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Supplemental Data for Commingled Plume Evaluation 3 ARCO#2133 and Chevron#9 4275 2905&2908 Benjamin Halt Drive Stockton California July 30 2001 Groundwater level and flow data suggest the shallow groundwater encountered at both sites is a consistent regional water table that flows from south to north under both sites with a consistent depth and hydraulic gradient. While commingling of TPHg and BTEX concentrations were not evident through review of maps and reports prepared for both sites, review of the MtBE data revealed relatively evident trends in concentrations contours over time. Review of the data suggests that following the 1995 release at ARCO, TPI-Ig and MtBE levels increased significantly in Chevron's monitoring well MW-9. Chevron's MW-9 is downgradient of the ARCO E-10 well (adjacent t6 the 1995 point of release). Figures 3 through 6 present MtBE isoconcentration contours from June 1996 through March 1997. June 1996 data (Figure 3) suggest disfinet, separate groundwater plumes of MtBE at each site. Data collected in September and December 1996, and winter 1997, (Figures 4, 5 and 6) can be interpreted to describe a single MtBE plume. Data from the four previous, consecutive quarters (Figures 7 through 10) also suggest MtBEe commingled plume is present five years following the suspected release at ARCO. CONCLUSIONS Based on SECOR's review of current and historic groundwater level, flow and analytical data at both locations, the groundwater plumes at ARCO #2133 and Chevron #9-4275 appear commingled at least with respect to MtBE concentrations in groundwater. Isoconcentration maps for MtBE (Figures 3 through 9) were prepared from data previously collected as part of the ongoing monitoring and reporting program. Data suggest a commingling of the MtBE plumes since late 1996, shortly after testing for the presence of MtBE began. Groundwater (potentiometric) maps (Figures 11 through 14)were also prepared from data previously collected as part of the ongoing monitoring and reporting program. Our interpretation suggests a relatively flat gradient, consistent flow direction, from beneath the ARCO site to the south of the study area toward the Chevron site to the north of the study area. Professional Review This document has been prepared by SECOR International Incorporated (SECOR) and has been reviewed by the Principal(s) and/or staff whose seal(s) and signature(s) appear hereon. The findings, interpretations, recommendations, specifications or professional opinions are presented, within the limits prescribed by the client, in accordance with generally lAARC0121331Misc Letters\Commingled letter Revision O.doe