Water Quality Assessment of the South Platte River Basin, Colorado , Nebraska, and Wyoming

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pound classes: carbamates, chlorophenoxy acid herbi- cides, organochlorine and organophosphorus insecticides, and triazine and other nitrogen- containing herbicides. The time period selected was water years 1980 -92. This 12 -year span is of sufficient length to detect trends while minimizing variability due to changes in analytical techniques. The data set was next screened in several ways to ensure only appropriate data were included: 1. Data were screened by medium, such as water or sediment; and by site type, such as surface water or ground water. 2. Data were screened to remove replicate samples, blanks, quality - control samples, and the like. 3. Where more than one sample was collected in a given month at a site, only the first sample of the month was retained, in order to remove temporal bias. 4. Data that were affected by changes in laboratory methods or otherwise determined to be biased were not used. For example, from 1980 -82, the National Water Quality Laboratory (NWQL) of the USGS had ammonia contami- nation that caused a positive bias for concentra- tions of ammonia species; and in May 1990, the NWQL changed method protocols for total phosphorus determinations (U.S. Geological Survey, written commun., 1992). Conse- quently, for trend analyses, the following data were used in this retrospective report: (a) for total organic nitrogen plus ammonia, dissolved organic nitrogen plus ammonia, and dissolved ammonia, only data for water years 1982 -92; and, (b) for total phosphorus, only data for water years 1980 -90. 5. After the above data screening, sites were accepted only if there were at least six data observations over a period of at least 2 years Next, sampling sites were screened and selected: 1. Where sites were closely spaced, for example due to a site - specific water - quality study, only one site was selected from the cluster. 2. Well sites were used only if they had aquifer and well -depth information available. 3. Where surface -water sites were closely spaced, sites having daily streamflow records were preferentially selected. As a result of data selection and screening, data were assembled for 54 surface -water sites and 107 wells (figs. 9 and 10). The agency source of data for selected surface -water sites is listed in table 5. Sur- face -water data primarily came from the USGS National Water Information System (NWIS) database (36 sites), which contains data collected for a variety of purposes by USGS personnel and their cooperators. Data from 12 surface -water sites originated in USEPA Storage and Retrieval ( STORET) data base; these data were collected by the Denver Water Department, the U.S. Army Corps of Engineers, the Colorado Depart- ment of Health, and the USEPA. Data from four sites were collected by the Denver Regional Council of Governments ( DRCOG), as part of the Bear Creek Reservoir Clean Lakes Study. Data for two sites were collected by the MWRD as part of their monitoring activities near their discharge point. The agency source of data for selected ground- water sites is listed in table 6. Ground -water data (75 sites) primarily came from the USGS NWIS data- base. Data from 32 ground -water sites were collected by the Northern Front Range Water Quality Planning Association (NFRWQPA) as part of a nitrates -in- ground -water monitoring effort. Quality Assurance of Data To document the quality of the data used in this report, information was collected about the quality - assurance programs for each agency source of data. Ideally, for comparisons of data, samples should have comparable collection, storage, preservation, analysis, and quality- control methods. The USGS collects equal- discharge- increment (EDI) or equal - width- increment (EWI) composite sur- face -water samples to ensure a representative sample is taken of the entire stream (Edwards and Glysson, 1988). The collection methods used by agencies con- tributing data to the USEPA STORET database often could not be ascertained. DRCOG samples were grab samples. Some of the MWRD samples were grab sam- ples collected just below the surface in flowing water (Roy Zimmerman, Metro Wastewater Reclamation District, written commun., 1992); however, the major- ity of samples collected were EWI composites. The USGS has protocols for ground -water sampling such as purging of wells, placement of pumps in wells, and equipment and collection methods for sample parame- ters (Wood, 1976), but the protocols used by other agencies are unknown. For this report, laboratory analyses were consid- ered reliable if they were performed by the USEPA or SOURCES AND CHARACTERISTICS OF AVAILABLE WATER - QUALITY DATA 19