EPA interim final report on WTC background study, April 2003

NYC 9/11 Public Portal Document

6.3 Minerals and Total Dust Results - Development of Background Concentrations The following sections present a summary of the analytical results, the calculated backgroimd values and a discussion that addresses the strengths and weaknesses for the crystalline silica, calcite, gypsum, portlandite and total dust data sets, as well as the corresponding background value that was derived for each analyte, matrix, type of space and type of surface. The statistical results and calculated UCLs are reported in Tables 6-4 through 6-6.

6.3.1 Crystalline Silica 6.3.1.1 Alpha-quartz Bulk Dust - There were nine bulk dust samples collected from residential areas. These samples were analyzed for alpha-quartz. There were no bulk samples collected from common areas. Eight of the samples detected alpha-quartz above the detection limit with a mean value of 2.2%. The 95% UCL for alpha-quartz in bulk dust is 3.66% and this value can be used as an estimate of the background concentration.

Air - There were forty-six air samples collected, with thirty-two collected from residential areas and fourteen collected from common areas. The frequency of detection for the residential areas (25%) and the common areas (29%) are similar, although the range of detection and mean concentrations are different. This is primarily due to one sample value from the residential areas being elevated above the rest of the samples in the data set. This value was determined to be a valid result and therefore was kept in the data set. There was some variability between the residential and common area, primarily due to one sample, however following the priorities identified earlier it is recommended to use residential data set for estimating the background concentration due the difference in the maximum detected concentrations. The 95% UCL for the residential data set is 61.9 pg/m’ which can be used as an estimate of background concentration for alpha-quartz in air.

Although the value estimated above has been identified as the recommended background concentration, an additional calculation was conducted without the elevated value to determine a UCL that is not influenced by the elevated value. The 95% UCL for the residential data that has the elevated value removed is 7.8 pg/m’. This value may be more representative of background, however it is unclear how common the elevated value may be in an urban environment, therefore the recommended UCL is 61.9 pg/m’ with the recognition that it may be much lower (i.e., 7.8 pg/m’).

Wipe - There were 114 wipe samples collected with eighty-one collected from residential areas and 33 collected from common areas. The wipe samples were collected from hard surfaces including counters, floors, and walls. The frequency of detection between the residential areas (38%) and the common areas (61%) for combined surfaces are not similar and the mean concentration for each type of

WORLD TRADE CENTER BACKGROUND STUDY 27

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