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 a high-flow runoff event at selected water-quality stations during the 1999 water year. The objective was to quantify spatial and temporal differences in constituent concentrations and loads over the ascending and descending limbs of a hydrograph depicting a high-flow runoff event. Discharge and water-quality data were collected during spring 1999 snowmelt runoff (May through early June) at nine water-quality stations, one on the North Fork Coeur d’Alene River and eight on the South Fork Coeur d’Alene River. The nine stations were sam- pled for whole-water recoverable and dissolved concentrations and loads of cadmium, lead, and zinc.
The concentrations and loads sampled during the 1999 snowmelt-runoff event represented near-normal conditions, not flood conditions, in that the recurrence interval for discharge near the hydrograph peak was about 2 years. The general trend among the nine stations was an inverse relation between discharge and dissolved concentrations of cadmium, lead, and zinc, and a direct relation between discharge and whole-water recoverable concentrations of these constituents. The smallest loads of dissolved and whole-water recoverable cadmium, lead, and zinc were measured at South Fork Coeur d’Alene River above Deadman Gulch; constituent concentrations at this site were some of the smallest among those sampled, and discharge was also relatively small. The largest loads of dissolved and whole-water recoverable cadmium, lead, and zinc were measured at South Fork Coeur d’Alene River at Pinehurst; constituent concentrations at this site were large and discharge was the second-largest of all the discharge measurements.
Hysteresis effects on concentrations and loads over the ascending and descending limbs of the snowmelt-runoff hydrograph were quite apparent, especially for whole-water recoverable constituents. Hysteresis is present when a property, such as constituent concentration or load, has different values for a given discharge over the ascending and descending limbs of a hydrograph. During this study, loads of whole-water recoverable constituents on the ascending limb were between 1.5 and 3.6 times larger than those mea- sured on the descending limb at nearly equal discharge. In contrast, dissolved constituents showed minimal hysteresis effects.
