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<br />Because many agencies collected the data
<br />for different purposes, nutrient parameters are
<br />reported in numerous ways. Nutrient parameters
<br />were combined to reduce the total number from 20
<br />to a more manageable 5 for data-analysis purposes.
<br />Procedures described by Mueller and others (1995,
<br />p. 7) for combining nutrient parameters were
<br />followed. The combined nutrient parameters
<br />summarized in this report, included on the data
<br />diskette with other nutrient and ancillary data,
<br />and used in interpretive work by Spahr and Wynn
<br />(1997) are:
<br />
<br />· Nitrate as nitrogen (hereinafter referred to as
<br />nitrate ).
<br />
<br />· Ammonia as nitrogen (hereinafter referred to
<br />as ammonia).
<br />
<br />· Total nitrogen as nitrogen (hereinafter referred
<br />to as total nitrogen).
<br />
<br />· Total phosphorus as phosphorus (hereinafter
<br />referred to as total phosphorus).
<br />
<br />· Orthophosphate as phosphorus (hereinafter
<br />referred to as orthophosphate).
<br />
<br />The data set was screened to include only sites
<br />that had a minimum of five observations for at least
<br />one of the five nutrients listed above. The resulting
<br />data set contains 4,927 samples at 123 sites located
<br />within the UCOL study unit. The locations of these
<br />123 sites are shown in figure 1. The sites labeled with
<br />map reference numbers 1 through 82 are from the
<br />USGS NWIS data base, and the remaining 41 sites are
<br />from the USEP A STORET data base. Map reference
<br />number, site number, and site name for each of the
<br />123 sites that met data-screening criteria are listed
<br />in table 2. The data set presented here was used
<br />for analysis of the spatial distribution, relation to
<br />land use, and temporal trends of nutrient concentra-
<br />tions in surface waters of the UCOL study unit.
<br />However, because of especially restrictive data
<br />requirements, temporal trend analysis was
<br />possible for fewer than 10 percent of the sites
<br />for each of the five combined nutrient parameters.
<br />
<br />DATA-SET SUMMARY
<br />
<br />The selected set of 123 sampling sites for
<br />analyzing surface-water quality has widely varying
<br />sampling periods. The sample-collection dates range
<br />
<br />from January 7,1980, to August 29,1994. The median
<br />sample period of record for individual sites is only
<br />2.5 years, whereas the seventy-fifth percentile is about
<br />12 years. The distribution of sampling dates for the
<br />USGS NWIS (map reference numbers 1-82) and
<br />USEPA STORET (map reference numbers 83-123)
<br />sites where nutrient data were collected are shown in
<br />figure 2.
<br />It is important to determine the similarity
<br />of sampling frequency and period of record
<br />before attempting to compare data values between
<br />sites. Sites 11 and 12 on the Williams Fork and
<br />sites 15 and 16 on the Blue River were sampled
<br />for short, but concurrent, time intervals and would,
<br />therefore, be good sites for comparing water quality
<br />between the two rivers. However, comparison of the
<br />data for these four sites with data from site 63 on
<br />Dry Creek, near Delta, Colo., would be misleading
<br />because data for site 63 were collected several years
<br />later. Sites 49 and 60 were appropriate choices for
<br />determining water-quality trends, in part, because
<br />they each have many samples collected over a
<br />14.5-year period.
<br />The number of samples and the number of
<br />samples collected for each of the five combined
<br />nutrient parameters (nitrate, ammonia, total nitrogen,
<br />total phosphorus, and orthophosphate) are listed, by
<br />site, in table 3. The total number of samples for the
<br />whole data set and for each of the five combined
<br />nutrient parameters is listed in the last row of table 3.
<br />None of the sites have data for all five nutrients for
<br />each and every sample. Therefore, for a given site,
<br />the number of samples in column 2 of table 3 usually
<br />is greater than the number of samples collected for
<br />nitrate, ammonia, total nitrogen, total phosphorus,
<br />or orthophosphate. Site 22 is typical: the number of
<br />samples is 76 but there are only 50 nitrate, 1 ammonia,
<br />o total nitrogen, 0 total phosphorus, and 69 orthophos-
<br />phate samples. The number of samples per site is
<br />variable. Because sites retained must have at least
<br />five samples for one of the combined nutrients, the
<br />minimum number of samples listed in table 3 is five.
<br />Sixteen of the sites (about 13 percent) have only
<br />5 samples. The median number of samples per site
<br />is 14 samples, whereas the seventy-fifth percentile
<br />is 65 samples. Site 100, Colorado River near Loma,
<br />has the most samples (263), collected from 1980
<br />to 1992.
<br />
<br />DATA-9ET SUMMARY 5
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