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Supplementary Site Assessment Report Former Mobil Bulk Plant 04-343 April 26, 1996 ' • Groundwater elevation has remained relatively constant at approximately 20 feet above mean sea level [NGVD-1929]; approximately 11 fbg. The gradient is generally to the north ' at 0.01 foot-per-foot. • During the past three quarters, no benzene has been detected in any well above Title 22 ' of the California Code of Regulations maximum concentration limit (MCL) for drinking water (1 ppb). Dissolved-phase TPH-G has been detected only in AW-3, at concentrations less than 100 ppb. Low to moderate concentrations of TPH-D have been detected in AW-1 ' through AW-5, and AW-12 during the past three quarters (Table 2)_ • The results of the background groundwater analysis are summarized in Table 3 and ' discussed below in Section 5.2.1 5.2 BIOREMEDIATION PARAMETER STUDY ' The purpose of this study is to evaluate the feasibility of enhancing biodegradation radation of hydrocarbon-affected soil and groundwater using an oxygen-releasing compound. Hydrocarbon- degrading microorganisms are widespread in the environment and occur in fresh and salt water, soil, and groundwater. Bacteria are the most numerous and biochemically active group. There exist hundreds of known hydrocarbon-degrading bacteria. Bacteria require a carbon source for ' cell growth and an oxygen source to sustain metabolic functions required for growth. Nutrients, including nitrogen and phosphorous are also required for cell growth. Bacteria that use organic ' compounds such as petroleum as their source of carbon are heterotrophic, and those that use oxygen are aerobic (EPA, 1995). Hydrocarbon degradation is essentially an oxidation-reduction reaction where the hydrocarbon ' is oxidized (donates electrons) and an electron acceptor (e.g. oxygen) is reduced (accepts electrons). There are a number of different compounds that can act as electron acceptors ' including oxygen, nitrate, iron oxides [Fe(OH)3], and sulfate. Dissolved oxygen (DO) is the most thermodynamically favored electron acceptor used in the biodegradation of petroleum hydrocarbons, and it also is the food source for the enzyme that can initiate the metabolism of ' hydrocarbon (Young, 1984). Under the right chemical conditions, the introduction of oxygen to hydrocarbon-affected groundwater can stimulate the growth of indigenous microbes and accelerate their metabolism of petroleum hydrocarbons. Metabolism of soil hydrocarbons present 1 below the static groundwater level may also benefit from the oxygen enhancement process and result in hydrocarbon concentration reduction. This process is dependent on various factors in addition to the electron acceptors described above, including the presence of an appropriate microbial population, nutrients, and pH. On August 16, 1995, additional groundwater samples were collected from Monitoring Wells AW-3 and AW-9 as part of the study. A summary of these background water analyses is presented in Table 3. These wells were selected in order to compare groundwater within a ;, 5