The Future of Water Quality in Coeur d'Alene Lake (2022) / Chapter Skim
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Pages 1-10
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The Plan was based on the assumption that increased nutrient loading from lakeshore development, land use changes in the basin, and other dynamics might pose a potential new threat to the Lake that could promote anoxic conditions in the bottom waters and release metals bound to Lake sediments that would then pose a threat to ecosystems and human health. Figure S-1 shows a map of CDA Lake, its major inflows (the CDA River and the St.
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2 THE FUTURE OF WATER QUALITY IN COEUR D'ALENE LAKE FIGURE S-1 IDEQ and CDA Tribe long-term monitoring locations in CDA Lake. SOURCE: Generated by the Committee using data from IDEQ, CDA Tribe, and the USGS National Map and associated datasets.
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. Chapter 7 reviews studies done to understand processes occurring in the Lake sediments to determine if deteriorating water quality conditions in the Lake might lead to metals release from the sediments.
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For zinc, remedial activities in the upper basin and particularly in the Box, including the continuous improvements at the Central Treatment Plant, have helped substantially lower concentrations and fluxes. Reductions of total lead fluxes from the South Fork of the CDA River were offset by processes in the lower basin that released lead between 2000 and 2010, such that present-day inputs to the Lake are still substantial.
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From page 5... ...
.3 Development of a powerful 3-D hydrodynamic and water quality model began over 15 years ago, but little has been done to use the model to better understand key processes within the Lake, the evolution of changes within the Lake, and the likely trajectory of the Lake under future climate changes. IN-LAKE PROCESSES: DISSOLVED OXYGEN AND NUTRIENTS Chapter 5 analyzes water column data from CDA Lake over the past 30 years to reveal trends in dissolved oxygen, nutrients, and lake productivity.
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From about 2003 to 2012, total lead concentrations in CDA Lake rose slowly but they have declined over the past eight to ten years. Most lead enters the Lake during periods of high discharge, such that the future trajectory of Lake lead concentrations will depend strongly on the extent of scouring of legacy sediment from the bed and banks of the CDA River.
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From page 7... ...
oxides having a dominant control on arsenic concentrations within the porewaters of Lake sediments, decreased oxygen concentrations leading to anoxia in bottom water could promote their reductive dissolution within the upper sediments and arsenic release into overlying Lake waters. A second threat of bottom water anoxia is release of phosphorus.
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From page 8... ...
FUTURE WATER QUALITY CONSIDERATIONS Chapter 10 reviews the recent climate history of the CDA region, examines climate projections that have been made through 2100, and considers how future changes in climate, population, and land use could affect the trends noted in Chapters 3–7. The chapter also projects long-term trends in metal enrichment in the water column of the Lake (assuming the rate of change in the past decade applies to the future)
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From page 9... ...
The mainstem CDA River and its watershed, along with the sediments of the Lake, contain an immense reservoir of sediment-bound metals. If changes in climate, population growth, or remediation activities result in greater metal inputs to the Lake or metal releases from Lake sediments, recovery of the Lake could be slowed or reversed.
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