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Geosyntec ° 3043 Gold Canal Drive,Suite 100 <br /> Rancho Cordova,CA 95670 <br /> PI1(916)637-8048 <br /> consultants www.geosyntec.com <br /> Site Characterization Work Plan Third Addendum <br /> Date: 8 January 2019 <br /> To: Sid Sewalia, Central Valley Regional Water Quality Control Board <br /> From: Nicole Gotberg P.G., Geosyntec Consultants Inc. <br /> Scott Forbess P.E., Geosyntec Consultants, Inc. <br /> Subject: Site Characterization Work Plan Third Addendum, Former Occidental <br /> Chemical Corporation—Lathrop Facility <br /> Geosyntec Consultants, Inc. (Geosyntec) has prepared this Third Addendum to the Site <br /> Characterization Work Plan (Work Plan)on behalf of Glenn Springs Holdings, Inc (GSH), for the <br /> Former Occidental Chemical Corporation (OCC) Lathrop Facility (Site) in Lathrop, California. <br /> The Work Plan was submitted on 22 June 2018 and approved by the Central Valley Regional <br /> Water Quality Control Board (CVRWQCB) on 27 June 2018. Fieldwork for the Site <br /> Characterization work was initiated on 24 July 2018. An Addendum to the Work Plan was <br /> submitted to the CVRWQCB on 10 September 2018 documenting changes to the monitoring well <br /> design and proposing two additional borehole/monitoring well locations (Location 9 and 10). The <br /> CVRWCB approved the Addendum on 25 September 2018. A Second Addendum was submitted <br /> to the CVRWQCB on 2 November 2018 proposing additional boreholes/monitoring wells at PW2, <br /> PW35 and PW36. This Third Addendum proposes one additional borehole/monitoring well at <br /> PW 16. <br /> One additional borehole/monitoring well is proposed to be installed between the Site and Lathrop <br /> Water District(LWD)water supply well LWD-6 at monitoring well clusters PW 16(Figure 1).The <br /> existing deepest monitoring well at the PW 16 monitoring well clusters above the Corcoran Clay <br /> is screened to 213 feet below ground surface (bgs).The Corcoran Clay was encountered at a depth <br /> of 279 feet bgs at Location 9. There may be up to a 65-foot-thick transmissive zone above the <br /> Corcoran Clay that is not monitored by the existing monitoring wells at the PW 16 monitoring well <br /> cluster. We are proposing to drill a borehole near PW 16 and, if the Corcoran Clay is encountered <br /> at a depth of 223 feet bgs or deeper, install a new monitoring well in the PW 16 monitoring well <br /> cluster screened immediately above the Corcoran Clay. <br /> engineers I scientists I innovators <br />