Scientific Investigations Report 2006-5188

Scientific Investigations Report 2006-5188

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Streamflow and trace-metal chemistry data collected at 10 U.S. Geological Survey streamflow-gaging stations in the Spokane River basin were used as input for the U.S. Geological Survey software, LOADEST, to estimate annual loads and mean flow-weighted concentrations of total and dissolved cadmium, lead, and zinc for 1999 to 2004. These estimates yielded valuable information about processes of metal storage and transport, and demonstrated that water quality data are a great aid in understanding these processes.

Cadmium composed less than 1 percent of the total metal load at all stations; mean annual total cadmium loads for 1999-2004 ranged from 39 kilograms at Amy Gulch to 3,400 kilograms at Harrison. Overall, mean annual total cadmium loads were lowest at the stations at Amy Gulch, Enaville, Ninemile Creek, and Canyon Creek, and highest at Harrison, Pinehurst, Post Falls, and Elizabeth Park.

Dissolved cadmium typically ranged from about 70 percent to about 100 percent of the total cadmium load. The ratio of dissolved to total cadmium was higher at stations where ground water was a volumetrically important component of the streamflow. Ratio of dissolved cadmium to total cadmium also was higher at most stations during low-flow years such as 2001, because of the greater relative contribution of ground water to streamflow. Dissolved cadmium/total cadmium ratios about 0.9 and greater were estimated at stations upstream from Pinehurst. The lowest dissolved cadmium/total cadmium ratio was estimated at Harrison.

Total lead constituted from 6 to 42 percent of the total load at stations upstream of Coeur d’Alene Lake and from 2 to 4 percent at stations downstream of the lake. Gaging stations at Amy Gulch, Ninemile Creek, and Enaville yielded the lowest mean annual estimated total lead loads in the basin. By far the highest overall mean total lead load in the basin was estimated at Harrison. Maximum total lead loads at eight stations in the basin were estimated for 2002, which may reflect flushing of accumulated lead-laden sediment stored in the stream channels during 2001, a low-flow year.

The ratio of dissolved lead to total lead varied in time and space and reflected the relative contribution of ground water to total streamflow and the variation in the amount of suspended particulate and (or) colloidal material. Dissolved lead ranged from less than 10 percent to nearly 30 percent of the total lead load.

Zinc composed more than 90 percent of the total metal load at 8 of the 10 gaging stations examined in this study. Mean annual total zinc loads for 1999-2004 ranged from 510,000 kilograms at Harrison to 14,000 kilograms at Amy Gulch. Low annual total zinc loads at Amy Gulch and at Enaville likely reflect the low availability of metals in the source areas upstream of these stations. Estimated mean annual total zinc loads at Harrison far exceeded those at any other station. The three highest estimated annual total zinc loads were at Harrison, Post Falls, and Pinehurst. Total zinc loads also were high at Outlet, Post Falls, and Long Lake.

Dissolved zinc/total zinc ratios typically were greater than 0.8 at most gaging stations. Although estimated total zinc loads were relatively low in 2001 due to the effect of streamflow on the load calculation, the ratio of dissolved zinc to total zinc load was higher in 2001 than in other years, owing to less dilution and a higher proportion of ground water input to streamflow.

Notable systematic differences between total metal loads at stations in different parts of the basin are partly due to the direct relation between streamflow and load. Above Coeur d’Alene Lake, gaging stations with higher streamflow generally yielded higher loads. However, the total metal loads at the three stations downstream of Coeur d’Alene Lake, where mean streamflow is 2-3 times higher than at Harrison, are relatively small. Based on the dissolved metal data and the relative proportions of metals, this observation was interpreted as lead trapped by sediments in Coeur d’Alene Lake and the transmission of cadmium and zinc.

Flow-weighted concentrations (FWCs) calculated from the estimated loads were examined to enhance the interpretation of metal load estimates, which were influenced by large spatial and temporal differences in streamflow. Total cadmium FWCs ranged from 14 to 0.04 µg/L and were highest at Ninemile and Canyon Creeks and lowest at Enaville. Both total cadmium loads and mean streamflow were low at Enaville, indicating overall low metal contribution from the North Fork Coeur d’Alene River. The combination of low total cadmium FWCs and high total cadmium loads at gaging stations downstream of Coeur d’Alene Lake was attributed to the effects of downstream dilution with higher streamflow.

Total lead FWCs were highest at Ninemile Creek, Canyon Creek, and Harrison. The elevated total lead FWCs at Harrison confirmed that the high total lead loads at this station were not simply due to higher streamflow. Conversely, relatively low total lead loads combined with high total lead FWCs at Ninemile and Canyon Creeks reflect relatively low streamflow but high concentrations of total lead.

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

An earlier version of the LOADEST software was used in a previous study to estimate loads for WY 1999-2001 for many of the same sites included in this report. Overall, there was good agreement between results of this study and the previous study. However, notable discrepancies are attributable to important differences in streamflow for 1999-2001 versus 1999-2004. Very low streamflow in 2001 strongly influenced Clark’s results because that year’s data constituted a greater proportion of total available data, whereas, 2001 data were a smaller proportion of the available data for this study. Because the software uses the relation between streamflow and concentration to calculate the regressions, estimated loads during the overlapping years were somewhat different for the two data sets.

Long-term regression models for 1991 to 2003 or 2004 were developed and annual trace-metal loads and FWCs were estimated for Enaville, Pinehurst, Harrison, and Post Falls to better understand the variability of metal loading with time. Long-term load estimates were compared with those for 1999-2004 to examine the robustness of the two differently calibrated models. Long-term estimates mirror the results for 1999-2004 in terms of the mean annual metal loads. The largest mean loads for the estimation period were measured at Harrison and Pinehurst. The largest estimated loads were in 1996 and 1997 (both very high streamflow years) at all four stations; high loads also were measured in 1991. The lowest loads prior to 1999 were in 1994 and 1998. Although low estimated loads in 1994 probably reflect low streamflow, estimated loads in 1998 may have reflected both low streamflow and lack of sufficient time for metals to accumulate upstream after unusually high flows had scoured the channels during 1996 and 1997.

The long- and short-term estimates for these four sites for 1999-2004, the interval common to both estimates, were similar, but with notable differences. Because the LOADEST software uses the relation between streamflow and concentration to perform the regressions, and the calibration data files for 1999-2004 represented a smaller data set than the 1991-2004 data set, different regression models resulted. Differing results demonstrate the importance of assuring that models not be extrapolated beyond the time interval and range of streamflow represented by the input data used to calibrate them.

LOADEST results suggested that statistically significant downward trends during 1991-2004 were at Enaville, Pinehurst, Harrison, and Post Falls for dissolved cadmium, total cadmium, total lead, and dissolved zinc. Additionally, data from Enaville and Post Falls showed significant downward trends for dissolved lead and total zinc loads; Harrison total zinc loads also diminished with time. The Mann-Kendall trend test also was applied to the load data and the results agreed with the LOADEST trend results in most cases, but gave contradictory results for total zinc at Pinehurst and Post Falls. The Mann-Kendall test indicated significant downward trends in FWCs for all constituents except dissolved lead and total lead at Harrison and for total lead at Pinehurst.

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