. MW-7