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Addendum to the Well Installation and
<br /> Hydraulic Testing Report and Additional
<br /> ' ARCADIS Well Installation Work Plan
<br /> Former Occidental Chemical Corporation
<br /> Lathrop, California
<br /> 1. Introduction
<br /> On behalf of Glenn Springs Holdings, Inc. (GSH),ARCADIS U.S., Inc. (ARCADIS)submitted a Well
<br /> Installation and Hydraulic Testing Report and Additional Well Installation Work.Plan(Report and Work
<br /> Plan)to the California Regional Water Quality Control Board—Central Valley Region (RWQCB)on
<br /> February 28,2012(ARCADIS, 2012).The RWQCB responded to the Report and Work Plan with a
<br /> letter dated August 6, 2012, requesting additional discussion and technical rationale(the RWQCB letter;
<br /> RWQCB, 2012).ARCADIS has prepared this Addendum to the Well Installation and Hydraulic Testing
<br /> Report and Additional Well Installation Work Plan(Addendum) in response to the RWQCB's request.
<br /> The Report and Work Plan and this Addendum pertain to the former Occidental Chemical Corporation
<br /> (OCC) manufacturing facility(Site)located at 16777 South Howland Road near the southwest corner of
<br /> Louise Avenue and South Howland Road in Lathrop, San Joaquin County, California(Figures 1 and 2).
<br /> Site contaminants of concern (COCs) include 2,3,4,5-tetrahydrothiophene-1,1-dioxide(sulfolane), 1,2-
<br /> dibromo-3-chloropropane(DBCP) and ethylene dibromide(EDB).A copy of the RWQCB letter is
<br /> provided in Appendix A.
<br /> The overall objective of the scope of the work presented in the Report and Work Plan and this
<br /> Addendum is to expedite the removal of COC-affected groundwater at the Site. COC contamination is
<br /> present in the water table zone(defined as first encountered groundwater, approximately 10 to 15 feet
<br /> below ground surface[ft bgs],to approximately 50 ft bgs), shallow zone(50 to 100 ft bgs), intermediate
<br /> zone(100 to 150 ft bgs)and deep zone(150 ft to the top of the Corcoran clay, approximately 200 ft
<br /> bgs).To meet this objective,ARCADIS proposes to expand the current on-site extraction and injection
<br /> well network using a phased approach.As the first phase of work, in December 2011,ARCADIS
<br /> installed extraction well EW-09 and three permanent monitoring wells(PW27-40, PW28-40, PW29-40).
<br /> EW-09 was placed in the area where the highest sulfolane concentrations were detected in soil,
<br /> following investigation and excavation of approximately 2,093 cubic yards(3,138 tons)of sulfolane-
<br /> affected soil(ARCADIS, 2011).
<br /> ARCADIS then conducted hydraulic testing at EW-09 between January 9 and January 13, 2012.
<br /> Pressure transducers were placed in pumping well EW-09 and observation wells PW27-40, PW28-40,
<br /> PW29-40, PW26-86, PW26-154, PW26-199, and PW18-199.The objective of the hydraulic testing was
<br /> to collect data within the water table zone to support the design basis for a groundwater remedy within
<br /> the sulfolane investigation and treatment area(ARCADIS, 2012).
<br /> Based on hydraulic testing results and groundwater model simulations that were presented to the
<br /> RWQCB on October 6, 2011,ARCADIS proposed to install the next phase of extraction and injection
<br /> wells, including ten extraction wells EW-10, 11, 12A, 12B, 13, 14A, 14B, 15A, 15B and 16 and four
<br /> injection wells INJ-11, INJ-12, INJ-13/EW-03 and INJ-14/EW-04(ARCADIS, 2012). Subsequently,
<br /> 2012-09-13 addendum well install work plan_hy test rpt.dm 1
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