The Future of Water Quality in Coeur d'Alene Lake (2022) / Chapter Skim
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6 In-Lake Processes: Metals
6 In-Lake Processes: Metals
Pages 189-214
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From page 189... ...
Understanding the status of metals concentrations in the Lake today is an important context for interpreting trends into the future. IN-LAKE PROCESSES RELEVANT TO METALS Status and trends of metals in lakes are controlled by interactions among a variety of complex processes, including hydrology, internal hydrodynamics, thermal stratification, vertical movement of particles, biological productivity, and biogeochemical reactions in sediments and the water column (Davison, 1993; Nriagu et al., 1995)
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From page 190... ...
It is important to understand that "total" metal in the water column does not represent all of the metal associated with particulate material because of methodological constraints.1 Thus, actual percentages of dissolved to total metal are lower than those found in Table 6-1. Another way to get at the true distribution of metals in the Lake is to consider, as geochemists do, the distribution between total particulate metal and dissolved metal using an operational partitioning coefficient.
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From page 191... ...
For each of the lake locations considered below, the raw data are shown, both in graphical form from the past five years and in tabular form for the entire dataset. This is followed by a graph of monthly averaged dissolved zinc concentration data from the photic and bottom zones to reveal seasonal patterns, and finally by a formal trends analysis of dissolved zinc concentration in photic and bottom waters over the past 26 years.
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From page 192... ...
For example, dissolved zinc concentrations in the euphotic zone in 2004 were 15–34 mg/L higher than in 2020 during the same month. Table 6-2 shows that photic zone zinc concentrations are beginning, in the most recent years, to approach the Lake Management Plan target of 36 mg/L during summer stratification.
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From page 193... ...
Zinc concentrations in the photic zone increase at that time through mixing with the enriched bottom water. Averaged over the year, the differences in concentration between bottom water and photic zone water as of 2020 are 12.8 mg/L, with bottom waters having higher concentrations of dissolved zinc (as much as 25 mg/L higher in bottom waters than in surface waters toward the end of the period of stratification)
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From page 194... ...
period is −35 percent. The close correspondence between the rate of decline of the Lake concentrations and the river inputs suggests that the water column of the Lake is responding rapidly to the declining inputs of zinc from the CDA River.
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From page 195... ...
SOURCE: Data courtesy of IDEQ and plotted by the committee. Dissolved Zinc Concentrations at C1 Data for dissolved zinc in the photic zone at C1 are available for 2004 to 2020, but only data from 2015 to 2020 are shown in Figure 6-5.
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From page 196... ...
photic zone begin to decline, and bottom water concentrations begin to increase, in May, such that by September bottom water concentrations exceed photic zone concentrations by 20 mg/L. This difference narrows as the Lake mixes in October through December.
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From page 197... ...
SOURCE: Data courtesy of CDA Tribe and IDEQ and analyzed and graphed by the committee. Thus, while zinc concentration in surface water at C4 peaks in January before starting to decline, at C5 the period of elevated discharge has low dissolved zinc concentrations, consistent with the proximity of C5 to unenriched inputs (Zn < 5 mg/L)
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From page 198... ...
Joe River reduce zinc concentrations at C5. An increase in dissolved zinc at C5 begins, on average, in May for surface waters and in April for bottom waters, as the influence of the St.
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From page 199... ...
The coincident timing of different processes and uncertainties about hydrodynamics make it difficult to differentiate their relative roles in the increase of dissolved zinc concentrations in both photic and bottom waters at C5 through the summer/fall. Formal Trend Analysis of Dissolved Zinc Statistical trends over the 14 years of record (considering all months in each year through the full water column)
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From page 200... ...
Joe River clearly drives water column zinc concentrations down in spring at C5, but this effect is mitigated as discharge recedes, such that zinc concentrations increase throughout the summer. Although one can postulate that the source of this high dissolved zinc at C5 is internal to the lake, it is not possible to quantitatively differentiate the contributions from advection of zinc-rich water from the north; influx of zinc into bottom waters from sediments following low pH or low dissolved oxygen conditions; or biogenic recycling and release of zinc by decomposition of organic material.
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From page 201... ...
Dissolved Cadmium Concentrations at C4 The entire record for cadmium in the photic zone of C4 is shown in Table 6-5 as a heat map that shows the rapid declines through the late spring and summer as well as the rapid decline in the last few years of the record. Figure 6-10 shows the final six years of the photic zone data from C4.
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From page 202... ...
Surface water data are shown in blue, and bottom water data are shown in red. Smoothed estimates of the trends are shown with red and blue lines.
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From page 203... ...
. The ratio of photic zone concentrations to bottom water concentrations averaged about 0.85 but was slightly larger around 2010 (a value of about 0.93)
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From page 204... ...
