The Future of Water Quality in Coeur d'Alene Lake (2022) / Chapter Skim
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10 Future Water Quality Considerations
10 Future Water Quality Considerations
Pages 303-338
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From page 303... ...
It then considers how future changes in climate, population, and land use could affect the trends noted in Chapters 3 through 7. Finally, the committee extends recent trends in metals concentration into the future to determine how much time will be needed before metals concentrations in CDA Lake fall below certain ecological thresholds.
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From page 304... ...
Specifically, the seasonal cycle of monthly average daytime maximum and minimum temperatures and monthly average precipitation volume, both averaged over the entire spatial region, were computed. Variability across all 20 climate model predictions was examined by computing the standard deviation of monthly average daily maximum temperature, daily average precipitation, and precipitation volume across the climate models.
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From page 305... ...
(B) The projected change from the historical monthly average daily maximum temperature.
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From page 306... ...
monthly average daily minimum temperatures are lowest in December and January and, across the region, average approximately −7°C, while this value is highest in July, at slightly less than 10°C. For the 2040–2060 period, model projections suggest that the monthly average daily minimum temperature in December would be approximately −4°C in the RCP4.5 scenario and slightly warmer yet in the RCP8.5 scenario.
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From page 307... ...
. These projected changes in average daily minimum and maximum temperatures in the region, particularly those associated with the RCP8.5 scenario, at century's end would be consistent with a "low- to no-snow future" that is of broader concern and study across the western United States (Siirila-Woodburn et al., 2021)
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From page 308... ...
(B) The projected change from the historical monthly average precipitation.
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From page 309... ...
Examining model projections for a moderate global greenhouse gas emissions scenario, the model suggests that, depending on the climate model used as input, peak snow water equivalent in the Pacific Northwest could decrease by 27–79 percent by the 2040s, with an average of a 51 percent reduction in snow water equivalent across all models. By the 2080s, the modeling experiments suggest that snow water equivalent could decrease by 44–96 percent depending on the specific climate model inputs to the Variable Infiltration Capacity model, with an average of a 73 percent reduction in peak snow water equivalent.
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From page 310... ...
. The choice of this number as a threshold is illustrated by looking at Figure 10-1-1, which shows total lead concentrations as a function of discharge for the period 2010–2020 for the CDA River near Harrison.
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From page 311... ...
(C) Time series of annual seven-day maximum discharge for the CDA River near Harrison, by water year 1996–2020; curve is a loess smooth of the time series.
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From page 312... ...
312 THE FUTURE OF WATER QUALITY IN COEUR D'ALENE LAKE B C FIGURE 10-1-2 Continued
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From page 313... ...
was used to route runoff simulated by each hydrologic model through the stream network. To the extent that there are clear hydrologic metrics that are informative about future water quality conditions in CDA Lake, these streamflow projections are a potentially valuable dataset for future analysis because they encompass a range of alternative future climate projections and hydrologic model configurations.
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From page 314... ...
Aquatic Thermal Regimes One of the major impacts of climate change on CDA Lake could be increases in lake temperature that would have a variety of biological, chemical, physical, and social effects both in the Lake and in the basin. Increased lake temperatures suggest that the duration of lake stratification will lengthen, which could promote longer p eriods of net oxygen consumption in bottom waters affecting both oxygen levels and pH at the Lake bottom and in underlying sediments (see Chapter 5)
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From page 315... ...
Thus, it is clear that surface water temperatures have been increasing at station C4. It is reasonable to assume that this is related to the general trend toward rising air temperatures, and this can be expected to increase in the future based on the expected increase in global greenhouse gas forcing of the climate.
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From page 316... ...
In terms of the physical impacts of lake warming, lake surface energetics are relatively well understood, and quantitative estimates of lake warming and stratification could be made for a range of future climates. Used in conjunction with the three-dimensional model of CDA Lake (Chapter 4)
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From page 317... ...
The committee asked Erin Brooks from the University of Idaho to opine on the potential for an increase in forest fires in the CDA region that might affect water quality in CDA Lake, summarized below (Brooks, 2021)
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From page 318... ...
