118 T. E. McHugh et al.Table 3<strong>VOC</strong> analyses <strong>of</strong> air and vapor samples at a Tex<strong>as</strong> apartment complex (ug/m 3 )Apt. 113 Apt. 123 Apt. 163Chemical IA SS IA SS IA SSOutdoorairDichlorobenzene, 1,4- 35 62
<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> 119As shown on Table 3, concentrations <strong>of</strong> methylene chloride and toluene <strong>in</strong> Apt. 123 and1,4-dichlorobenzene <strong>in</strong> Apt. 113 were higher <strong>in</strong> below-foundation samples than <strong>in</strong> <strong>in</strong>doorsamples, <strong>in</strong>dicat<strong>in</strong>g that the migration <strong>of</strong> <strong>VOC</strong>s from <strong>in</strong>door air to below-foundation soilvapors occurred prior to penetration <strong>of</strong> the foundation for the sample collection. Theseresults are consistent with the model simulations <strong>in</strong>dicat<strong>in</strong>g that elevated concentrations <strong>of</strong><strong>VOC</strong>s can persist <strong>in</strong> the subsurface follow<strong>in</strong>g a transient <strong>in</strong>door rele<strong>as</strong>e even after the <strong>VOC</strong>shave dissipated from <strong>in</strong>door air. Because the orig<strong>in</strong>al goal <strong>of</strong> the sampl<strong>in</strong>g program w<strong>as</strong> todeterm<strong>in</strong>e the presence or absence <strong>of</strong> a subsurface vapor <strong>in</strong>trusion impact at the apartmentcomplex, no additional sampl<strong>in</strong>g w<strong>as</strong> conducted to further evaluate the transport <strong>of</strong> <strong>VOC</strong>sfrom <strong>in</strong>doors to the below-foundation soils.Discussion and RecommendationThe model simulations and field data presented above support the conclusion that <strong>in</strong>door airsources <strong>of</strong> <strong>VOC</strong>s can impact the below-foundation soil g<strong>as</strong>, complicat<strong>in</strong>g the field evaluation<strong>of</strong> vapor <strong>in</strong>trusion at corrective action sites. A simple advection model h<strong>as</strong> been developedthat allows for both positive and negative build<strong>in</strong>g pressures relative to below the foundation.The model provides for the simulation <strong>of</strong> reversible air exchange between the <strong>in</strong>door air andthe sub-foundation soil g<strong>as</strong> allow<strong>in</strong>g an evaluation <strong>of</strong> the impact <strong>of</strong> <strong>in</strong>door <strong>VOC</strong> sources onbelow-foundation soil g<strong>as</strong>. This model <strong>in</strong>dicates that transient <strong>in</strong>door <strong>VOC</strong> sources can result<strong>in</strong> significant concentrations <strong>of</strong> <strong>VOC</strong>s below the build<strong>in</strong>g foundation. Transient simulations<strong>of</strong> a constant source and a first-order source yielded below-foundation concentrations <strong>of</strong>30% and 20% <strong>of</strong> the peak <strong>in</strong>door air concentrations, respectively. As a result, a build<strong>in</strong>g thathad recently experienced a peak benzene concentration result<strong>in</strong>g from a transient <strong>in</strong>doorsource equal to the 90% percentile <strong>in</strong>door air benzene concentration (15.3 ug/m 3 ;Sextonet al., 2004) could also have a below-foundation benzene concentration <strong>of</strong> 4.6 ug/m 3 , greaterthan the US EPA benzene screen<strong>in</strong>g value for below-foundation samples (3.1 ug/m 3 ;USEPA, 2002). The model simulations further <strong>in</strong>dicate that the below-foundation benzenecould persist at concentrations above the US EPA screen<strong>in</strong>g values even after the <strong>in</strong>door airconcentration falls below the analytical detection limit (0.5 ug/m 3 ). As a result, application<strong>of</strong> the US EPA screen<strong>in</strong>g values to below-foundation sample results could erroneously<strong>in</strong>dicate a potential for subsurface vapor <strong>in</strong>trusion impacts even at sites where <strong>in</strong>door rele<strong>as</strong>esrepresent the only source <strong>of</strong> <strong>VOC</strong>s at the site.Field data collected from an apartment complex <strong>in</strong> Tex<strong>as</strong> confirm model predictionsthat <strong>in</strong>door <strong>VOC</strong>s can impact the below-foundation soil g<strong>as</strong>. Four <strong>VOC</strong>s commonly <strong>as</strong>sociatedwith <strong>in</strong>door sources (1,4-dichlorobenzene, methylene chloride, toluene, and xylene)were detected <strong>in</strong> <strong>in</strong>door and below-foundation samples collected from three vacant apartments.<strong>Indoor</strong> sources were identified <strong>as</strong> the most likely source <strong>of</strong> these <strong>VOC</strong>s at this site.In two <strong>of</strong> three apartments sampled, the below-foundation <strong>VOC</strong> concentration w<strong>as</strong> higherthan the <strong>in</strong>door <strong>VOC</strong> concentration, confirm<strong>in</strong>g that <strong>VOC</strong>s orig<strong>in</strong>at<strong>in</strong>g from <strong>in</strong>doors canpersist <strong>in</strong> the subsurface after dissipat<strong>in</strong>g from <strong>in</strong>door air. These results <strong>in</strong>dicate that <strong>in</strong>doorsources can result <strong>in</strong> a mislead<strong>in</strong>g concentration gradient between below-foundationsamples and <strong>in</strong>door samples, with below-foundation <strong>VOC</strong> concentrations exceed<strong>in</strong>g <strong>in</strong>doorconcentrations <strong>in</strong> some c<strong>as</strong>es. These f<strong>in</strong>d<strong>in</strong>gs should be used to update the conceptual modelfor vapor <strong>in</strong>trusion <strong>in</strong> order to account for bi-directional vapor exchange across build<strong>in</strong>gfoundations.B<strong>as</strong>ed on the f<strong>in</strong>d<strong>in</strong>g that bi-directional transport <strong>of</strong> <strong>VOC</strong>s can occur between <strong>in</strong>doorair and the subsurface, the conceptual model <strong>of</strong> vapor <strong>in</strong>trusion and <strong>as</strong>sociated regulatoryguidance should <strong>in</strong>clude the follow<strong>in</strong>g considerations:
Change language