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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5144

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Quality Assurance

Several measures were taken to ensure the procedures used for sample collection, processing, transport, and analysis did not affect the concentrations of compounds of interest in the samples. Types of quality assurance used during this study included using materials, samplers, and methods known to be appropriate for analytes of interest, collecting blanks and replicate samples in the field, and a high level of laboratory quality assurance (Pirkey and Glodt, 1998; Maloney, 2005).

For VOC samples, field blanks were collected during each major sampling trip and consisted of pouring nitrogen-purged VOC-free water into the VOC sampler containing four VOC vials, letting the water contact the sampler for 5 minutes, then processing the samples as if they were environmental samples. Reproducibility of sample collection, processing, and analysis was assured by collecting one replicate sample during each sampling trip. These replicate samples were collected sequentially, that is, they were collected one after the other. The results of the blank and replicate samples show that contamination was not introduced by the materials and methods used for sample collection and the methods provided a reproducible sample. The results of the field blanks and replicate samples are listed in tables 5 and 11 (at back of report), respectively.

For SPMD samples, field, dialysis, and laboratory blanks were collected and processed for each deployment (2004 and 2006). Field blanks were collected at each site during 2004 and at two sites during 2006 (one site for each lake). A field blank consisted of exposing an SPMD to the atmosphere during the time the SPMDs that were placed into the water were exposed to the atmosphere (typically 1 to 2 minutes). This was done during deployment and retrieval of the SPMDs. The exposed SPMDs were sealed in tin cans and accompanied the environmental samples to the laboratory. Dialysis blanks were a check on whether any contamination was introduced during the extraction procedure of the SPMDs, and laboratory blanks assured the sample processing and analysis at the laboratory did not introduce contaminants to the sample. Duplicate SPMD samples were collected at two sites during the 2004 collection by placing them in separate stainless-steel containers attached side-by-side to the buoy chains. Unexposed SPMDs spiked with PAH compounds in the laboratory (laboratory spikes) were analyzed to determine the recoverability of PAH compounds during the extraction and analysis of the SPMD samples. The results of the blank and duplicate samples are shown in table 6 and show that minor contamination was present in field blanks during the 2006 collection period. The compounds detected in both field blanks were alkyl naphthalenes, naphthalene, and phenanthrene; pyrene and fluoranthene also were detected in one field blank. The concentrations, however, were far less than those reported for the environmental samples. The dialysis blank had detectable concentrations of the same compounds as the field blanks at concentrations near those detected in the field blanks, thus, the dialysis process likely did not contaminate samples. PAH compounds were not detected in field blanks collected in 2004, however, the laboratory reporting limits were much higher (250 µg/kg) as compared to those in 2006 (95 µg/kg). Laboratory blanks were consistently without detections and laboratory spikes had recoveries that generally were between 70 and 90 percent (table 6).

Lake-bottom sediment was quality assured using a laboratory blank, spike recovery, and several duplicate samples (table 7). The estimated concentration of naphtathalene in the laboratory blank was 4.81 µg/kg, which was subtracted from the concentrations detected in the environmental samples. Laboratory spike recoveries ranged from 43 to 108 percent. The same compounds generally were detected in duplicate samples but the concentrations were sometimes different by a factor of two.

Overall, the results of the quality assurance indicate that the collection, processing, and analysis of samples collected during this study produced samples that were free of gross contamination and representative of the water or sediment sampled. Contamination was minor and always far less than the concentrations detected in environmental samples. The reproducibility of VOC samples was very good and that of the SPMD and sediment samples was slightly more variable, but reasonably good.

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