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SECOR I�=RNATIQNAL INCORPORATED www.secorcom I' S E C 0 R in" e m o r a n d u m 14998 W.6 Avenue Suite 800 Golden,Co 80401 303.278.7878—phone 303.278.7866 fax to: Melissa Green from: Sonya Cadle re: Technical summary of modeling activities, BP P RCO#4493, Stockton, CA date: October 13, 2006 I have examined the data for the above-referenced site with respect to groundwater concentrations of gasoline range organics (GRO), benzene, and methyl tertiary-butyl ether 19 (MTBE). The following analyses have been performed: • Concentration trends for GRO, benzene, and N TBE in groundwater for wells MW-4, MW-5, MW-8M, MW-81D, and MW-9M were plo ted. These wells are located at or near the plume centerline and so can be'considered representative of the maximum plume .. , concentrations for the constituents in question. The resulting concentration graphs with plotted exponential trendlines generated by Microsoft Excel are included electronically with this memo. Boring logs and cross-section data were examined along the plume centerline to determine geologic influences on the plume. • Relative solubilities and estimated remaining mass of the constituents were compared between the constituents. Several important points may be distilled from the analyses performed. First, the concentration trendlines indicate that all three constituents are decreasing in concentration in each of the wells considered. This indicates that the plume is shrinking, and source concentrations are decreasing. The rate of decrease varies by constituent Benzene and MTBE are decreasing '- more quickly, and can be expected to reach groundwater standards for all the wells considered within approximately the next 10 years, provided that current trends of decreasing concentration continue. In many of the wells, either one or both of these compounds is already below standards. Concentrations of GRO are also on a decreasing trend, but are not decreasing as quickly as benzene and MTBE. There are likely several reasons or the slower rate of decrease. GRO is not a single compound, but is comprised of many com ounds mixed together. Each compound has a different solubility and different partitioning coeffi 3ients for soil, water, and vapor phases. The more soluble and more volatile compounds will "st-ip" out of the GRO mixture preferentially into the soil gas and groundwater, leaving less soluble and less volatile compounds behind. Thus, initial decreases in GRO concentration will be more rapid than later concentration changes. So over time, there will be less GRO present, but the remaining amount will also I:1BP-ARC0104 BP Valley Portfolio144931Reports\NFARIAddendumlCOC ry odeling tech memo.doc ! 1 of 2