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This section presents a summary of the concentration trends for VOCs detected in the Upper, Middle,and Lower Horizons.The primary VOCs of interest in all three horizons are carbon tetrachloride and chloroform.Target cleanup goals are the California Drinking Water Program maximum contaminant levels (CA MCLs) of 80 pg/L for total trihalomethanes (chloroform as an individual compound does not have a primary or secondary MCL) and 0.5 pg/L for carbon tetrachloride (California Environmental Protection Agency,2018). Upper Horizon Carbon tetrachloride concentrations in March 2023 are within historical ranges and below the MCL.Carbon tetrachloride concentrations have been generally stable or declining in the Upper Horizon wells since 2014 (see Figure 10). Chloroform concentrations in the Upper Horizon are generally stable to decreasing compared to historical values,with the exception of MW-9. Although chloroform concentrations measured in MW-9 have increased in the previous two consecutive sampling events,they remain below the 80 pg/L MCL.Chloroform concentration trends in the Upper Horizon wells are presented on Figure 15. Middle Horizon Carbon tetrachloride concentrations are generally declining,stable,or not detected above laboratory reporting limits in the Middle Horizon wells since 2005 (Figure 11).The highest concentrations of carbon tetrachloride occur in well MW-12,which is located over 2,000 feet down gradient of the site.The carbon tetrachloride concentration detected in MW-12 in 2023 represents the lowest detection since 2013,and a generally decreasing trend since 2015. Chloroform concentrations in the Middle Horizon are generally decreasing or stable compared to historical values (Figure 16).The chloroform concentration detected in well MW-12 in 2023 represents the lowest concentration yet detected in that well,consistent with the long-term decreasing trend. Lower Horizon Carbon tetrachloride concentrations have been stable or declining in Lower Horizon wells since 2005(Figure 12).Carbon tetrachloride was not detected in Lower Horizon wells sampled in March 2023. The chloroform concentration detected in MW-15 in March 2023 represents a local increase in concentration; however,this detection remains within historic range and well below the MCL(Figure 17). Laboratory Quality Assurance/Quality Control All groundwater samples were received by the analytical laboratory on ice and intact.All samples were analyzed within the method holding times. Based on the laboratory QA/QC results,the March 2021 data arejudged to be valid for their intended use. Field Quality Assurance/Quality Control https://woodplc.sharepoint.com/teams/olintracy/shared documents/general/03.0 docctrl/2021 biennial report/01_text/tcy 2021 biennial_text_rev.docx Page 6