Quality-control data

Quality-control samples were collected in the Potomac River Basin to estimate the bias and variability in sample collection. Bias is the systematic error that is manifested as a consistent positive or negative deviation from the known or true value. Precision (variability) is the random error associated with the reproducibility of a value without reference to the known true value. Three types of quality-control samples were used in the Potomac River Basin study using the methods described in, "Quality-Control Design for Surface-Water Sampling in the National Water-Quality Assessment Program" (Mueller and others, 1997), in Selected References. Quality-control samples are associated with about 10 percent of the environmental samples collected from 1992-1996.

Field blanks are quality-control samples which measure bias. Field blanks were used to make sure the sampling and processing equipment were not introducing contamination to the sample. Field blanks were also used to assure that there was no cross-contamination occurring after sequential sampling (for example, to assure no cross contamination from site to site during a subunit synoptic survey). Reagent water, free of the analytes to be measured, was passed through the sampling and processing equipment as if it was an environmental sample. The sample was shipped with the environmental samples and analyzed at the same laboratory.

Spikes are environmental field samples fortified with a known concentration of selected analytes to measure bias in analyte degradation during shipping or in analyte recovery. In the field, a set of 3 or 4 water samples was processed. One is designated as the environmental sample. The remaining 2 or 3 water samples were spiked with a known concentration of selected analytes and then shipped to the laboratory. Spikes were performed on samples collected for analysis of dissolved pesticides in water.

Replicates are quality-control samples used to measure the variability in water samples during sample processing and analysis. A composite water sample is collected in the field and split into subsamples. Each of the subsamples are considered to be identical in composition. The subsamples are individually processed sequentially using the same processing equipment and then shipped to the laboratory for analysis. The Potomac River Basin study unit used split replicate samples for surface water and tissues. Samples collected one after another are called sequential replicates. Sequential replicates were used primarily for ground water.

Streamwater
Field blank samples	view frequency table	download data
Replicate, spike, and associated environmental samples	view frequency table	download data

Streambed sediment and aquatic tissues
Replicate and associated environmental samples	view frequency table	download data

Ground water
Field blank samples	view frequency table	download data
Replicate, spike, and associated environmental samples	view frequency table	download data