SITE INFORMATION AND CORRESPONDENCE

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Mr. Jim Barton • • 6421 W. Capitol Avenue Closure Consideration Page 3 of 3 The long-term importance of the MTBE impact to MW-8A is not clear to EHD. For a two-year period, while GWE was in operation, groundwater samples from the well were ND for all analytes of concern, after which time until present they have been continuously impacted by MTBE in the mid-hundreds ppb. It is not known if first water above MW-8A is impacted. MW-19B and MW-20B toward and near New West Shell were initially impacted but their MTBE concentrations have rapidly declined. Possible reason: differing hydraulic conductivities in the area proximal to the New West area and the MW-8A area and response to the reestablished `natural' gradient. EHD recommends that case closure consideration be delayed until adequate data has been collected to confidently address the concerns raised herein with the MW-8A and MW-2 data. Recommended actions to take to resolve the issues include: • Collection of additional data from the groundwater extraction wells and selected monitoring wells to assess for a significant rebound in the pumping area and compare to MW-2 data • Collection of grab water samples or new monitoring well samples (preferred) installed a short distance down-gradient of MW-2 and MW-8A to test EHD's hypothesis regarding MW-2 and to assess for the presence of a plume extension from the NWS site to MW-8 that is potentially migrating in an easterly direction under the current groundwater flow conditions. • Monitoring the site concurrently with the New West Shell to have more time- correlative data to expand the relative database and address potential issues with the latter site. • Checking monitoring wells on both sites with level-loggers and record pumping times at the CSA wells to check for an indication of a pumping influence from the CSA wells. The delay of case closure will provide EHD, and hopefully AGE and Apex,time to reevaluate the pumping test data from both sites and calculate hydraulic properties of the tested zones and calculate seepage velocity and possibly the distance traveled by the plume in various plume geometries. It would be hoped that such reviews will lead to valid and testable technical models or explanations for the MW-8A and MW-2 data. Questions or comments may be directed to the undersigned at(209)468-3436 or by mail (address on the letterhead). Donna Heran,REHS, Director EnvironmenHealth Department Nuel C.C. Henderson, Jr., PG Marg�rioVS Engineering Geologist,Unit IV Supervisor,Unit IV