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t August 24, 2001 Mr.Mike Bowles Pilkington North America 500 East Louise Avenue Lathrop,CA 95330 Subject: Addendum to Work Plan for Installation of Monitoring Wells CVRWQCB Waste Discharge Requirement Order No. 5-01-114 Pilkington North America,Lathrop, San Joaquin County Dear Mr. Bowles: We received a copy of the Conditional Approval of Groundwater Monitoring Workplan, Pilkington North America, Lathrop, San Joaquin County (Conditional Approval), and drafted the following response. The Conditional Approval includes four comments by Mr.O'Brien of the Central Valley Regional Water Quality Control Board (CVRWQCB), with the requirement that each of the comments be addressed in an addendum work plan. Response to the first comment: The proposed monitoring well locations were determined assuming an average northerly gradient. This was based in part on our experience at the site related to the diesel spill investigation south of the cullet tunnel pumping and from information obtained by telephone conversations with groundwater consultants working on the groundwater plume associated with the Simplot property adjacent to the facility to the southeast (Tom Prevost of ConestogaRovers & Associates in Stockton and Paul Horton of The Source Group in Walnut Creek). Mr. Horton maintains a groundwater model of the area and indicated to Condor that the general gradient in the near surface aquifer of that area varies in direction seasonally, ranging from northwest to northeast depending on pumping from the City of Lathrop wells to the northeast. From this information, Condor placed the wells in the center of the northern extent of the discharge areas. These locations are optimal for remaining down gradient of discharge areas regardless of the azimuth of groundwater flow at different times of the year. From the same information, the well south of Site A was placed to monitor up-gradient groundwater flowing into the discharge area. This well would also monitor any water migrating southward from groundwater mounded beneath the Site A application area. Response to the second comment: The boreholes will be logged by evaluation of drill cuttings. In addition, at the request of the CVRWQCB, a split-spoon sampler will be used every five feet to augment logging with the drill cuttings.