Scientific Investigations Report 2006-5188
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5188
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Estimated annual trace-metal loads of dissolved and total Cd, Pb, and Zn for 1999-2004 (referred to as “short-term” loads) for each site are presented in table 3. The overall mean total metal loads (the sum of TCd, TPb, and TZn) and the mean percentage of each metal of the sum are presented in table 3. Graphs showing mean streamflow and mean dissolved and total Cd, Pb, and Zn loads for each site for 1999–2004 are shown in figure 5.
Estimated mean annual TCd loads were relatively low at Amy Gulch (39 kg/yr), Enaville (58 kg/yr), Ninemile Creek (200 kg/yr), and Canyon Creek (510 kg/yr). Conversely, estimated mean annual TCd loads were relatively high at Harrison (3,400 kg/yr), Pinehurst (2,600 kg/yr), Post Falls (1,500 kg/yr), and Elizabeth Park (1,200 kg/yr). The lowest single annual TCd load, 18 kg, was estimated at Amy Gulch in 2001. Results for 2001, the lowest-flow year, marked the lowest TCd loads at every station in the basin.
The maximum annual TCd load, 5,100 kg, was at Harrison in 1999. The same year also marked maximum loads at Pinehurst (3,400 kg), Post Falls (2,100 kg), and Elizabeth Park (1,800 kg) (table 3). TCd loads for the entire estimation period were high at these four sites.
Dissolved Cd typically ranged from about 70 to 100 percent of the TCd load (fig. 5). In general, the DCd/TCd ratio was higher at stations where ground water was a volumetrically important component of the streamflow, for example, on tributary streams farther upstream in the basin or at stations at the outlet of Coeur d’Alene Lake or Long Lake. DCd also is higher at most stations during low-flow years such as 2001, because of the greater contribution of ground water to streamflow. Relatively high DCd/TCd ratios (about 0.9 and greater) were estimated at stations 1 through 4. The lowest DCd/TCd was estimated at Harrison, where less than 70 percent of the TCd was in the dissolved state.
In a few instances, DCd loads exceeded TCd loads because DCd concentrations were reported to be slightly higher than TCd concentrations when Cd levels were at or near the minimum analytical detection limit. Again, these situations indicate that essentially the entire Cd load is in the dissolved state.
Stations 4 (Amy Gulch), 2 (Ninemile Creek), and 6 (Enaville) had relatively low mean annual estimated TPb loads: 1,700, 2,100, and 3,600 kg, respectively. The lowest single annual TPb load, 64 kg, was at station 4 in 2001. Minimum TPb loads for 1999-2004 occurred mainly in 2001 at most stations, although 2003 also was characterized by low TPb loads.
The highest overall mean TPb loads in the basin were at Harrison (station 7). Mean annual TPb loads at Harrison exceeded those at all other stations by a factor of 6 or more. The maximum annual TPb load in the basin, about 590,000 kg, was at Harrison for 2002. Water year 2002 also marked peak TPb loads at seven other stations in the basin, which may reflect flushing of accumulated Pb-laden sediment that was stored in the stream channels during the previous year.
The DPb/TPb ratio varied in time and space and reflected the relative contribution of ground water (high in DPb) to total streamflow and the presence (low DPb/TPb) or absence (high DPb/TPb) of Pb-carrying suspended particulate and (or) colloidal material. DPb/TPb ranged from less than 10 percent to nearly 30 percent of the TPb load. The highest DPb/TPb loads were at stations 8, 9, and 10 (downstream of Coeur d’Alene Lake) and at stations 1 and 2, all stations where ground water contribution to streamflow is high and suspended sediment tends to be low under normal flow conditions.
Zinc is the primary constituent of metal loads in the Spokane River basin, composing more than 90 percent of the total metal load at 6 of the 10 stations. Relatively low mean annual TZn loads occurred at Amy Gulch (14,000 kg) and at Enaville (17,000 kg). The low loads at these two stations likely reflect the low availability of zinc in the source areas upstream of these stations.
Mean annual loads of TZn at Harrison were about 510,000 kg/yr and far exceeded those at any other station. The single highest annual TZn load in the basin (760,000 kg) was at Harrison in 1999. High TZn loads also were measured at Pinehurst and Post Falls, with mean annual loads greater than 350,000 kg. Mean annual loads of about 265,000 and 280,000 kg of TZn were estimated at the stations at Coeur d’Alene Lake outlet and Long Lake, respectively.
