| Description |
A water-quality investigation of the Clark Fork-Pend Oreille and Spokane River Basins began in 1997 as part of the U.S. Geological Survey National Water-Quality Assessment Program. As part of the investigation, selected streams in the Spokane River Basin were sampled for trace metals during water years 1999-2001. These data, combined with data collected as part of a U.S. Environmental Protection Agency Remedial Investigation/ Feasibility Study, were used to assess the occurrence, loads, and transport of cadmium, lead, and zinc at 21 streamflow-gaging stations in the Spokane River Basin. Concentrations of dissolved and total cadmium, lead, and zinc varied widely both at and among stations. At most stations, dissolved cadmium and zinc composed most of the total concentrations; dissolved lead generally composed less than 10 percent of the total lead concentration. From the South Fork Coeur d'Alene River near Mullan downstream to the South Fork Coeur d'Alene River near Pinehurst, concentrations of trace metals increased 2 to 4 orders of magnitude. The mean flow-weighted concentrations of total cadmium, lead, and zinc near Pinehurst for water years 1999-2001 were 5.7, 80, and 810 micrograms per liter ( g/L), respectively. On the Coeur d'Alene River near Harrison, downstream from the confluence of the metal-enriched South Fork and the relatively dilute North Fork Coeur d'Alene River, the mean flow-weighted concentrations of total cadmium, lead, and zinc were 1.6, 88, and 240 g/L, respectively. Trace-metal concentrations were smaller in the Spokane River than in the Coeur d'Alene River because of dilution and retention in Coeur d'Alene Lake. The mean flowweighted concentrations of total cadmium, lead, and zinc in the Spokane River near Post Falls were 0.32, 3.1, and 71 g/L, respectively. Regression models relating the mass transport, or load, of trace metals to changes in stream discharge and time were successful in simulating the variability in trace-metal concentrations and loads. The median coefficient of determination for the load models for the 21 stations was largest for total lead (92 percent) and smallest for dissolved and total cadmium (82 percent). Whereas most of the cadmium and zinc loads in the Spokane River Basin were derived from the South Fork Coeur d'Alene River, most of the lead load was derived from the Coeur d'Alene River downstream from the confluence of the North and South Forks. Major tributary sources of trace metals to the South Fork Coeur d'Alene River were Canyon Creek, Ninemile Creek, and Government Gulch. These three tributaries contributed about 3,000 pounds of cadmium, 23,000 pounds of lead, and 310,000 pounds of zinc annually to the South Fork Coeur d'Alene River. Erosion and transport of sediment-bound lead in the Coeur d'Alene River was the primary source of total lead, accounting for almost 400,000 pounds annually during water years 1999-2000. Ground-water discharge in the area of the Bunker Hill Superfund site was a major source of zinc in the South Fork Coeur d'Alene River, contributing more than 250,000 pounds per year. During water years 1999-2000, the average annual loads of cadmium, lead, and zinc transported from the Coeur d'Alene, St. Joe, and St. Maries Rivers to Coeur d'Alene Lake were 8,900, 500,000, and 1.4 million pounds, respectively. The Coeur d'Alene River accounted for more than 99 percent of the total load of each of these three metals entering the lake. About 4,600 pounds of cadmium, 44,000 pounds of lead, and 980,000 pounds of zinc were transported from Coeur d'Alene Lake into the Spokane River. Between the Spokane River near Post Falls, Idaho, and the Spokane River at Long Lake, Washington, there was an annual net loss of about 2,600, 20,000, and 250,000 pounds of cadmium, lead, and zinc, respectively. About 2,000 pounds of cadmium, 24,000 pounds of lead, and 730,000 pounds of zinc were transported annually downstream from Long Lake toward the Columbia River. |
