The Remedial Investigation/Feasibility Study conducted by the U.S. Environmental Protection Agency within the Spokane River Basin of northern Idaho and eastern Washington included extensive data-collection activities to determine the nature and extent of trace-element contamination within the basin. The U.S. Geological Survey designed and implemented synoptic sampling of the 1999 snowmelt-runoff event at 42 water- quality stations during the 1999 water year. The distribution of the 42 stations was as follows: North Fork Coeur d’Alene River and tributaries, 4 stations; South Fork Coeur d’Alene River, 13 stations; Canyon, Ninemile, and Pine Creeks, 4 stations each; other tributaries to South Fork Coeur d’Alene River, 10 stations; and main stem Coeur d’Alene River, 3 stations. The objective was to synoptically collect discharge and water-quality data in order to significantly improve the estimation of trace-element loads from multiple contributing source areas during the snowmelt-runoff event. Discharge and water-quality data were collected near the peak discharge during late May 1999. Each station was sampled for whole-water recoverable and dissolved concentrations and loads of cadmium, lead, and zinc.
Three general concentration levels of cadmium, lead, and zinc were noted among the 42 stations. Dissolved cadmium concentrations were less than 1 microgram per liter (μg/L) at 26 stations, exceeded 10 μg/L at 1 station, and ranged from 1 to 10 μg/L at the remaining 15 stations. Whole-water recoverable cadmium concentrations were less than 1 μg/L at 23 stations, exceeded 10 μg/L at 4 stations, and ranged from 1 to 10 μg/L at the remaining 15 stations. Dissolved lead concentrations were less than 1 μg/L at 22 stations, exceeded 10 μg/L at 7 stations, and ranged from 1 to 10 μg/L at the remaining 13 stations. Whole-water recoverable lead concentrations were less than 10 μg/L at 13 stations, exceeded 500 μg/L at 20 stations, and ranged from 10 to 500 μg/L at the remaining 9 stations. Dis- solved zinc concentrations were less than 10 μg/L at 14 stations, exceeded 500 μg/L at 6 stations, and ranged from 10 to 500 μg/L at the remaining 22 stations. Whole-water recoverable zinc concentrations were less than 10 μg/L at 9 stations, exceeded 500 μg/L at 15 stations, and ranged from 10 to 500 μg/L at the remaining 18 stations.
The accounting of tributary loads between two South Fork stations at O’Brien Gulch and Pinehurst revealed differences between dissolved and whole-water recoverable loads, as well as differences among the three trace elements. Tributary loads accounted for an average of 29 percent (range of 27 to 31.6 percent) of the differences in whole-water recoverable loads of the three trace elements between the O’Brien Gulch and Pinehurst stations. This result implies that the main stem of the South Fork Coeur d’Alene River is an important source of sediment-associated trace elements under elevated streamflows. In the case of dissolved loads of cadmium and zinc, the tributary loads accounted for about one-half (range of 47.3 to 55 percent) of the differences between the two South Fork stations. As with whole-water recoverable loads, this result indicates an important source of dissolved cadmium and zinc within the main stem. The picture is much different for dissolved lead loads: About 94 percent of the load difference between the O’Brien Gulch and Pinehurst stations was accounted for by loads from the 13 tributaries.
The Coeur d’Alene River near Harrison transported 924 pounds of dissolved lead per day, of which 82.8 pounds came from the South Fork and 11.7 pounds from the North Fork. Only 10.2 percent of the load at Harrison was measured at the Pinehurst and Enaville stations; therefore, a substantial load of dissolved lead is being contributed downstream from the confluence of the North and South Forks.