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Streamwater Quality at Selected Sites in the Fraser River Basin, Grand County, Colorado, Water Years 1991-2000 EXPLANATION A5 Sampling site with site number Wastewater treatment plant Continental divide Median is greater than the 75th percentile Median is greater than the 50th percentile and less than or equal to the 75th percentile I Median is greater than the 25th percentile and less than or equal to the 50th percentile 0 Median is less than or equal to the 25th percentile No data available Nitrite Chloride Ammonia plus nitrate Dissolved Ortho- Total phosphorus phosphorus phosphorus 25th 50th 75th Percentiles of nutrient concentrations in milligrams per liter Nitrite Plus Dissolved Ortho- Total Chloride Ammonia nitrate phosphorus phosphorus phosphorus 0.525 0.002 0.005 0.006 0.002 0.018 2.925 0.006 0.064 0.014 0.010 0.041 6.510 0.014 0.125 0.029 0.023 0.059 105°52'30" 106° 6 Granby 105°45' ?? Ta ernl;h - 5 ? t ree? J ? 13 rf?• pole Creek A ?? ' - 9 X70 B *4"- ti1-2 - - _--./ ?w Fri 11 p raw Ir +V? ' ~YZ? J ? 3 0?- Q L r, W111i?1 ??1 ?? Far;` 1 Z 39°52'30.. U cO/V 2fVC DMDE 0 2 4 6 MILES 0 2 4 6 MILES Figure 2. Spatial distribution and magnitude of median concentrations of selected water-quality constituents in the Fraser River Basin, 1991-2000. instantaneous chemical concentration by the instantaneous dis- charge. Loading values then were multiplied by a conversion factor to yield values in tons per day. Median instantaneous loads were calculated for samples at each site from all data for the period of record. Analysis of the instantaneous load data on the main stem of the Fraser River showed similar results to the concentration data: the majority of the inputs of nutrients in the streamwater was between sites 3 and 4 (fig. 3). The reduction of nitrite plus nitrate and orthophosphorus loads downstream from site 4 indicated that physical, chemical, or biological processes that remove nutrients from the streamwater were greater than input sources. The continual increase in total phosphorus load may be a result of increased sediment loads in the river, which are com- monly high in insoluble phosphorus. Data collected for chloride show that about one-third of the total load of chloride in the entire Fraser River is present by the time the water reaches site 1. The gradual increase in loads may be an indication of ground-water discharge to the stream, inputs from the wastewater-treatment plants, and inputs from tributaries. In order to assess the monthly loading of nutrients and chloride in the Fraser River, median instantaneous loads for all monthly data from site 4 are shown in figure 4. This site is used because monthly samples are available for the entire period of study, and the site is located downstream from the wastewater-treatment plants. Monthly median stream dis- charge is included in figure 4 to show the relation between loads and streamflow conditions. Peak nutrient loads began in early spring before the onset of spring runoff, and loadings to the Fraser River were largest between January and May. The large loads in the Fraser River between January and May most likely resulted from the increased seasonal population and