Distributions of Median Nutrient and Chlorophyll Concentrations across the Red River Basin, USA

Acquisition and compilation of water-quality data for an 11-yr time period (1996–2006) from 589 stream and river stations were conducted to support nutrient criteria development for the multistate Red River Basin shared by Arkansas, Louisiana, New Mexico, Oklahoma, and Texas. Ten water-quality parameters were collected from six data sources (USGS, Arkansas Department of Environmental Quality, Louisiana Department of Environmental Quality, Oklahoma Conservation Commission, Oklahoma Water Resources Board, and Texas Commission on Environmental Quality), and an additional 13 parameters were acquired from at least one source. Median concentrations of water-quality parameters were calculated at each individual station and frequency distributions (minimum, 10th, 25th, 50th, 75th, 90th percentiles, and maximum) of the median concentrations were calculated. Across the Red River Basin, median values for total nitrogen (TN), total phosphorus (TP), and sestonic chlorophyll-a (chl-a) ranged from <0.02 to 20.2 mg L⁻¹, <0.01 to 6.66 mg L⁻¹, and 0.10 to 262 μg L⁻¹, respectively. Overall, the 25th percentiles of TN data specific to the Red River Basin were generally similar to the USEPA-recommended ecoregion nutrient criteria of 0.31 to 0.88 mg L⁻¹, whereas median TP and chl-a data specific to the Red River Basin showed 25th percentiles higher than the USEPA-recommended criteria (0.010–0.067 mg TP L⁻¹; 0.93–3.00 μg chl-a L⁻¹). The unique location of the Red River Basin in the south-central United States places it near the boundaries of several aggregate ecoregions; therefore, the development of ecoregion nutrient criteria likely requires using data specific to the Red River Basin, as shown in these analyses. This study provided basin-specific frequency distribution of median concentrations of water-quality parameters as the first step to support states in developing nutrient criteria to protect designated uses in the multijurisdictional Red River Basin.