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Michael Kith [EH] From: Beatty, Jenifer <Jenifer.Beatty@arcadis.com > Sent: Monday, September 18, 2017 1:34 PM To: Michael Kith [EH] Cc: Roger Smith; Hinkley, Darcy Subject: Well Destruction Permit Application - GSH Wells on the Former Pilkington Property, Lathrop Attachments: 2005_EDB-DBCP-Sulfolane Contours.pdf; 2012_Lathrop COC Figures.pdf; 2016_Lathrop COC Figures.pdf; Lathrop Aerials.pdf Hello Michael, Thank you for your email and discussing this request last week. We have attached the additional information requested in support of our request to pressure grout the wells located on the former Pilkington property to prepare the site for development by the adjacent property owner. As requested, historical isoconcentration figures from 2005, 2012, and third quarter 2016 for the primary contaminants of concern (EDB/DBCP/Sulfolane) at the site are attached for your review. The 2005 figures were prepared by the previous consultant (CRA). Please note that Arcadis does not classify the aquifer in the same manner. CRA classified the aquifer into three zones which include the shallow (31-83 feet bgs), intermediate (84-150 feet bgs), and deep (151-218 feet bgs). Arcadis assumed responsibility for the project in 2011 and divided the aquifer into four zones which includes the water table (first encountered groundwater to 50 feet bgs), shallow (50-100 feet bgs), intermediate (100-150 feet bgs), and deep (150 to the top of the Corcoran Clay [approximately 250 to 300 feet bgs]). As we discussed there is no soil data for the area where the wells to be destroyed are located. As we have also discussed, the former source areas for groundwater contamination on the Simplot property (former Occidental property) are located on the Simplot property and are a significant distance from the wells we are proposing to destroy. The wells that are to be destroyed were installed to assess offsite migration of the groundwater plume from the Former Occidental/now Simplot property. We are not aware of any known or suspected sources of soil contamination on the Former Pilkington property. We have also attached historical aerial photographs that show that the property in question has never been developed other than for agricultural use. Although there are no Water Table wells on the former Pilkington property, COC concentrations in the Water Table on the Simplot property (see attached figures) do not extend far from the wells on the Simplot property, suggesting that the source areas in soil on the Simplot property are localized. The 2005 Shallow groundwater isoconcentration figures indicate site COCs are present on the former Pilkington property. However, this is based on wells screened from 71 to 76-feet bgs (PW08) (i.e. not Water Table), 75 to 80-feet bgs (PW09), and EW-4 which is screened from 103 to 120-feet bgs and 170 to 190-feet bgs (which is actually in the intermediate and deep zones) and not indicative of local soil contamination on the former Pilkington property. Please also note that the COC concentration contours presented in the 2016 figures are significantly lower than the contours presented in the 2012 figures. Please let us know if you would like me to arrange a call so that we can discuss this in detail. As you are aware, the RWQCB has approved the workplan to destroy the wells. Thank you, Jenifer Jenifer J. Beatty PG, CHG I Program Manager, Vice President I Jenifer.Beattyarcadis.com Arcadis 101 Creekside Ridge Court, Suite 200 Roseville CA I 95678 I USA T. +1 916 786 7971 I M. + 1 916 747 7971 1.