Quality Assurance

Data-collection and analytical procedures used in this study incorporated practices designed to control, verify, and assess the quality of sample data. Methods and associated quality control for collection and field processing of water-quality samples are described by Ward and Harr (1990), and Horowitz and others (1994).

Replicate samples were obtained in the field to provide data on precision (reproducibility) for samples exposed to all sources of variability. Precision of analytical results for field replicates is affected by many sources of variability within the field and laboratory environments, including sample collection, processing, and analysis. Replicates were collected by splitting a single composite sample into two subsamples. Each subsample was then processed and analyzed separately.

The differences between replicates can be used to estimate the precision of analytical results for a constituent by calculating a standard deviation according to the following equation (Taylor, 1987):

                    (1)

where
         S = standard deviation of the difference in concentration between replicate analyses,
         d = difference in concentration between each pair of replicate analyses, and
         k = number of pairs of replicate analyses.

Precision also can be expressed as a relative standard deviation (RSD), in percent, which is computed from the standard deviation and the mean concentration for all the replicate analyses. Expressing precision relative to a mean concentration standardizes comparison of precision among individual constituents. The RSD, in percent, is calculated according to the following equation (Taylor, 1987):

                                                 (2)

where
        RSD = relative standard deviation,
        S = standard deviation, and
        x = mean of all replicate concentrations.

Three replicate water-quality samples and two replicate streambed-sediment samples were collected. Paired analyses along with the calculated RSD for each constituent are presented in table 4 for water-quality samples and table 5 for streambed-sediment samples (back of the report). RSDs were calculated using values that were rounded to standard reporting levels for the particular constituent and its analytical method. The RSD for constituents analyzed in water samples were all less than 20 percent (table 4). RSDs for total- recoverable metals were consistently greater than for dissolved metals. This difference was possibly caused by splitting of suspended sediment in the whole-water composite sample into two subsamples. Variability in the distribution of sediment among the subsamples could have resulted in unequal quantities of particulate-bound metal. The RSD for constituents analyzed in the streambed-sediment replicates were 12 percent or less, except for copper, which was 22 percent (table 5).

Table 4. Quality-assurance data for surface-water samples collected in the Miller Creek watershed, Montana, August 30, 2000 (click here for pdf file) (click here for xls file)

Table 5. Quality-assurance data for streambed-sediment samples collected in the Miller Creek watershed, Montana, August 2000 (click here for pdf file) (click here for xls file)

Two field-blank samples of deionized water certified to be free of detectable concentrations of trace elements were processed during this study. Blanks were processed through the same sampling equipment used in the collection of stream samples. Blank samples were subjected to the same processing (sample splitting, filtration, preservation, transportation, and laboratory handling) as stream samples. Each blank was analyzed for the same constituents as those of stream samples. All constituent concentrations in the blanks samples were less than the minimum reporting level for the analytical methods used during this study (table 4), indicating that the collection, field processing, and laboratory analyses were free of significant contamination that could bias results.

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