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

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a laboratory certified by the USEPA or were done by the USGS or by a laboratory participating in the stan- dard- reference - sample quality - control program of the USGS. The MWRD analyzes samples internally; their laboratory follows USEPA guidelines, and they suc- cessfully passed the USGS audit (Carl Calkins, Metro Wastewater Reclamation District, personal commun., 1992). Samples collected for NFRWQPA by the Weld and Larimer County Health Departments in Colorado were sent to the Colorado State University Soils Labo- ratory for analysis. Duplicate samples were collected from randomly selected wells and were analyzed by the Colorado Department of Health and the Weld County Health Department. There were no significant differ- ences in the results from the three laboratories (Dave Dubois, Northern Front Range Water Quality Planning Association, personal commun., 1992). All of the agencies used similar quality- control methods that included standards, blanks, replicates, and spikes; and all of the agencies had quality- assurance programs. SITE CHARACTERIZATION When using water - quality data from selected sites to assess basin -wide conditions, it is useful to characterize each site relative to environmental factors that affect water quality. Geology, land use, and signif- icance of wastewater - treatment -plant discharges were selected as important surface - water -site characteristics for this analysis (table 5). Typically, surface -water sites are characterized by selecting the predominant type for each environmental factor within a site's entire drainage basin. For example, land use for the 3,861 -mil drainage basin for the South Platte River at Denver (site 19) consists of 44% forest, 32% range- land, 8% agricultural, 8% urban, and 8% other land uses; therefore, this might be characterized as a for- ested site. Closer examination of this site, however, reveals that water contributed from the forested portion of this basin is diverted out of the stream before it reaches Denver, and water at the site consists of waste- water- treatment -plant discharges and urban runoff. Therefore, this site is more correctly characterized as urban. The characterization of sites for this analysis takes into account the hydrologic modifications, which affect the source of water at each site. Geology was characterized for surface -water sites as crystalline bedrock for sites in the mountains, alluvium for sites in the plains, or sedimentary bedrock for sites in the transition zone between the mountains and plains. This geologic grouping is paralleled to some degree by other site characteristics such as phys- iographic province and ecoregion (for more details, see Dennehy and others, 1993). Land use was character- ized as forest, rangeland, urban, built -up, agricultural, or mixed agricultural and urban. Built -up land use is a subcategory of urban land use but was separated out for this analysis. Built -up areas consist of low- density housing and other development, often located along stream valleys, as compared with urban land use, which consists of high - density housing and commercial and industrial development encompassing quite large pop- ulation centers. The mixed agricultural and urban land use was assigned to locations along the Front Range corridor downstream from Denver where agriculture and population centers are commingled. The impor- tance of point discharges was evaluated by the propor- tion of streamflow contributed by a point discharge at a site and by the proximity of those point discharges to the site. Geology, land use, well depth, and primary use of well were selected as important characteristics for ground -water sites for this analysis (table 6). For ground -water sites, the geology is defined by the aqui- fer (fig. 7) from which water is withdrawn. The wells were distributed between five aquifers: the alluvial aquifer of the South Platte River and its tributaries, the High Plains aquifer, the Denver aquifer, the Arapahoe aquifer, and the Laramie/Fox Hills aquifer. Land use at ground -water sites was assigned by overlying the land - use map with a map of well locations, under the assumption that water in shallow wells can be affected by recharge from the overlying land surface. However, this assumption cannot be made for deeper wells, so land use was not assigned to wells in the deeper aqui- fers, like the High Plains aquifer and the Denver Basin aquifer system. Ground -water sites also were charac- terized by well depth; where available, the location of the open interval is listed in table 6, but for most wells only the well depth was known. Ground -water sites also were characterized according to the primary use of the water, which was stock wells, domestic use, public supply, irrigation, or observation wells. DATA - ANALYSIS METHODS Various methods were used to summarize and analyze data in this report. Non- parametric statistical techniques, such as boxplots, primarily were used because such methods require few assumptions about the statistical properties of a data set and are suited for data sets having few observations that might not be normally distributed. Boxplots were drawn to compare constituent concentrations in water (Helsel and Hirsch, 1992); box - plots (fig. 11) graphically display the median or 50th percentile (the center line of the box), interquartile range (the box representing the range between the 25th and 75th percentiles), the 10th and 90th percentiles 32 Water - Quality Assessment of the South Platte River Basin, Colorado, Nebraska, and Wyoming -- Analysis of Available Nutrient, Suspended- Sediment, and Pesticide Data, Water Years 1980 -92