. There are fewer potential impacts of forest fire on drinking water in the CDA region, given that CDA River and Lake are not primary sources of drinking water for the region's population.
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From page 319... ...
FUTURE WATER QUALITY CONSIDERATIONS 319 FIGURE 10-7 Linear trends in wetting rain days and area burned for forested areas in the western United States from 1984 to 2015. SOURCE: Holden et al.
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From page 320... ...
The CDA region is just west of an area of Idaho and Montana that suffered more than 3 million acres burned by forest fire in 1910. Dubbed the Great 1910 Fire, the conflagration decimated the town of Wallace and other areas.
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From page 321... ...
; and dissolved oxygen, nutrient, and metals concentrations in CDA Lake (Chapters 5 and 6)
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From page 322... ...
, will strengthen and lengthen 3 This threshold could only be derived for the St. Joe River due to lack of continuous time series data on the CDA River (see Chapter 4)
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From page 323... ...
. The fourth climate effect that could impact CDA Lake water quality is an increased frequency and size of forest fires (Gray Box 4)
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From page 324... ...
TABLE 10-2 How Climate Change Will Impact Water Quality Trends Noted in Chapters 3 Through 7 324 CLIMATE CHANGES AND OTHER IMPACTS OF CONCERN TO WATER QUALITY (GRAY BOXES) 5. Increased Conclusions from 1. Increased magnitude and frequency 2. Forward shift in lakeshore Chapters 3–7 of high runoff events timing of flow 3. Warmer lake 4. More fires populations 3.1 Land cover has No effect No effect No effect Could burn No effect changed from barren revegetated areas to revegetated 3.4 Loading of Pb Could slow or reverse trend by No effect No effect Could increase No effect has decreased mobilizing sediment and associated Pb delivery of Pb enriched sediments 3.5 Loading of Could slow or reverse trend by No effect No effect Could increase No effect sediment has mobilizing sediment across the CDA delivery of sediments decreased basin 3.6 Loading of Cd Could slow or reverse trend by No effect No effect Could increase No effect has decreased mobilizing sediment and associated delivery of Cd metals but less effect than for Pb/P enriched sediments 3.6 Loading of Zn Could slow or reverse trend by No effect No effect Could increase No effect has decreased mobilizing sediment and associated delivery of Zn metals but less effect than for Pb/P enriched sediments 3.7 Loading of P has Could slow or reverse trend by No effect No effect Could increase Could increase decreased mobilizing sediment and associated P delivery of sediments bioavailable P and P loading near the Lake 4.2 The date when No effect Threshold reached Enhanced No effect No effect St.
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From page 325... ...
could increase reverse and [total Pb] over the past could increase 8–10 years 7.3 Risk that No effect Will increase/ Increase risk: Same as column 3 Same as column 3 enhanced anoxia will enhance risk of As Increased DO utilization leads to for a warmer lake for a warmer lake release arsenic release increased biomass/organic carbon, which favors enhanced anoxia; lower DO leads to greater anoxia; N and P increase at sediment– water interface 7.4 Risk that No effect Will increase/ Increase risk: Same as column 3 Same as column 3 enhanced anoxia enhance P release Increased DO utilization leads for a warmer lake for a warmer lake will release P, which to increased biomass/organic through feedback carbon, which favors enhanced loop could release anoxia; lower DO leads to greater more arsenic anoxia; N and P increase at sediment–water interface 7.5 Risk that No effect Potential for increase Unclear effect on risk: Same as column 3 Same as column 3 decrease in pH will in period over which N-fixing cyanobacteria could lower for a warmer lake for a warmer lake drive Zn release pH can decline pH at the sediment–water interface resulting in Zn 1. Ammonia to nitrate → decrease release in pH 2. Denitrification with organic matter (N03 → N2)
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From page 326... ...
that have accompanied decreases in phosphorus loading to CDA Lake. Likewise, dissolved oxygen conditions in the Lake are also showing signs of improvement over time (Table 5-5)
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From page 327... ...
