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�_�__---==rte—=- Zy♦�+w illlL VtfJ�a+ii7L_��_ ____ _ �_-_-_ �_..` _ � _ - _ _ _- _-_ �-"-�___ __ 65-17 � ravztatina and capillazy forces decreases evanoracion. _sduces the -=art--* of some traas=ormac2.oa pathtirays (see below) . a:iz_ �t lead to ground-water contaminatian. 65.2.2 Transformation Processes in Soil/Ground-water System- 55.2-2.1 Chemical Transformation No data were available on chemLeal transformation of gasoline in -- the -envzronmeat. However, as discussed in Chapter 64, phocaoxida=ion - - has been reported to play a sLg=tLficant role in the chemical degradation of soma petroleum hydrocarbons in the sunlit environment (1845,1848,2252.2259) . Alkanes. benzenes. and mono-substit'ucad betzzanes have been shown to be relatively resiscant to photolysis in aqueous systems; xylenes photoiyzed slowly while trisubstiturad benzenes and naphthalenes phocolyzed at rates competitive with volatilizacion (1845) . Penetration of o11 below the soil surface limits exposure Cc solar radiation while extensive lateral spreading of all over impermeable or rocky surfaces may promote substantial - phocooxLdative degradaeion. The oxygenated products of phocooxLdacxon ars generally more water-soluble than the parent hydrocarbons and are thus more likely to be leached from soil. enhanced toxLcitY of the oxygenated hydrocarbons has also been observed (2248,2252) . 65.2.2.2 Biological Degradation Natural ecosystems have considerable exposure cc pocroleum hydrocarbons from natural emissions. accidental cont=Lnaeion through oil spills and storage tar}c leoks, and doliberace application to land in dizpoual accivitiag such as land-farming waacg; therefore, choir biodegradation is of environmental importance. Numerous authors have observed the biodegradation of pecroleum hydrocarbons, and sovaral extansivs rraviewo and reports ara avaiiable (1846,2252.2255,2249,2253) . An extensive and diverse group of petroleum hydrocarbon-degrading _ bacteria and fungi are widely distributed in the environment. The reader is referred cc Chapter 64 for a more detailed summary of the biodegradation of petroleum hydrocarbons. The qualitative hydrocarbon content: of petroleum mLxtures largely datermines their dagradability. In general. microorganisms exhibit decreasing ability to degrade aliphatic hydrocarbons with increasing chain length, n•At-**-s are considered more easily biodegraded than branchad or cyclic alkanes; aromatics are generally more rapidly biodegraded than alkanes. The composLtion of gasoline suggests that aosc of the aromatic species w111 be highly biodegradable, and many of the aliphatic species chat are not volatilized will be moderately biodegradable. In a study of the biodegradation of individual components of gasoline using microorganisms isolated from ground water, the aliphacics and aromatics were shown to be sources of carbon for Nocari and Pseudomonas cultures, respectively (2331) . Very few of the remaining components supported baccera.al growth; ca-axidation was suggested as a possible mechanism for removal of non-growth components. 6/87 t. r..