Figure 6-16 shows the relationship between the monthly mean values of observed surface water concentration of total lead at C4 (shown as a time series in Figure 6-14) in relation to the estimated average daily flux into the Lake for the months for which there are surface water concentration data at C4.
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From page 205... ...
SOURCE: Data courtesy of IDEQ and plotted by the committee. TABLE 6-7 Concentrations of Total Lead in C4 Surface Waters by Month, in ug/L Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TPb flux 2004 2.9 5.1 2.3 1.1 0.7 0.6 0.6 1.9 69.0 2005 4.1 9.0 1.5 1.2 1.0 0.5 1.1 1.0 73.0 2006 6.1 7.0 5.9 2.0 1.3 0.6 256 2007 0.6 0.8 0.6 1.0 1.1 382.0 2008 4.0 61.7 7.2 2.4 1.5 1.2 378.0 2009 8.5 4.4 1.7 1.8 0.7 0.9 220.0 2010 1.8 2.7 2.6 1.8 1.2 0.8 0.8 1.3 65.0 2011 21.9 23.6 25.5 4.2 2.7 1.4 0.9 1.1 979.0 2012 3.8 28.7 14.7 4.8 2.7 1.7 1.0 493.0 2013 8.9 4.3 1.8 1.3 0.7 1.4 1.3 199.0 2014 29.7 8.5 3.6 1.4 1.4 1.0 1.2 1.3 460.0 2015 7.7 29.2 1.8 0.9 0.8 0.8 1.3 0.8 299.0 2016 4.1 1.8 1.0 0.8 0.6 0.5 1.2 254.0 2017 52.8 8.7 4.3 1.8 1.2 0.8 0.9 1.1 1.6 1009.0 2018 6.4 6.2 4.7 2.1 0.7 0.4 0.7 1.0 1.2 191.0 2019 3.4 3.5 1.6 0.9 0.6 0.7 0.6 1.4 0.7 62.0 2020 2.1 2.7 1.4 0.8 0.6 0.5 0.7 68.0 NOTES: The colors range from red for the highest values, through brown, then green, and then blue for the lowest values.
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From page 206... ...
SOURCE: Data courtesy of IDEQ (concentration) and flux based on USGS National Water Information System (NWIS)
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From page 207... ...
Smoothed estimates of the trends are shown by the red and blue lines. SOURCE: Data courtesy of IDEQ and analyzed and graphed by the committee.
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From page 208... ...
Total Lead Concentrations at C1 At C1 and farther from the mouth of the CDA River, the patterns of total lead seasonality and trends are similar to those observed at C4 in surface waters. Figure 6-18 shows the most recent six years of surface water data.
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From page 209... ...
The committee investigated the reason for the difference in lead concentration in surface versus bottom waters at C5. Given lead's chemistry and lack of biogenic cycling, the most likely process creating this difference is hydrodynamic advection of lead from the northern Lake, with desorption from suspended particles and/or upward transfer from the Lake bed to the water column being less likely processes.
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From page 210... ...
Smoothed estimates of the trends are shown by the red and blue lines. SOURCE: Data courtesy of CDA Tribe and plotted by the committee.
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From page 211... ...
Stratification makes little difference, as evidenced by similar total lead concentrations in surface water and bottom water TABLE 6-8 Summary Table for Trends in Total Lead at Sites C1, C4, and C5 Total record Change Change over past 2020 median 2020 median Site evaluated since start 10 years deep water surface water C1 17 years −11% −61% 0.5 mg/L 0.4 mg/L C4 17 years −41% −71% 0.9 mg/L 0.9 mg/L C5 13 years −42% −45% 1.0 mg/L 0.5 mg/L NOTES: See Appendix B for methods of calculation. The shading indicates the significance level of the result.
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From page 212... ...
Total lead concentrations in bottom waters exceed concentrations in surface waters after stratification begins, but dissolved lead concentrations are not different between bottom and surface waters for much of the summer/fall, consistent with the strong binding of lead to particulate material and a lack of benthic flux of lead from the sediments to the water column. Hence, where there are differences in dissolved lead between surface and bottom waters, it could be attributable to north-to-south advection of lead to C5 during the period of low discharge.
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From page 213... ...
For zinc, after the onset of stratification, dissolved zinc concentrations decline in surface waters at C1 and C4 but increase in bottom waters. At C5 in summer, zinc concentrations in bottom waters increase faster than in surface waters, such that bottom-water zinc concentrations at C5 eventually equal those at C4.
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From page 214... ...
2000. Benthic flux of metals and nutrients into the water column of Lake Coeur d'Alene, Idaho: Report of an August 1999 pilot study.
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