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Nutrients National Synthesis Project

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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:

    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

FIELD DEFINITION
SU Study unit identifier
SITE NAWQA site identifier
STATION USGS station identifier
DATE Date of sampling event or measurement
TIME Time of sampling event or measurement
PURPOSE Purpose code for sample collection
P00060 Mean daily streamflow, feet cubed per second
P00061 Instantaneous streamflow, feet cubed per second
P00010 Water temperature, degrees Celsius (C.)
P00095 Specific conductance, microsiemens (µs)/cm @ 25 degrees C.
P00300 Dissolved oxygen, mg/L
P00400 pH, whole water, field, standard units
P00915 Calcium, dissolved, mg/L as Ca
P00925 Magnesium, dissolved, mg/L as Mg
P00930 Sodium, dissolved, mg/L as Na
P00935 Potassium, dissolved, mg/L as K
ALK Alkalinity, mg/L as CACO3 (mostly field titration of filtered water)
P00940 Chloride, dissolved, mg/L as Cl
P00945 Sulfate, dissolved, mg/L as SO4
P00950 Fluoride, dissolved, mg/L as F
P00955 Silica, dissolved, mg/L as SiO2
P01046 Iron, dissolved, µg/L as Fe
P01056 Manganese, dissolved, µg/L as Mn
P70300 Residue, dissolved, at 180 degrees C., mg/L
P00608 Ammonia, dissolved, mg/L as N
P00613 Nitrite, dissolved, mg/L as N
P00631 Nitrite and nitrate, dissolved, mg/L as N
P00623 Ammonia and organic nitrogen, dissolved, mg/L as N
P00625 Ammonia and organic nitrogen, total, mg/L as N
P00671 Orthophosphate, dissolved., mg/L as P
P00666 Phosphorus, dissolved, mg/L as P
P00665 Phosphorus, total, mg/L as P

STREAMFLOW

FIELD DEFINITION
SU Study unit identifier
SITE NAWQA site identifier
DATE Date of sampling event or measurement
P00060 Mean daily streamflow, feet cubed per second

RIVERS AND STREAMS SITE DESCRIPTION

FIELD DEFINITION
SU Study unit identifier
SITE NAWQA site identifier
NAME Site name
SAMP_TYPE Sampling type
LU_TYPE Land use type
LATITUDE Latitude; degrees, minutes, seconds
LONGITUDE Longitude; degrees, minutes, seconds
AREA Watershed area, in square kilometers
URBAN Percent of watershed classified as urban land use
AGRICULTURAL Percent of watershed classified as agricultural land use
RANGELAND Percent of watershed classified as rangeland
FOREST Percent of watershed classified as forest
WATER Percent of watershed classified as water
WETLANDS Percent of watershed classified as wetlands
OTHER Percent of watershed classified as other land use or cover

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