Field blanks (FB) were collected at 136 sites between May 2004
and March 2010, and source-solution blanks (SSB) were collected at
77 of those sites.
FBs were collected using the same protocols used for
collection of groundwater samples; SSBs were collected at the field
site by pouring blank water directly into the sample bottle.
FBs assess potential contamination during sample collection, field processing,
shipping, and laboratory processing and analysis; SSBs assess
potential contamination during shipping and laboratory processing
and analysis.
The maximum detection frequency for the 13 compounds detected in FBs and the 3 compounds detected in SSBs was 7.4% and 3.9% (both diphenhydramine).
The reason the FBs and SSBs were cleaner than the LSBs is unknown. No additional quality-control censoring of data was required on the basis of FB detections because
all periods with high detection frequencies in FBs corresponded to
periods with high detection frequencies in LSBs.