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Technical Report

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nutrient criteria, USEPA Region 6, water quality, Red River Basin, Gulf of Mexico


Acquisition and compilation of water quality data for a ten year time period (1996 – 2006) from 589 stream and river stations was conducted to support nutrient criteria development for the multi–state Red River Basin shared by Arkansas, Louisiana, New Mexico, Oklahoma and Texas, USA. Twenty–three water quality parameters were collected from five data sources (USGS, ADEQ, LDEQ, OCC, OWRB, and TCEQ) and an additional 13 parameters were acquired from at least one source. Data for the primary biological parameter of interest, chlorophyll a, was sparse and available from only two sources. Following compilation of data, medians were calculated for the ten year period and median distributions (min, 10th, 25th, 50th, 75th, 90th percentiles and max) were presented for several different spatial scales including state specific data, HUC8 designated watersheds, and various ecoregions. Across this 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 26 µg L⁻¹, respectively. Overall, the 25th percentiles of median TN data specific to the Red River Basin were generally similar to the USEPA recommended eco–region nutrient criteria. Whereas, median TP and chl–a data specific to the Red River Basin showed 25th percentiles greater than the USEPA recommended criteria. The unique location of the Red River Basin in the south–central USA places it near the boundaries of several aggregate eco–regions; therefore, the development of eco–region nutrient criteria likely requires using data specific to the Red River Basin, as shown in these analyses. This study provided basin–specific distribution of medians as the first step supporting states in developing nutrient criteria to protect designated uses in the multi–jurisdictional Red River Basin and in potentially reducing nutrient export from the Red River Basin to the Gulf of Mexico.

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