The DZn/TZn ratio typically was greater than 0.8 at most stations, indicating that zinc occurs primarily in the dissolved state. Although TZn loads were relatively low in 2001 due to low streamflow, the ratio DZn/TZn was higher in 2001 compared to other years resulting from high concentrations of DZn in ground water and less dilution of ground water inflow from snowmelt runoff.
Mean total metal load (the sum of TCd, TPb, and TZn) for 1999-2004 ranged from about 15,000 kg/y at station 4 to about 750,000 kg/y at station 7 (table 3). The percentage of Cd of the total metal load at all stations was less than 1 percent. The percentage of Pb of the total metal load increased from about 6 percent at station 2 to about 32 percent downstream at station 7. Downstream of Coeur d’Alene Lake the percentage of Pb dropped to 2 to 4 percent at stations 8 through 10. Zn accounted for the highest proportion of the total metal load at all stations. In contrast to Pb, Zn regularly decreased from about 93 percent at stations 1 through 3 to about 68 percent at station 7. At stations 8 through 10, the proportion of Zn was the highest in the basin, about 96 percent.
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. Generally, the further downstream the station’s location in the Coeur d’Alene River basin, the higher the total metal load, because streamflow increases downstream. (Enaville is an exception; although it has relatively high mean streamflow, loads are relatively low because of low concentrations). Total metal loads at the three stations downstream of Coeur d’Alene Lake (stations 8, 9, and 10) are much less than metal loads at station 7. Only about 4,200 kg/y of metals enter Coeur d’Alene Lake from the St. Joe River via mean annual streamflow of about 2,900 ft3/s (Clark, 2002).
The decrease in metal loads at gaging stations downstream of Coeur d’Alene Lake likely is due to the well-documented process of retention of metals, particularly Pb, in lakebed sediments (Horowitz and others, 1995; Clark, 2002). This study did not consider metals in the St. Joe River, but because Pb is primarily carried by particulate matter, a substantial amount of Pb is likely being deposited in the lakebed sediments. The relative proportions, as well as quantities, of metals upstream and downstream of the lake support this inference. The relative proportions of Cd, Pb, and Zn change notably upstream and downstream of the lake. For example, the percentage of TPb of the total metal load ranges from 6 percent at station 2 to 32 percent at station 7. However, TPb is only 2-4 percent of the total metal load at stations 8 through 10.
Conversely, Zn, which generally occurs in the dissolved state, accounts for a much higher proportion of the total metal load at stations 8 – 10 than at station 7. Indeed, the DZn/TZn load ratio rises sharply from 0.68 at stations 7 to 0.97 at station 8. These data suggest that metals are affected by different storage and transport processes, and that water-quality data aid significantly in understanding these processes.
Clark (2002) used LOADEST software to estimate loads for WY 1999-2001 at many of the same sites studied in this report. The input calibration data used by Clark were identical to the calibration data used in this study for 1999-2001, but these data also included data for 2002-04. Comparing Clark’s results with those from this study is another opportunity to compare load estimates for the same time interval based on two different regressions.
Metal load estimates from this study for nine sites during 1999–2001 (produced by regressing data from 1999-2004) were compared with Clark’s (2002) estimates for the same interval (table 4). Overall, there was fairly good agreement between the two sets of results. However, notable discrepancies, including 2001 TPb results, were identified in estimates for Enaville. TPb annual load estimate from this study was higher than Clark’s estimate by a factor of almost 2. Smaller differences in TPb estimates for other years occurred at Harrison, Amy Gulch, and Elizabeth Park; most TPb discrepancies are negative, that is, estimates from this study were greater than Clark’s estimates. One important difference between the two time intervals is that very low streamflow in 2001 strongly influenced Clark’s results because that year’s data constituted a greater proportion of the total available data, whereas, 2001 data were a smaller proportion of the available data for this study. Because the software uses the relation between flow and concentration to perform the regressions, including the subsequent 3 years’ TPb and streamflow data probably resulted in a regression line with a steeper slope, causing estimated loads to be somewhat higher. In addition, this difference was probably more noticeable with Pb than with Cd or Zn because of the effect of streamflow on sediment transport and consequently, Pb load.
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