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MEMORANDUM CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD & CENTRAL VALLEY REGION 3443 Routier Road, Suite A Panne: (916)255-3000 Sacramento, CA 95827-3098 CALNRT: 8-494-3000 TO: GLB FROM: HAT DATE: 10 September 199.1 SIGNATURE: SUBJECT: IN-SrM BIOREMEDIATION (BIORESTORATION) OF SOIL AND GROUND WATER CONTAMINATED WITH GASOLINE Hydrocarbons are organic carbon compounds composed of carbon, oxygen, and hydrogen. Normal, branched, and cyclic alkanes are the most abundant hydrocarbon components of gasoline. Normal alkanes such as methane, pentane, and butane, are readily degradable. Branched alkanes, such as methylpentane and dimethylbutane, and cyclic alkanes, such as cyclopropane, are not as degradable as the normal alkanes. As molecular size and complexity increase the rate of degradation of the molecules decreases. Other major components of gasoline are aromatic compounds, such as benzene, and alkylaromatic compounds, such as toluene, ethylbenzene, and xylene. Like alkanes, the larger and more complex molecules take longer to biodegrade. N-alkanes, n-alkylaromatics, and aromatic compounds in the C1O - C'a range are generally considered the least toxic and easiest to biodegrade. Counterparts in the CS - 0 range are easily removed by volatilization and can be toxic to microbes in high concentrations. N-alkanes, n- alkylaromatics, and aromatic compounds above 1722 have low toxicity but are difficult to break down. Branched alkanes and cycloalkanes in the C10 - C' range are less biodegradable than the a- alkanes and aromatics- Branching creates multiple carbon bonds which interfere with biodegradation. Highly condensed aromatic and cycloparaffinic compounds with four or more rings, such as benzo(a)pyrene, tar, and asphalt, are resistant to biodegradation. Breakdown or biodegradation of petroleum hydrocarbons may be accomplished by enzymes produced by microbes. Enzymes are specialized protein molecules that accelerate chemical reaction rates. Enzymes work at catalytic sites within the microbe cell or cell membrane, The enzyme molecule contains a catalytic site designed for particular substrate molecules. When a substrate molecule encounters a catalytic site, a chemical bond is formed. However, the new bond causes the enzyme to deform slightly, straining the geometry of the substrate molecule and eventually breaking some of its chemical bonds. The enzyme molecule then returns to its original shape releasing the breakdown products of the substrate molecule. The enzyme is then available to repeat the process and the breakdown products can be subsequently acted on by other microbes. Microbial degradation of gasoline components can occur by aerobic respiration, anaerobic respiration, or fermentation. Breakdown of petroleum hydrocarbons by these pathways requires an electron acceptor. Oxygen, nitrate, sulfate, and carbon dioxide are the most common electron acceptors. If the reaction takes place in an environment containing free oxygen as the electron acceptor, the reaction is aerobic and the end products are CO, and water. If the reaction is lacking 90 ' d 800' uN £2:6 £6` 9T daS ' oho 131