SITE INFORMATION AND CORRESPONDENCE

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Mr. W. T. Nickerson • . - 3 - contains nitrate-nitrogen concentrations ranging from 1.5 to 3.5 m 12 December 2000 have not been detected in the four years of monitoringhisto for MW-7 may be reduced to annually. �' Other monitored toning requ history. We concur that the monitoring frequency Sierra-Pacific proposes to remove MW-6 from the monitoring schedule. The monitoring history shows that MW-6 is at the center of mass of the plume that originated from the northern pond. Therefore, this well should remain in the monitoring network. The history also shows that in MW-6, constituent this well without loss of critical data. concentrations are maximum during the fall, so monitoring frequency may be reduced to annually for Well MW-5 is hydrologically positioned between the southern pond and MW-9. Constituents present in MW-5 are also present in MW-9, so MW-5 may be removed from the monitoring schedule at this time. The wells that will be removed from the monitoring schedule should remain available for future monitoring events. If constituent migration patterns change, or when UNOCAL implements remediation, UNOCAL may benefit from reinstating these wells in the monitoring network. Reduction of analyses for chlorinated herbicides (EPA 815 1) in the furthest downgradient wells and MW-12) is warranted, since these constituents have not yet arrived at the terminus of the plume. Sierra-Pacific proposes to initiate analyses in MW-9 and MW-12 if the constituents are detected in W 9 MW-6 or MW-5. However, these constituents were present in MW-6 during the 1999 monitoring events, and have never been detected in MW-5. Therefore, we propose that the EPA 8151 analysis be Performed annually at MW-9 in the fall, and not be required at MW-12 until chlorinated herbicides are detected in MW-9. At that time, the analytical frequency shall be semi-annually for both MW-9 and MW-12. The former evaporation ponds were capped in 1996 and a maintenance and monitoring schedule for the maintenanc pond caps is required in by the MRp. UNOCAL did not propose changes to the pond cap e schedule, and this section of the draft revised MRP remains unchanged. The draft is enclosed. Please review the enclosed draft revised MRP and provide your comments as well as a replacement site map (Figure 1) by 26 January 2000. We will then proceed with finalizing the MRP in order to have it in effect for the winter monitoring event. If you have any questions, you may contact me at IT 16)/255-3080. I AMY ERRELL Site Cleanup Unit Enclosure cc: Mr. Michael Infuma, San Joaquin County Public Health Services, Stockton Mr. Jan Wagoner, UNOCAL, Elk Grove Mr. Eric Chase, Sierra-Pacific Group, El Dorado Hills