. A forward shift in the timing of flow and a warmer lake, as well as population growth in lakeshore areas, can eventually lead to eutrophication and a lowering of dissolved oxygen and pH in the lake bottom waters, which could enhance internal loading of dissolved zinc and cadmium (but not lead, owing to its higher affinity for iron mineral phases and particulate organic matter)
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From page 328... ...
Rather, mixing of high metal loads from the CDA River with lake water, lake sediments, and biologically generated particulate material will determine the concentration of metals in the water, suspended particulates, and sediments of CDA Lake. Exchange of metals between sediments and water are driven by concentrations, and organisms respond to metal concentrations in water and particulate material rather than loads (Luoma and Rainbow, 2008)
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From page 329... ...
Here, the concentrations appear to be responding to internal cycling processes rather than responding to the on-going downward trends driven by the downward trends in the external loading to the Lake. The bottom water TABLE 10-3 The Year in Which Dissolved Lead Concentrations Will Fall Below a Target of 0.54 mg/L, Assuming the Trend from the Past Ten Years Continues Surface Water Deep Water Site Spring Summer Spring Summer C1 C4 2023 2025 C5 C6 NOTES: A dark blue entry indicates that the median curve lies below the critical value in 2020, and the curve is declining over time.
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From page 330... ...
TABLE 10-6 The Year in Which Dissolved Zinc Concentrations Will Fall Below a Target of 5 mg/L, Assuming the Trend from the Past Ten Years Continues Surface Water Deep Water Site Spring Summer Spring Summer C1 2080 2088 2102 2112 C4 2075 2069 2082 2129 C5 2045 NOTES: A dark blue entry indicates that the median curve lies below the critical value in 2020, and the curve is declining over time. A number indicates that the median curve is declining over time, and the number represents the year in which this curve would fall below the critical value.
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From page 331... ...
Dissolved Zinc, Spokane River below Lake Outlet For the spring season, the flow-normalized mean concentration in 2020 is 39.3 mg/L, and the ten-year trend is −2.9 percent per year, a highly significant downward trend. The dissolved zinc concentration would fall below the 36 mg/L LMP target in 2024, it would cross 15 mg/L in 2054, and it would cross 5 mg/L in 2092.
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From page 332... ...
Dissolved Zinc, CDA River near Harrison For the spring season, the flow-normalized mean concentration in 2020 is 98 mg/L, and the ten-year trend is −3.2 percent per year, which is a highly significant downward trend. The concentration would cross below 36 mg/L in 2052, below 15 mg/L in 2079, and below 5 mg/L in 2113.
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From page 333... ...
As discussed in Chapter 7, such exchange is largely driven by redox processes that are controlled by dissolved oxygen levels and the potential for lower pH in bottom waters that might accompany longer periods of stratification or greater productivity in surface waters. This section considers the potential for lake sediments to become a source of zinc as zinc loading to the Lake is reduced (as a result of remediation in the basin and other factors)
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From page 334... ...
As described in previous chapters, there is a large reservoir of zinc in the sediments of CDA Lake. The Lake serves to remove zinc from the water column, and as shown in Chapter 3, fluxes leaving the Lake are lower than those entering it.
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From page 335... ...
and the trends in dissolved oxygen, phosphorus, and metals concentrations within CDA Lake (discussed in Chapters 5 and 6) , and it may increase the potential for metals release from lake sediments (discussed in Chapter 7)
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From page 336... ...
As shown in Chapter 3, CDA Lake is still a net sink for zinc. If zinc concentrations in the water column decrease owing to decreased loading from the basin, there is the potential for release of zinc from the Lake sediments to the water column, which will depend on the concentration gradient as well as the accessibility of the zinc in the sediments to the overlying water.
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From page 337... ...
1994. A simple hydrologically based model of land-surface water and energy fluxes for general-circulation models.
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From page 338... ...
2015. Changes in winter atmospheric rivers along the North American west coast in CMIP5 climate models.
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