U.S. Geological Survey

Data Series 152

National Water-Quality Assessment Program

Water-Quality, Streamflow, and Ancillary Data for Nutrients in Streams and Rivers Across the Nation, 1992–2001

Sample Collection and Laboratory Analysis

The basic sampling frequency at stream and river sites was monthly. In high-intensity sampling years, several additional high-flow samples were collected to characterize that part of the hydrograph where concentrations might be more variable. At subsets of sites, samples were collected more frequently during time periods when concentrations of certain constituents, primarily pesticides, were expected to be high. These sampling frequencies varied from biweekly to as often as every other day. Routine sampling included field blanks and replicates to assess measurement bias and variability (Mueller and others, 1997).

Typically, samples were collected using a depth-integrating sampler at multiple vertical locations in the stream cross section (Shelton, 1994; U.S. Geological Survey, variously dated). Other methods of sample collection are noted in the data file. Samples for analysis of dissolved constituents were filtered in the field, within 2 hours of collection, through either a nitrocellulose filter or a polyether-sulfone medium with a pore size of 0.45 micrometer. Prior to water year 1995, nutrient samples were preserved with mercuric chloride in the field and then chilled and maintained at 4 degrees Celsius until analyzed at the laboratory. Starting in water year 1995, nutrient samples were no longer preserved but were chilled and maintained at 4 degrees Celsius. Beginning in water year 1999, samples collected for the determination of total kjeldahl nitrogen and total phosphorus were preserved with 1 milliliter of 4.5 normal sulfuric acid and then chilled and maintained at 4 degrees Celsius.

All nutrient samples were analyzed at the U.S. Geological Survey National Water-Quality Laboratory (NWQL) in Colorado. Analyses were made for a suite of nitrogen (N) and phosphorus (P) constituents.

• Dissolved ammonium plus un-ionized ammonia, hereinafter referred to as "ammonia"
• Dissolved nitrite
• Dissolved nitrite plus nitrate as nitrogen, hereinafter referred to as "nitrate"
• Dissolved Kjeldahl nitrogen, consisting of ammonia plus organic N
• Total Kjeldahl nitrogen
• Dissolved orthophosphate
• Dissolved phosphorus
• Total phosphorus

Ammonia, nitrite, nitrate, and orthophosphate were analyzed according to methods described by Fishman (1993). Kjeldahl nitrogen was analyzed as described by Patton and Truitt (1992). During 1992–98, total and dissolved phosphorus were analyzed using a modified Kjeldahl procedure (Patton and Truitt, 2000). In 1999, these analyses were changed to U.S. Environmental Protection Agency method 365.1, low-level persulfate digestion (U.S. Environmental Protection Agency, 1993).

The U.S. Geological Survey operates several state and regional sediment laboratories. Sediment samples were analyzed according to methods described by Guy (1969). Sediment samples collected for the NAWQA program can be subsamples of the depth integrated composite sample collected for chemical analysis (Shelton, 1994, page 20) or separate depth integrated composite samples.

Dissolved organic carbon was analyzed according to methods described by Brenton and Arnett (1993). Suspended and particulate carbon analysis methods are described by Wershaw and others (1987) and the U. S. Environmental Protection Agency (1997). Changes in the field filtering methods occurred near the end of water year 2000 and are described in U.S. Geological Survey Office of Water Quality Technical Memorandum 2000.08 (U.S. Geological Survey, 2000).