Quality Control Procedures

To properly interpret water-quality sampling data, it is necessary to have information about the sources of variability and possible contamination associated with sample collection, processing, and analysis. All analyses of NASQAN water samples except for suspended-sediment concentration and particle size are provided by the USGS National Water Quality Laboratory (NWQL) in Lakewood, Colorado. Quality control procedures for the NWQL are available at http://nwql.usgs.gov/quality.shtml. Suspended sediment analyses are provided by several regional USGS sediment laboratories. Each regional laboratory has Quality control procedures specific to that laboratory.

In addition to laboratory-specific quality control programs, the quality of all NASQAN analytical data are independently monitored by the USGS Branch of Quality Systems (BQS). Blind samples are made by BQS and submitted to the various laboratories as routine environmental samples. The bias and variability of analytical results are reported for suspended sediment, inorganic constituents, and organic constituents. The frequency and magnitude of contamination also is measured.

Field quality control for the NASQAN program is modeled after that used by the USGS National Water Quality Assessment program. (See Quality-Control Design for Surface Water Sampling in the National Water-Quality Assessment Program for details.) Field quality-control procedures require the collection and analysis of field blanks, field replicates, and field matrix spikes. Field quality-control samples are intended to measure sources of bias or variability attributed to field activities. The following definitions are excerpted from the previously linked NAWQA Quality Control Design report:

Field Blanks A field blank is prepared in the field and used to demonstrate that: (1) equipment has been adequately cleaned to remove contamination introduced by samples obtained at previous sites, (2) sample collection and processing have not resulted in contamination, and (3) sample handling and transport have not introduced contamination. In addition, because the field blank is treated like an environmental sample at the laboratory, it includes potential contamination introduced during laboratory handling and analysis.

Field Replicates Field replicates are two or more samples collected or processed so that the samples are considered to be essentially identical in composition. Split replicates are prepared by dividing a single volume of water into multiple samples. These replicates provide a measure of the variability introduced during sample processing and analysis.

Field Matrix-Spikes A field matrix spike is a specific type of spiked sample that is fortified in the field prior to shipping. These samples are used to assess the potential bias for analytes in a particular sample matrix. This bias also might include analyte degradation during shipping and handling. Bias is estimated from spiked samples by calculating the percentage of the added analyte that is measured (recovered) in the sample. Recovery can be either greater than or less than 100 percent, so the bias can be either positive or negative; however, matrix interference and analyte degradation generally result in a negative bias.

Examples of USGS field quality-control data analyses are presented in the following publications:

Mueller, D.K., 1998, Quality of nutrient data from streams and ground water sampled during 1993-95--National Water-Quality Assessment Program: U.S. Geological Survey Open- Report 98-276, 28 p.

Martin, J.D., 1999, Quality of pesticide data for environmental water samples collected for the National Water-Quality Assessment Program, 1992-96, and examples of the use of quality-control information in water-quality assessments

Martin, J.D., Gilliom, R.J., and Schertz, T.J., 1999, Summary and evaluation of pesticides in field blanks collected for the National Water-Quality Assessment Program: 1992-95: U.S. Geological Survey Open-File Report 98-412, 102 p.

Kelly, V.J., Hooper, R.P., Aulenbach, B.T., and Janet, Mary, 2001, Concentrations and annual fluxes for selected water-quality constituents from the USGS National Stream Quality Accounting Network (NASQAN), 1996-2000: U.S. Geological Survey Water-Resources Investigations Report 01-4255

Martin, J.D., 2002, Variability of pesticide detections and concentrations in field replicate water samples collected for the National Water-Quality Assessment program, 1992-97: U.S. Geological Survey Water-Resources Investigations Report 01-4178, 84 p.

Mueller, D.K. and Titus, C.J., 2005, Quality of nutrient data from streams and ground water sampled during water years 1992-2001: U.S. Geological Survey Scientific Investigations Report 2005-5106, 27 p.

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