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Nutrients National Synthesis Project

Data on nutrients in the streams, rivers, and ground water of the United States

Explanation of Stream Data

    The stream data are the results from 8459 water samples collected at 213 surface water sites across the conterminous United States. All of the sampling sites are located in the 20 major watersheds that comprise the first group of NAWQA study units. Samples were collected from October 1991 through December 1996. About 90 percent of the samples were collected during 1993, 1994, and 1995. All samples were flow-weighted and cross-sectionally integrated by standard USGS methods (Ward and Harr, 1990). Complete descriptions of sample collection and processing methods are provided by Shelton (1994).

    NAWQA stream sampling sites are designated as either integrator or indicator sites. Integrator sites are on large streams and rivers with mixed land-use influences. Indicator sites are on small to medium sized streams influenced by one predominant land use.

    The NAWQA design employs two major sampling strategies of varying scope and intensity. The 213 sites with nutrient data consist of:

1. 59 Intensive Fixed Sites (IFS) - Intensive fixed sites are the NAWQA sites where the most detailed data were collected. The sampling frequency at the IFS was higher than at other sites, primarily to characterize the distribution of pesticides. Generally, samples were collected weekly or semi-weekly during periods when pesticide levels were expected to be highest and monthly or semi-monthly during the remainder of the year. In addition to pesticides, samples at the IFS were analyzed for pH, alkalinity, specific conductance, temperature, major ions, nutrients, organic carbon, and suspended sediment.
2. 154 Basic Fixed Sites (BFS) - Basic Fixed Sites were generally sampled monthly, with additional samples collected during extreme flow conditions. Samples at the BFS were analyzed for the same non-pesticide constituents as samples from the IFS.

   NAWQA sites are identified by a site number that corresponds to the USGS station number where most of the data were collected. In a few cases, data for a site were collected at more than one station. Both the NAWQA site number and the USGS station number (where data were collected) are listed in the data tables.

    All samples included in the surface water data file are assigned a sample purpose code. These codes help to identify the reason a particular sample was collected, and can be helpful in aggregating groups of samples for analysis and interpretation. The sample purpose codes are defined as follows:

• ff - Fixed Frequency -- water samples collected for chemical analysis on a predetermined time schedule, without regard to flow conditions or other significant biases. The frequency varies, such as from twice weekly, to as coarse as once every two months or quarterly, depending on the site and season. Although the scheduling may vary widely, the most important feature of "fixed frequency" samples is that time of collection was not modified to target specific high or low flow conditions within the scheduled time period for the sample.
• sh - Storm Hydrograph -- water samples that are part of a storm hydrograph sampling, usually involving multiple samples over one or two days, but sometimes involving 3-5 days. (One of these storm samples is coded as hf).
• hf - Extreme High Flow -- an extra sample taken at extreme high flow for a particular season, collected specifically to characterize conditions at a higher than normal flow for the particular season. These are individual samples in addition to routine fixed-frequency samples.
• lf - Extreme Low Flow -- an extra sample taken at extreme low flow for a particular season. These are individual samples in addition to the routine fixed-frequency samples.

    Sampling at many sites was not spaced evenly throughout the year. Sampling at the intensive fixed sites was more frequent during periods when pesticide levels were expected to be elevated. At the basic fixed sites, extra samples were collected during extreme flow conditions. Thus, care must be used in the interpretation of these data. For example, summary statistics such as the mean or median concentration for a specific compound may be biased unless procedures are used to account for the uneven sampling frequency.

    In addition, the number of samples collected varied considerably among the sites. Thus, the results shown in  the surface water data file are influenced more by conditions at some sites than others. Data analysis procedures that account for this unequal distribution of samples among the sites must be used to avoid biased interpretations.

    Concentrations are in milligrams per liter (mg/L) or micrograms per liter (µg/L).   A negative value indicates that the concentration in the sample is below the value which follows, the laboratory method reporting limit, with the absolute value of this number giving the method reporting limit, and a "." in a field indicates there are no data available.

    Definitions of fields in the data sets are provided in the following lists. "P" codes are the five-digit numbers used to identify water-quality constituents and properties in the USGS National Water Information System (NWIS) and the U.S. Environmental Protection Agency Data Storage and Retrieval System (STORET).    Most "P" codes are preceded by a remark field which contains important information about that measurement.   A "<" indicates that the concentration in the sample is below the value which follows, the laboratory method reporting limit, a concentration preceded by "E" indicates an estimated value, and a concentration preceded by ">" indicates a value "greater than" the maximum calibration value for the method.

    The Rivers and Streams Site Description table gives precise information (latitude and longitude) about the geographic location as well as the land use type and percent cover characteristics of each site.

NUTRIENTS IN RIVERS AND STREAMS

STREAMFLOW

RIVERS AND STREAMS SITE DESCRIPTION