114 T. E. McHugh et al.Figure 4. Predicted <strong>in</strong>door and subsurface <strong>VOC</strong> concentrations follow<strong>in</strong>g transient <strong>in</strong>door <strong>VOC</strong>rele<strong>as</strong>e. Detection limit represents typical detection limit for <strong>VOC</strong> analysis <strong>of</strong> bulk air samples. Ph<strong>as</strong>e1—<strong>VOC</strong>s detectable <strong>in</strong> <strong>in</strong>door air and subsurface with higher concentration <strong>in</strong> <strong>in</strong>door air; Ph<strong>as</strong>e2—<strong>VOC</strong>s detectable <strong>in</strong> <strong>in</strong>door air and subsurface with higher concentration <strong>in</strong> subsurface; Ph<strong>as</strong>e3—<strong>VOC</strong>s detectable <strong>in</strong> subsurface but not <strong>in</strong>door air; Ph<strong>as</strong>e 4—<strong>VOC</strong>s not detectable <strong>in</strong> <strong>in</strong>door air orsubsurface.between the <strong>in</strong>door air <strong>VOC</strong> concentration and the below-foundation <strong>VOC</strong> concentrationcan be described <strong>in</strong> four ph<strong>as</strong>es: 1) <strong>VOC</strong>s detectable <strong>in</strong> both compartments with <strong>in</strong>door airconcentrations greater than below-foundation concentrations, 2) <strong>VOC</strong>s detectable <strong>in</strong> bothcompartments with <strong>in</strong>door air concentrations less than below-foundation concentrations, 3)<strong>VOC</strong>s detectable below the foundation but not detectable <strong>in</strong> <strong>in</strong>door air, and 4) <strong>VOC</strong>s notdetectable <strong>in</strong> either compartment.
<strong>Indoor</strong> <strong>Air</strong> <strong>as</strong> a <strong>Source</strong> <strong>of</strong> <strong>VOC</strong> <strong>Contam<strong>in</strong>ation</strong> 115For the purpose <strong>of</strong> evaluat<strong>in</strong>g subsurface vapor <strong>in</strong>trusion, ph<strong>as</strong>es 2 and 3 are <strong>of</strong> particularconcern. Sampl<strong>in</strong>g conducted dur<strong>in</strong>g these two ph<strong>as</strong>es would <strong>in</strong>dicate a <strong>VOC</strong> concentrationgradient from below the foundation <strong>in</strong>to the build<strong>in</strong>g even though the orig<strong>in</strong>al <strong>VOC</strong> sourcew<strong>as</strong> <strong>in</strong>doors. In both model scenarios, ph<strong>as</strong>es 2 and 3 cover more than 60% <strong>of</strong> the 5-daysimulation period, <strong>in</strong>dicat<strong>in</strong>g a significant potential for me<strong>as</strong>ur<strong>in</strong>g a higher <strong>VOC</strong> concentration<strong>in</strong> the subsurface than <strong>in</strong> the build<strong>in</strong>g follow<strong>in</strong>g the rele<strong>as</strong>e <strong>of</strong> a transient <strong>in</strong>door<strong>VOC</strong> source. These me<strong>as</strong>urement results could e<strong>as</strong>ily be mis<strong>in</strong>terpreted <strong>as</strong> an <strong>in</strong>dication <strong>of</strong>subsurface <strong>VOC</strong> vapor <strong>in</strong>trusion.Model Sensitivity AnalysisTwokey sources <strong>of</strong> uncerta<strong>in</strong>ty <strong>in</strong> the model <strong>in</strong>puts are: i) the advective air flow rate throughthe build<strong>in</strong>g foundation under positive and negative build<strong>in</strong>g pressure conditions (Q bg andQ gb ), and ii) the prevalence <strong>of</strong> positive versus negative build<strong>in</strong>g pressure conditions. Inorder to evaluate the applicability <strong>of</strong> the model results to a variety <strong>of</strong> build<strong>in</strong>g conditions, <strong>as</strong>ensitivity analysis w<strong>as</strong> performed for these two model <strong>in</strong>puts.The model <strong>in</strong>put values used for the b<strong>as</strong>e model simulations yield values <strong>of</strong> Q bg andQ gb , which vary from 0 to 14 L/m<strong>in</strong> over the range <strong>of</strong> build<strong>in</strong>g pressures specified. Forcomparison, Ett<strong>in</strong>ger (2005) suggests that advective flow through the build<strong>in</strong>g foundationtypically ranges between 1 and 10 L/m<strong>in</strong> for s<strong>in</strong>gle-family residences, while Johnson (2002)suggests that advective flow through the build<strong>in</strong>g foundation can be estimated <strong>as</strong> 0.0001to 0.05 times Q ab , correspond<strong>in</strong>g to a range <strong>of</strong> 0.1 to 50 L/m<strong>in</strong> for a typical s<strong>in</strong>gle-familyresidence. In order to evaluate the effect <strong>of</strong> variations <strong>in</strong> Q bg and Q gb on the model results, themaximum pressure gradient (P max )w<strong>as</strong> varied to achieve maximum Q bg and Q gb valuesrang<strong>in</strong>g from 0.5 to 50 L/m<strong>in</strong>. For low advective flow rates, the maximum subsurface <strong>VOC</strong>concentration (C g,max )w<strong>as</strong> lower than the b<strong>as</strong>e c<strong>as</strong>e, but the time period over which the<strong>VOC</strong> persisted below the build<strong>in</strong>g foundation above detectable concentrations w<strong>as</strong> greater.For high advective flow rates, C g,max w<strong>as</strong> higher than the b<strong>as</strong>e c<strong>as</strong>e, but the time period overwhich the <strong>VOC</strong> persisted below the build<strong>in</strong>g foundation above detectable concentrations w<strong>as</strong>lower. In all c<strong>as</strong>es, there were periods <strong>of</strong> time dur<strong>in</strong>g which subsurface <strong>VOC</strong> concentrationswere higher than <strong>in</strong>door <strong>VOC</strong> concentrations.Although the b<strong>as</strong>e model scenarios <strong>as</strong>sumed equal frequency <strong>of</strong> positive and negativebuild<strong>in</strong>g pressure conditions, the prevalence <strong>of</strong> positive versus negative build<strong>in</strong>gpressures will vary from build<strong>in</strong>g to build<strong>in</strong>g. To evaluate the impact <strong>of</strong> predom<strong>in</strong>atelypositive or negative build<strong>in</strong>g pressures on the model results, build<strong>in</strong>g pressure w<strong>as</strong> variedfrom +4 Pato−4 Pa<strong>in</strong><strong>as</strong>tep-wise f<strong>as</strong>hion rather than the s<strong>in</strong>usoidal variation <strong>of</strong> theb<strong>as</strong>e c<strong>as</strong>e. The prevalence <strong>of</strong> positive pressure conditions w<strong>as</strong> varied from 5% to 95%.In other words, positive pressure conditions were simulated for a period <strong>of</strong> 6 m<strong>in</strong>utesout <strong>of</strong> every 2 hours to 114 m<strong>in</strong>utes out <strong>of</strong> every 2 hours. As the prevalence <strong>of</strong> positivepressure conditions <strong>in</strong>cre<strong>as</strong>es, both the maximum subsurface <strong>VOC</strong> concentration (C g,max )and the percentage <strong>of</strong> the simulation time for which the subsurface <strong>VOC</strong> concentrationis greater than the <strong>in</strong>door <strong>VOC</strong> concentration <strong>in</strong>cre<strong>as</strong>es. C g,max is directly proportionalto the prevalence <strong>of</strong> positive pressure conditions, so for a positive pressure prevalence<strong>of</strong> 75%, C g,max is 50% higher than the b<strong>as</strong>e c<strong>as</strong>e and for a positive pressure prevalence<strong>of</strong> 25%, C g,max is 50% lower than the b<strong>as</strong>e c<strong>as</strong>e. The sensitivity analysis also <strong>in</strong>dicatedthat transient <strong>in</strong>door <strong>VOC</strong> sources can result <strong>in</strong> subsurface <strong>VOC</strong> concentrationsthat are higher than <strong>in</strong>door <strong>VOC</strong> concentrations under a wide variety <strong>of</strong> pressure scenarios.When positive build<strong>in</strong>g pressure conditions occurred less than 15% <strong>of</strong> the time, subsurface<strong>VOC</strong> concentrations were never higher than <strong>in</strong>door <strong>VOC</strong> concentrations. However, when
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