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According to the California Regional Water Quality Control Board — Central Valley <br /> Region (CRWQCB, 2003), the lowest water quality goals for acenaphthene are: <br /> • the taste and odor limitz of 20 pg/L, and <br /> • the reference dose as drinking water leve 13 of 420 pg/L. <br /> To date, the highest acenaphthene concentration detected in the groundwater samples <br /> collected from the Site was 140 pg/L (MW-23R) during the March 15, 2001 event. <br /> Acenaphthene concentrations in MW-23R were most recently detected at 110 pg/L <br /> during the September 15, 2004, sampling event. Although the groundwater in the area <br /> of the Site is not used for drinking water due to elevated TDS concentrations (as a result <br /> of saltwater intrusion) unrelated to the Site, an acenaphthene concentration of 20 pg/L <br /> is used for evaluation purposes in this assessment of groundwater conditions in the <br /> vicinity of MW-26R. <br /> Based on the most recent (September 2004) groundwater monitoring data (ENV <br /> America, 2005b), the estimated extents of acenaphthene concentrations exceeding 20 <br /> pg/L in groundwater in the A and B zones at the Site are shown on Figure 3. <br /> Acenaphthene concentrations in the groundwater samples collected from the C-zone <br /> wells in September 2004 (ENV America, 2005b) did not exceed 20 pg/L. The <br /> acenaphthene concentrations in groundwater in the A, B, and C zones at the Site are <br /> summarized below. <br /> A Zone <br /> The extent of acenaphthene concentrations exceeding 20 pg/L in groundwater is limited <br /> to the vicinity of wells MW-9R and MW-8 (Figure 3). The acenaphthene concentrations <br /> in the groundwater samples collected from well MW-7, which is located less than 80 feet <br /> downgradient from MW-8, are less than 20 pg/L (Exhibit C of the Third Quarter 2004 <br /> Groundwater Monitoring Report, ENV America, 2005b). <br /> B Zone <br /> Acenaphthene concentrations of the groundwater samples collected from 5 of 8 B-zone <br /> wells in September 2004 exceeded 20 pg/L Figure 3). The highest acenaphthene <br /> concentration in groundwater beneath Area I (Figure 2) was 41 pg/L (MW-19R). <br /> Acenaphthene concentrations in the groundwater samples collected from monitoring <br /> 2 USEPA National Recommended Ambient Water Quality Criteria for Human Health and Welfare <br /> Protection <br /> a USEPA Integrated Risk Information System (IRIS) <br /> 8 ENV,^: <br /> A M E R I C A <br /> P:\PG&BPGE-05-01 Stockton MW26R Investigation\MW26 Work Plan\PGE Stockton MW26R WP.4oc <br />