Scientific Investigations Report 2006-5188

Scientific Investigations Report 2006-5188

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Model Input


Streamflow at 9 of the 10 gaging stations in the sampling network was measured using standard USGS methods (Buchanan and Somers, 1968, 1969; Carter and Davidian, 1968), using a continuous record of water stage calibrated to periodic measurements. Station 7, however, is situated within the backwater created by Coeur d’Alene Lake and therefore does not have a valid stage-streamflow relation. To estimate streamflow at this station, the USGS streamflow model FourPt was used (Delong and others, 1997). The model uses channel geometry and water-stage data at upstream and downstream gaging stations in the stream reach being modeled. The model was calibrated using streamflow measurements over a wide range of streamflows and lake stage and was used to compute a daily mean streamflow for station 7 for WYs 1991–2004. Streamflow at station 8 was not available for WY 2003–04; therefore, streamflow for station 9 was used for 2003–04.

Trace-Metal Concentrations

Short-term regression models were calibrated at 10 sites using trace-metal concentration data collected during 1999–2004. Long-term regression models also were calibrated for four of the sites using concentration data collected during 1991–2004. Trace-metal load estimates were compared for 1991–2004 based on two different regression models. This aspect of the study is discussed in the section “Estimated Trace-Metal Loads, 1999-2004”

The trace-metal concentration data used in this report were collected as part of a number of studies by the USGS. These studies included the Northern Rockies Intermontane Basins study of the National Water Quality Assessment program (NROK NAWQA) and the Remedial Investigation/Feasibility Study (RI/FS) of the Spokane River basin (URS-Greiner, 2001; Woods, 2001b; Clark and others, 2004). Because these studies had somewhat different objectives, the number of trace metal samples varied widely among the 10 sites (table 1). Nevertheless, all trace-metal concentration data were collected by USGS personnel and analyzed in USGS laboratory facilities using consistent methods and quality control measures.

The sampling approach for the NAWQA and RI/FS studies was to allocate samples over the full range of the station hydrograph to develop a robust relation between trace-metal concentration and streamflow. Generally, samples were collected on a fixed-interval frequency, and additional samples were collected during low or high streamflow. Additional samples were collected as part of the RI/FS study during significant streamflow events such as rain-on-snow, spring snowmelt runoff, and thunderstorms to characterize trace-metal transport during those times.

Samples at all stations were collected using nonmetallic samplers and cross-sectional, depth-integrated sampling procedures (Edwards and Glysson, 1988). Samples were composited and subsampled using a polyethylene churn or Teflon® cone-splitting device. Samples for whole-water recoverable (total) analyses were withdrawn directly from the splitting device. Samples for dissolved (smaller than 0.45-µm diameter) analyses were withdrawn directly from either the churn splitter or a subsample of the cone splitter and passed through a pre-rinsed, 0.45-µm pore size, disposable Gelman capsule filter. All trace-metal samples were preserved with 2 mL of Ultrex nitric acid. Samples were shipped in plastic coolers to the USGS National Water Quality Laboratory (NWQL) in Denver, Colo. About 10 percent of the samples were submitted as blanks, field spikes, and duplicates for quality assurance purposes as described by Friedman and Erdmann (1982) and Mueller and others (1997).

Samples at all stations were analyzed at the NWQL using similar analytical techniques. At the NWQL, samples were analyzed for total and dissolved concentrations of selected trace metals. Samples for total analysis were digested by heating with dilute hydrochloric acid and were filtered prior to analysis. Trace-metal concentrations were determined by atomic absorption spectrometry in conjunction with a graphite furnace and inductively coupled plasma-mass spectrometry (Fishman, 1993). Quality assurance/quality control procedures used at the NWQL were documented by Pritt and Raese (1995).

Trace-metal concentrations generally vary in relation to streamflow. For example, in samples collected during WYs 1999-2004 from the South Fork Coeur d’Alene River at Elizabeth Park, concentrations of TCd and TZn generally decreased and TPb increased with increasing streamflow (fig. 4). This relation between streamflow and concentration is typical of many mining-affected streams in the region (Clark, 2002).

Some samples having Cd concentrations near the minimum analytical detection limit were reported to have DCd concentration slightly higher than the TCd concentration, a situation that probably is an artifact of the analytical methods at low concentration. In most cases, the difference between DCd and TCd was less than 10 percent of the total concentration; therefore, the Cd in these samples was considered to be entirely dissolved.

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