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M 310 PETADLEt1IN COt+l7kktINATEO SOILS ESTIMATES POR HYDROCARBON VAPOR EMISSIONS 311 surface and no biodegradation of the vapors(eonservatia a asstirnphons),the steady- Treble 2 Casnllnuod state on"imensional solution to the governing diffusion equation yields the fol- lowing expression for the vapor flux, � Soume {Ifh x Eardsatoa hate Vapor Conc i t Source 1 (WCM 1 p0�pt�a �Fiumkoarbcd=Ica ve* 1 (23) groundwater NC 580 32x10-6 lin -,groundwater w11h t ppm honzem NC 32 1 ex 10-' —Iree-hailed gasoline NC 026 3200-11 10-e Gratindwatar Trealmene where C„w is given by Equation 4, and H„and (D,d% are the thicl.ness and --carbon beds with 1 ppb i�ksclraige NC 01 NC effective porous media vapor diffusion eoeflicmnt(see Equation 12)for each dis- --air3lppper with gas sat water INC 1965 2 4 x 10'7 tinct soil layer above lite source —alt-topper with 1 ppm benwrie water NC 109 s y Equation 23 is a generalized farm of an aqua- t.3x 1tl- tion that is often used for estimating diffusive vapor fluxes from landfills r 13 in Leaving Soils In Place9 59X10-5 219 NC writing Equation 23 we assume that the vapor concentration is zero at ground NC.uotcarculated at Flatapplrcabta level(this yields the maximum emissions rate), and any contribution from diffu- •reaeavalu►s oorsesposd 10 speci&r wMai9nh;ilsm,bob a StQ °r r 2o•C 1 mole%benzene S ro deep euoav9lim 6 1 m x 12 2 m area il through the Sail InDtSIt1iC 15 Insignificant The latter 35 a valid dSSniitptton rr.2D C 1 mole%barLwne,ioDw maks TPm bice gaerdue!paaoUmr 61 m x e 1 m me1L for volatile hydrocarbons, but is not necessarily applicable for very nonvolatile '1 20 C 00 1 male%bervww 161Jmrn tso seFtit vapor eudreclroa rare i compounds 'T-2D C 1 mold%benzene 10 man r4hurww crone in grou wa aw 7a vmin too Spm}prouadwaler pumpmp ratE OOM pmtp(D D1 Sr1m)1r094quid Pmdutt pimping rine 'T-20 C 1 moil%benn9ne,16 mgir sarrrrelad 03m h orauwwaw 70 pmrh t2o Bp&#Benund morPuP [01918 561'x7 renin i2Do 9DFW ail Ammim =20'C 1 mole 4S lfannerla 5 m depth to cWammadlon,rn sud”seri,s 1 m x 61 m,Kell ill. EMISSION RATE CALCULATIONS n To illustrate the use of the models presented in§112,benzene vapor emissions estimates ane calculated below for a hypothetical service station cleanup consist- I/ !a Emissions Assoeiafed with Excavated Pits ing of a tank excavation and replacern eat,soil venting,free-liquid product pump- ing,and a groundwater pump and treat operation It will be assumed that gasoline Emissions from the empty pit are estimated by assuming vapors diffuse up- containing 1 mole%(—I%by weight)benzene 1s the source of soil and ground- ward from the pit bottom through a stagnant layer of air and are swe t awl b water contamtnalion "Fresh" gasolines usually contain between 0 5`� to 3% f; p z e the v toil at round level Equations 4 and were used to generate the benzene � by weigh(benzene Table 2 summarizes The results presented in this section,these emissions estimates that appear in Figure 7 for a range of CXCavation depths and are inputs to the air dispersion models discussed in §IV 1 benzene niole fractions For the air dispersion analyses appearing in §IV 2 the- emissions rate corresponds to the paranteter values Table 2. Suirintery of Benzene Vapor Emissions Predlctlona° X, = mole fraction of benzene in the Flux Emission Rate Vapor Cone at Source hydrocarbon spill 0 01 Source (91cm2-d) (gfdl (gfcm3l _ T = absolute temperatum = 0 0 K 20 C Empty Tank PO ) --maximum steady-state 3 7 x 10-$ 5$ NC P,° = vapor pressure of pure benzene = U 10 stili —three-day avenge 2 9 x 1d-5 22 NC M,, = molecular weight of benzene = 78 girnole Soil P418, R = Universal Gas Constant = 82 1 erriz-ataifmate-"K 119 average Ior 8 hrsfd 3 tlx td-a 370 ntc D,^ = vapor phase diffusion coefficient in —1 (24 h average) air l cartr/dI = 7270 cmz/d Solt Veering d H = depth of excavatton lcml = 305 cm (10 ft) unlreateid vapors, constant source NC 6532 3 2 x 10-11 --treated vapors,oonstera)source l9=0 Bs) NC 327 1 6x 10-F The result 1s —untreated vapors.5110 gal spill NC 84 4 2x 10-9 —treated vapors,500 gal spdi(v=o 95} NC 47 2 1 x 10-9 RnIi 1�.=7 7 x:10-5 glcm2-d