Occurrence and distribution of dissolved trace elements in the surface waters of the Yakima River Basin, Washington, 1999-2000

The occurrence, distribution, and transport of dissolved (filtered-water) trace elements in the surface waters of the Yakima River Basin were assessed using data collected between 1999 and 2000 as part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program. Samples were collected at 34 sites throughout the basin in August 1999, using a Lagrangian sampling design. From May 1999 through January 2000, samples also were collected weekly during the irrigation season and once per month during the nonirrigation season at three intensive fixed sites. Although the focus of this study was on 9 trace elements (aluminum, arsenic, barium, copper, iron, manganese, nickel, uranium, and zinc), 14 additional elements were analyzed in filtered water. Concentrations for most trace elements in filtered water generally were low and there were no exceedances of the U.S. Environmental Protection Agency (USEPA) freshwater aquatic-life water-quality criteria. The USEPA drinking-water standard for arsenic (10 µg/L) was exceeded in two samples that were collected under base-flow conditions during the nonirrigation season at Granger Drain. Over 40 percent of all filtered-water samples collected during this study exceeded the USEPA health advisory level of 2.0 µg/L for arsenic. Arsenic concentrations in agricultural drains were highest when the drains were primarily fed by shallow ground water, and concentrations in the Yakima River were highest when the river was fed primarily by agricultural return flow. The USEPA secondary maximum contaminant level for manganese (50 µg/L) was exceeded in three samples collected at Granger Drain during the nonirrigation season. Instantaneous arsenic loads calculated for August 1999 were similar to mean monthly loads determined in August 1989 at two intensive fixed sites located on the Yakima main stem. In August 1999, arsenic loads increased twofold between the Yakima River at river mile 72 above Satus and the Yakima River at Kiona at river mile 29.9. The dissolved arsenic loads for the Yakima River at Euclid Bridge at river mile 55 near Grandview and Yakima River at Kiona were within 13 percent of the August 1989 levels.