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2" frNIC CONTAMINANTS tN SUBSURFACE ENVIRONMENTS SECT 22' ' r r nisms primarily responsible for biodegradation cannot survive in large mg/L The value for oxygenated fuels (i e.. those containing MTBE numbers to this locus. Biodegradation of contaminants in the ground. TSA,ethanolmethanol,etc.)is poorly constrained at present, but is 2 water,therefore,is usually an insignificant means of removal However, least an order of magnitude greater than that for non-oxygenated fuel:w if contaminants are transported to the water table or vadose zone(due to The general effect of co-solvents on solubility can be stated as w fLuctuattont to the groundwater elevation) which are much more well oxygenated, biodegradation can occur much more rapidly 1n S = In S' + of A S 9 2 4 2 Chemical Oxidation(Details in Sechm 3.2 4 2) - mo � where , le fraction solubility in water(dimensionless) So = mole fraction solubility in mixed solvents Contaminants ise (dimemionless) groundwater can undergo abiotic transformation o = parameter related to solutes'surface area and interfacia m ors (provided precurssuch as phenol and catechol are present)as a result w tree energy cn r of aerobic microbial process. f . volume traction of co-solvent(0<f<q(dmietisioniess) n W 8.3 S70RAGE CAPACITY IN LOCUS The value of 200 mg/L stated above for the maximum commnU*.d of gasoline without additives dissolved in groundwater is an approximi = r 8.3.1 lunbrctlon and Basic Equations tion based on a laboratory study(API, 1985)and solubility calculatioc from literature—reported values(Lyman, I"?). The actual concentrt Some of the information below considers the case for gasoline,which tions found in the field are typically much lower than the maximus is the product most commmonly held in an UST.The maximum concen- estimated value. The maximum values in the field typically diminis tration of an unadulterated gasoline that can be dissolved in ground- with time, as advection and dispersion serve to spread the plume an water is directly related Io the solubilities of each constituent in a gaso• dilute the concentrations line blend (for further discussion of dissolution, see Section 7). Hydrocarbon solubility in water is not significantly affccted by moi ` For unoxidszed hydrocarbon in contact with pure water the concen- est changes in temperature, ionic strength, pH, or pressure Temper; tration of the solute in aqueous solution is proportional to the concen- tune and pressure are nearly constant for most grotitidwaters.Solubilll tration of the conslituent in gasoline and the solubility of the solute. is reduced somewhat with increasing ionic strength in aqueous soluteol_n t• Solubility may be affected, perhaps quite significantly, by co-solvei C'. Xy 3�" (g.12) effects Garrett Hal. (1985)note that in gasoline's that contain MTB' i = 1, 2, 3 . . . a constituents the BTEX compounds are more soluble than in gaso6na not container , MTBE, because BTEX are almost completely soluble in ether, where Ci, - concentration of constituent i in the water phase solvent values for a (see equation 8.13) we imp'Wically-0enved; hN (mg/[.) availability is limited for gasoline products In general, solubility it Xy = mole fraction of constituent i in gasolene creases with increasing dissolved organic carbon. (dimensionleu) The composition of the dissolved gasoline is quite different from 11 S„ - solubility of constituent i in pure water(mg/L) composition of the bulk product (see Table 3-2). For non-oxygenau • Calculations are more diffycult it significant amounts of oxygenated gasolines,the most soluble components are the light aromatics:benzen additives are preselet toluene, xylene, and ethylbentcne (BTEX). The literature Includes se _ oral studies of the water soluble fraction (WSF) of gasoline and oth 8.9.2 Guidance on Inputs for, and Calculation of, Maximum petroleum products often stored in underground storage tanks. 71 n Value results show that BTX compounds comprise 23-55%of the bulk pro� uet but 42-74% of the WSF, benzene eoncentrodons we ennched to An approximate maximum for non-oxygenate)gasoline blends or for times in the WSF versus the bulli product(Guard A d., 1983) Colem;m hydrocarbon constituents of oxygenated fuels is an the order of 200 et al (1984) performed solubilization experiments and found that tl I