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NNE ) Conceptual Site Model and MRP Status The Conceptual Site Model (CSM) report was developed in response to a letter from the CVWB to Olin dated June 20,2017 (CVWB,2017a).The CSM recommended continued monitored natural attenuation (MNA)and modifications to the MRP.The CVWB provided comments on the CSM on November 09,2017(CVWB,2017b).A response to these comments was submitted on December 12,2017 (Amec Foster Wheeler,2017b). In a letter dated July 30,2018,the CVWB found the CSM and recommended remedy acceptable. The CVWB's July 30,2018 letter also issued the Revised MRP(CVWB,2018), which incorporated MRP revisions recommended in the CSM.The CVWB modified the MRP sampling schedule via electronic mail dated August 13, 2018,confirming that no sampling was required in 2018. The CVWB requested that Olin propose changes to the MRP in the September 23,2021 Response to Comments 2027 and 2079 Biennial Monitoring and Sampling Report(CVWB,2021).Olin submitted proposed changes to the MRP in the Proposed Revisions to Monitoring and Reporting Program letter,dated December 17,2021. The Revised MRP is based on a CVWB letter dated June 2,2022, and includes 8 monitoring wells(Figures 2 and 3)sampled biennially, as summarized in Table 1.Well construction details are presented in Table 2. Monitoring and reporting activities completed during the reporting period have been conducted in accordance with the Revised MRP. Groundwater LEVELS During the biennial monitoring conducted in March of 2023,groundwater levels were measured in eight monitoring wells.Water levels were measured from the top of the well casing to the nearest 0.01-foot using an electronic water level meter.Water level measurements are presented in Table 3. Upper Horizon well MW-6 is damaged,so water was measured in nearby Upper Horizon well MW-5 as a substitute until MW-17 is installed,as described in Response to Central Valley Regional Water Quality Control Board Review of 2079 Biennial Monitoring and Sampling Report(Wood, 2019). MW-6 will be decommissioned and replaced with MW-17 pursuant to the Revised Well Decommissioning and Replacement Well Installation Work Plan (Wood,2022). Groundwater Sampling and Analyses Groundwater samples were collected from eight wells (MW-5, MW-9, MW-10, MW-11, MW-12, MW-13, MW-14,and MW-15) in accordance with the Revised MRP.Groundwater samples could not be collected from MW-6 in either event because the well had been damaged, but MW-5 was sampled as a substitute. Groundwater samples were collected using the low-flow purging and sampling technique. During the purging procedure, measurements of field parameters including temperature,electrical conductivity,and pH of the purge water were recorded on sample collection logs.Once the field parameters stabilized,a groundwater sample was collected from the monitoring well using a laboratory supplied container.Groundwater sample https://woodplc.sharepoint.com/teams/olintracy/shared documents/general/03.0 docctrl/2021 biennial report/01_text/tcy 2021 biennial_text_rev.docx Page 2