Scientific Investigations Report 2006-5188

Scientific Investigations Report 2006-5188

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Flow-Weighted Concentrations of Trace Metals, 1999-2004

Constituent loads are calculated as the product of streamflow and concentration. Because load calculation results are strongly governed by variations in streamflow, a calculated load may not be the best measure by which to analyze metal transport and behavior. Interpretation of calculated annual loads is improved by examining mean annual Flow-Weighted Concentrations (FWCs) of trace metals. A FWC is an estimate of the mean actual concentration in a total volume of water flowing past a site in a specific period, such as a month or year.

FWCs permit a better understanding of metal transport in the context of variable streamflow conditions and relative location in the basin. For example, table 5 shows mean daily streamflow from 1999 to 2004 ranged from less than 1 m3/s at Ninemile Creek to greater than 200 m3/s at Long Lake. Mean annual streamflow at Amy Gulch varied by more than a factor of 3 during this period. By examining FWCs, a better understanding of the true differences in metal transport can be gained.

Mean annual FWCs for total and dissolved Cd, Pb, and Zn are presented in table 5; the overall mean FWCs for the 10 sites are presented in figure 6. Mean annual FWCs were calculated by the following method. Daily load estimates for each constituent, in kilograms, were summed to obtain annual total loads for each year. Annual sums were divided by the total streamflow, in cubic meters, for that year. Appropriate conversion factors were applied to obtain mean annual FWCs in micrograms per liter.

Total Cd FWCs in the Coeur d’Alene basin ranged from 14 to 0.04 µg/L. Concentrations were highest at Ninemile Creek and Canyon Creek, where estimated loads were relatively low due to low mean streamflow. TCd FWCs were lowest at Enaville; TCd loads also were low at this station, indicating overall low metal contribution from the North Fork Coeur d’Alene River. Low TCd FWCs were measured at stations downstream from Coeur d’Alene Lake, even though TCd loads at these sites were relatively high. This demonstrates that although concentrations may be low because of high streamflow, the overall transport of trace metals can be high.

Total Pb FWCs were highest at Ninemile Creek, Canyon Creek, and Harrison. Elevated TPb FWCs at Harrison confirmed that the high TPb loads at this station were not simply due to higher streamflow; downstream dilution did not compensate for additional sources of metal. Conversely, relatively low TPb loads together with high TPb FWCs at Ninemile and Canyon Creeks reflect low streamflow but high concentrations of TPb.

Total zinc FWCs also demonstrated the profound effect of streamflow on load calculations, and indicated source areas for zinc in the basin. Total Zn FWCs at Ninemile and Canyon Creeks are by far the highest in the basin but contribute among the lowest TZn loads due to their relatively low streamflow. Conversely, stations downstream from Coeur d’Alene Lake exhibit among the lowest TZn FWCs, but TZn load estimates are high because of their high mean streamflow relative to other stations in the basin.

Effects of dilution were recognized by examining TZn loads together with FWCs at Pinehurst and Harrison. Mean annual flows at Pinehurst generally were about 20 percent of those at Harrison. Although mean annual TZn loads at Pinehurst were slightly less than loads at Harrison, the mean annual FWCs of TZn at Pinehurst were more than 3 times greater than FWCs at Harrison, due to downstream dilution of metals by inflow of the North Fork Coeur d’Alene River just downstream of Pinehurst (fig. 6).

Total metal loads and FWCs at Enaville are among the lowest in the basin. This likely reflects an upstream source area low in metals in addition to the effects of dilution by relatively high streamflow at this site. Overall, these data confirm the low metal contribution of this part of the basin to the whole.

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