Indoor Air as a Source of VOC Contamination in Shallow Soils - GSI ...
Indoor Air as a Source of VOC Contamination in Shallow Soils - GSI ...
Indoor Air as a Source of VOC Contamination in Shallow Soils - GSI ...
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116 T. E. McHugh et al.positive pressure conditions occurred more than 30% <strong>of</strong> the time, subsurface <strong>VOC</strong> concentrationsexceeded <strong>in</strong>door <strong>VOC</strong> concentrations for at le<strong>as</strong>t 50% <strong>of</strong> the 5-day simulationperiod.In addition to the pressure <strong>as</strong>sumptions described above, other model <strong>in</strong>put parameterscan significantly affect the results. A lower below-foundation soil permeability greatly<strong>in</strong>cre<strong>as</strong>es the persistence <strong>of</strong> <strong>VOC</strong>s <strong>in</strong> the subsurface follow<strong>in</strong>g removal <strong>of</strong> the <strong>in</strong>door <strong>VOC</strong>source. For example, a soil permeability representative <strong>of</strong> a f<strong>in</strong>e-gra<strong>in</strong>ed sand (i.e., 1 ×10 −12 m 2 ) results <strong>in</strong> the presence <strong>of</strong> detectable <strong>VOC</strong> concentrations <strong>in</strong> the subsurface formore than 30 days follow<strong>in</strong>g the rele<strong>as</strong>e <strong>of</strong> a transient <strong>in</strong>door <strong>VOC</strong> source. In contr<strong>as</strong>t,although not considered <strong>in</strong> the model, w<strong>in</strong>d-driven advection <strong>of</strong> outdoor air through thebelow-foundation soils (Fischer et al., 1996) could serve to reduce the magnitude andduration <strong>of</strong> subsurface <strong>VOC</strong> impacts from <strong>in</strong>door sources.The sensitivity analysis <strong>in</strong>dicates that the advection model presented here providesqualitatively similar results over a broad range <strong>of</strong> build<strong>in</strong>g pressure conditions. However,the quantitative model results vary significantly depend<strong>in</strong>g on the selection <strong>of</strong> model <strong>in</strong>putparameters. As a result, the model should be considered a qualitative tool for the evaluation<strong>of</strong> potential <strong>VOC</strong> exchange between a build<strong>in</strong>g <strong>in</strong>terior and the subsurface. <strong>Indoor</strong> <strong>VOC</strong>sources <strong>of</strong> higher m<strong>as</strong>s flux or longer duration result <strong>in</strong> higher below-foundation <strong>VOC</strong>concentrations. The specific model results presented for the b<strong>as</strong>e c<strong>as</strong>e scenarios are <strong>in</strong>tendedto illustrate the types <strong>of</strong> <strong>VOC</strong> concentration gradients that may be observed between <strong>in</strong>doorair and subsurface vapors follow<strong>in</strong>g the rele<strong>as</strong>e <strong>of</strong> a transient <strong>in</strong>door <strong>VOC</strong> source. Theresults are not <strong>in</strong>tended to quantify the magnitude or duration <strong>of</strong> these relationships <strong>in</strong>specific build<strong>in</strong>gs.Field Evaluation <strong>of</strong> Below-Foundation <strong>VOC</strong> ImpactsIndependent <strong>of</strong> model simulations, field me<strong>as</strong>urements <strong>of</strong> <strong>in</strong>door and below-foundation<strong>VOC</strong> concentrations have also <strong>in</strong>dicated the migration <strong>of</strong> <strong>VOC</strong>s from <strong>in</strong>door air <strong>in</strong>to thesubsurface. <strong>Indoor</strong> air and below-foundation samples collected from three apartments ata complex <strong>in</strong> Fort Worth, Tex<strong>as</strong>, show that <strong>VOC</strong>s orig<strong>in</strong>at<strong>in</strong>g <strong>in</strong>doors have migrated <strong>in</strong>tothe subsurface and are present at detectable concentrations. Although this is a limited dat<strong>as</strong>et from a s<strong>in</strong>gle site, the results provide a prelim<strong>in</strong>ary <strong>in</strong>dication that <strong>VOC</strong> transport from<strong>in</strong>door to the subsurface can occur.Sample Collection and AnalysisAn <strong>in</strong>door air and below-foundation vapor sampl<strong>in</strong>g program w<strong>as</strong> conducted <strong>in</strong> three vacantapartments with<strong>in</strong> an apartment complex <strong>in</strong> Forth Worth, Tex<strong>as</strong>. The <strong>in</strong>itial purpose <strong>of</strong> thesampl<strong>in</strong>g program w<strong>as</strong> to evaluate the potential for vapor <strong>in</strong>trusion impacts <strong>as</strong>sociated withmethyl-tertiary butyl ether (MTBE) detected <strong>in</strong> a groundwater monitor<strong>in</strong>g well upgradient<strong>of</strong> the property at an adjacent former g<strong>as</strong> station. Groundwater samples from the adjacentproperty have been analyzed for benzene (16 to 4,800 ug/L), ethylbenzene (3 to 490 ug/L),toluene (