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296 PETROLEUM OONTAMIt+tATED SOILS ESTIMATES FOR HYDROCARBON VAPOR EMiSSIONS 297
<br /> free-liquid residual gasoline,and in-situ soil venitng of the remaining contaminat- As stated above, components in the residual contaminant partition be-
<br /> er soil A model that estimates the emissions for the case in which gasoline- tween vapor, adsorbed, soluble (dissolved rn soiI moisture), and free-liquid
<br /> contaminated si►,is are left in place is also presented to the second main section (or solid) residual phases Mathematically, this can be described for aa} com-
<br /> tlhese models are used to estimate the benzene emissions associated with this ponent i
<br /> hypothetical remediation, and with leaving the soils in-place (no treatment) The
<br /> vapor fluxes arc used as air dispersion model inputs in the third inam section,and M, _ a,Pl"CAMite Mii,o
<br /> ambient air concentrations ate calculated for a nearby community For compare- A71 Yi R T a,h1 +
<br /> scan, the anhbient air hydrocarbon concenir~duons due to hydrocarbon vapor cinis- � �.� "'�� Mw,i M`j1,0sions from undisturbed underground gasohne-eontattunated soils are also computed where-
<br /> 11, MODEL DEVELOPMENT
<br /> !►9, = total tholes of i to soil matrix
<br /> y, = mole fraction of i in soil moisture phase
<br /> a, = activity coefficient for i in water
<br /> li 1 Vapor Equilibrium Models k, = sorption coetTclen( for i [{mass-iltnass-soil)!(mass-ilrriass-
<br /> An integral part of any vapor transport model is the calculation of vapor con- 1420)]
<br /> centrations at the source based on measured residual Boil contamination levels, P.` = pure component vapor pressure of i
<br /> Lontammant composition. soil properties (organic carbon content, soil moisture (.,1 = vapor-filled void fraction in soil matrix
<br /> content),and enviroruitental factors(temperature) Two main approaches are used A*.1 = soil matrix density
<br /> in vapor transport models,but rarely is their ice justified by the authors Before R = gds constant (=82 1 cm'-atmtmole-K)
<br /> resentin vapor emissions models, therefore. it is useful to briefl review the N = absolute temperature
<br /> various methods for calculating vapor concentrations and Justify the approach MH o = total moles of fretd residual contaminant
<br /> Iii � = total moles to soilt mmoioisture phase
<br /> used in this work
<br /> The influenoe of sod type, moisture content, chemical type, temperature, and Ma = rinse of soil matrix
<br /> residual soil contaminant levels has been the focus of studies by Chiou and Shoup,' M�rel,, = molecular weight of water
<br /> I
<br /> Spencer," Poe et al ,3 and Valsaraj and ThitWeaux ` in each study the effect
<br /> I 1
<br /> of the parameters listed above on the equilibrium vapor concentration above aThe tiler term on the right-hand side of Equation l represents the number of mole.;
<br /> soil matrix was studied for a single component Briefly,changes in the moisture of t to the vapor phase, the second represents the number of moles of i in the
<br /> free-liquid residual phase, the Ibird term is the number of tholes of i dissolved
<br /> content significantly influence the vapor concentration when the soils are"dry", in the soil moislum, and the last term is the number of moles of i sorbed to the
<br /> that is, the moisture content is less than that required to provide a complete
<br /> monolayer coverage of water inolectiles on file soil particle surfaces This air- soil particles In writing Equation i we assume equilibrium between an ideal gas I
<br /> responds roughly to the '`wiitiug point" of a snit, and for sandy sad tapes is vapor phase,ori ideal mixture free-liquid hydrocarbon phase,and a nonideal still
<br /> moisture phase When contaminant
<br /> in the 0.02 to D OS g-H"a!g-sod moisture content large It has been observed contaminant levels are great enough that arise-liquid(or
<br /> that the sorptive capacity of soil is greatly increased when the soil is dry 'Y solid) residual phase is present, hien irqudtion 1 must be solved iteratively, sub-
<br /> When the contaminant concentration is low enough that free adsorptive sites are }set to the condition that Erx,y,=1
<br /> available on the snit (= <100 msoil), the adsorbed eontami- Once Biluation 1 is solved,the vapor concentration in equilibrium with the con-
<br /> J~-contaminantfl.mg-contaminant/kg-soil),
<br /> equilibrium can be modeled by a modified Brauner-Emmett-Teller taminantlsoil matrix, C,.-q, Iniass-ilvolume-vapor) is obtained fronh
<br /> (131':1) equation S If the moisture content is great enough that there is more than
<br /> a monlayer of water molecules adhering to the soil surface,then the vapor equihbri- Cl eg=a3',Ma.P." (2)
<br /> um appears to be governed by the partitioning between four phases vapor, dis- RT
<br /> solved in the sod moisture, sorbed to the soil particles, and free-residual (when
<br /> concentrations are great enough) " More often than not,the moisture content of where Mw,denotes the molecular weight of component i 1n the limits of low
<br /> soils buried more than a foot below ground surface will be greater than the wilt- and high residual contaminant soil concentrations Equation 1 reduces to forms
<br /> ing point, so we will focus on modeling the partitioning of contaminants in this that do not require iterative solutions- In the lo% concentration limit (h e , no
<br /> nichiuiire content reginic free-liquid or solid precipitate Plias DrC-41111 1-1111 Will, 1 tY, �•�����
<br />
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