Assessment of total nitrogen and total phosphorus in selected surface water of the National Park Service Northern Colorado Plateau Network, Colorado, Utah, and Wyoming, from 1972 through 2007

Brown, Juliane; Thoma, David

Nutrients are a nationally recognized concern for water quality of streams, rivers, groundwater, and water bodies. Nutrient impairment is documented by the U.S. Environmental Protection Agency (USEPA) as a primary cause of degradation in lakes and reservoirs, and nutrients are related to organic enrichment and oxygen depletion, which is an important cause of degradation in streams. Recently (2011), an effort to develop State-based numeric nutrient criteria has resulted in renewed emphasis on nutrients in surface water throughout the Nation. In response to this renewed emphasis and to investigate nutrient water quality for Northern Colorado Plateau Network (NCPN) streams, the U.S. Geological Survey (USGS), in cooperation with the National Park Service (NPS), assessed total nitrogen (TN) and total phosphorus (TP) concentration data for 93 sites in or near 14 National Park units for the time period 1972 through 2007. For this study, nutrient (specifically, TN and TP) data collected from 1972 through 2007 from 93 stream water-quality sampling sites were screened for exceedances of nutrient standards or criteria, and 52 of these sites were analyzed for temporal, monotonic trends in ambient concentrations and flow-adjusted concentrations (FAC) of TN and TP. Concentrations of TN and TP at the 93 water-quality sampling sites were compared to applicable State of Utah standards or USEPA recommended nutrient criteria for aggregated Ecoregions II (forested mountains) and III (xeric west), and descriptive statistics were compiled for each site. Notable exceedances and median concentrations from water-quality sampling sites on the Colorado, Fremont, Green, Gunnison (and tributaries), Virgin, and Yampa Rivers were described. Not all sites had TN and(or) TP data for comparison. Sixty-two of 65 sites for TN and 92 of 93 sites for TP had one or more exceedances, and exceedances ranged from 2 to 100 percent of nutrient samples. The average frequency of exceedance for TN for all 65 sites was 74 percent and for TP for all 93 sites the average frequency of exceedance was 61 percent. Moreover, 45 percent of the 65 sites evaluated for TN exceedances and 35 percent of the 93 sites evaluated for TP exceedances had exceedances greater than or equal to 85 percent of samples. Throughout the NCPN, median TN concentrations ranged from 0.106 to 2.42 mg/L (overall median 0.633 mg/L), and median TP concentrations ranged from 0.006 to 0.367 mg/L (overall median 0.076 mg/L), excluding data from the one wastewater treatment plant outfall included in the dataset. Median concentrations were equal to or less than the applicable standard or criterion for data from 8 TN (12 percent) and 35 TP (38 percent) sites.

Assessment of total nitrogen and total phosphorus in selected surface water of the National Park Service Northern Colorado Plateau Network, Colorado, Utah, and Wyoming, from 1972 through 2007

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