SITE INFORMATION AND CORRESPONDENCE

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GEOMATRIX Mr. Robert Mihalovich Chevron Environmental Management Company July 12, 2001 Page 2 Outlined below are responses to the RWQCB comments and an evaluation of the effective- ness of an ORC pilot study. RESPONSE TO COMMENTS Comment 1: The Continued Occurrence of Natural Attenuation To date, Chevron has collected ten years (1991 through 200 1) of groundwater monitoring data from the site. These monitoring data indicate stable hydraulic conditions (groundwater eleva- tions and flow directions) at the site. The analytical data consistently show declining concen- tration trends of petroleum hydrocarbons, demonstrating a stable and continuously shrinking plume at the site. Benzene concentrations of groundwater samples collected from the wells have declined one to three orders of magnitude since 1991 (Figure 1). In addition to groundwater monitoring for petroleum hydrocarbons, Chevron has been col- lecting intrinsic bioremediation data from the site since 1995 (Geomatrix, 2000; Table 2 and Figure 5). These data consistently indicate the depletion of electron acceptors (dissolved oxygen, nitrate, and sulfate), the accumulation of metabolic byproducts of intermediates (dis- solved iron and alkalinity), lower pH, and reducing conditions in groundwater where benzene, toluene, ethylbenzene, and xylenes (collectively referred to as BTEX) are detected. These results are evidence of the continued intrinsic bioremediation of dissolved petroleum hydro- carbons in the saturated zone at the site. NA is the most technically and economically feasible remediation alternative for the site (Geomatrix, 2000) and it is estimated that the continued presence of NA will result in attain- ment of current water quality objectives (WQOs) in less than 20 years with no additional active remedial action necessary. The continued monitoring of NA parameters would not provide significant information regarding site conditions and would result in additional expenditures for Chevron(the estimated difference in contracted costs between NA and monitored NA is $65,000 [Geomatrix, 2000]). Based on this information,NA is recom- mended to be the remedial action for the site. Response to Comment 2: Estimated Rate of Declining Benzene Concentrations The rate of declining benzene concentrations at the site, which ranged from 0.1 to 0.29 per- cent, was estimated based on a first-order process (Geomatrix, 2000,p. 7). Researchers have shown that NA of BTEX approximates a first-order process. In a first-order process, the reaction rate (i.e., the rate of BTEX degradation via NA) is directly proportional to the first power of the reactant(i.e., BTEX) concentrations (Metcalf& Eddy, 1979).a A first-order a Metcalf&Eddy, 1979, Wastewater Engineering: Treatment, Disposal, Reuse,2nd Edition,McGraw-Hill,Inc.