Summary
Coeur d’Alene Lake (CDA Lake, or the Lake) in northern Idaho is an invaluable natural, recreational, and economic resource to residents of Idaho and eastern Washington. The 3,740-mi2 watershed that drains into the Lake can be divided into five major subbasins: the North and South Forks of the CDA River (collectively called the upper basin), the lower CDA River basin, the St. Joe River basin, and the nearshore subwatersheds directly surrounding CDA Lake. The entire watershed lies within the homeland of the Coeur d’Alene Tribe (the CDA Tribe). Starting in the late 1880s, the area along the South Fork of the CDA River (the Silver Valley) was mined for lead, silver, and zinc. Mineral extraction and beneficiation wastes, laden with heavy metals, were discharged to the South Fork of the CDA River and flowed downstream, subsequently contaminating more than 75 million metric tons of CDA Lake sediments with lead, cadmium, arsenic, and zinc. Although mining activities have declined significantly and metal inputs to CDA Lake have also declined, metal concentrations in CDA Lake and its sediments remain at or above ambient water quality standards set by the State of Idaho and the CDA Tribe, with arsenic, cadmium, lead, and zinc concentrations orders of magnitude higher than in most lakes in the United States.1
Coincident with the diminution of mining, in 1983 the Bunker Hill mining district in the Silver Valley was designated as a regulated hazardous waste site under the nation’s Superfund law, and remediation of the affected parts of the CDA basin began. Although all mineral extraction activities were located ~60 km or more upstream, the Lake continues to be a repository for wastes heavily contaminated with metals. Nonetheless, CDA Lake itself was not included as a target of remediation under Superfund. Rather, protection of water quality in the Lake was left to a Lake Management Plan to be implemented by the CDA Tribe, which owns the bed and banks of the southern third of the Lake, and the State of Idaho, which controls the northern two-thirds. 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. Joe River) and outflow (the Spokane River), and the monitoring stations where water quality data have been collected variously over the past 30 years by the U.S. Geological Survey (USGS), the Idaho Department of Environmental Quality (IDEQ), the CDA Tribe, the U.S. Environmental Protection Agency (EPA), independent researchers, and others. Despite sustained efforts to collect data on various water quality parameters in the Lake and watershed, comprehensive analyses of the monitoring data and explicit testing of hypotheses are rare. Given the uncertainties in the data
___________________
1 Text here and throughout the report was modified after report release to clarify that mining has not completely ceased in the CDA watershed.
analysis activities, along with concerns of various stakeholder groups, in late 2020, IDEQ, EPA, and Kootenai County requested that the National Academies of Sciences, Engineering, and Medicine analyze available data and information about CDA Lake water quality and provide insights about future Lake conditions. The statement of task for the National Academies’ study is found in Box S-1.
Chapters 1 and 2 introduce CDA Lake and its watershed and current long-term monitoring efforts. Chapter 3 reviews the watershed’s historical mining activities, the Superfund remedy, and land use changes, and it analyzes 30 years of data on metal and nutrient inputs to the Lake from the watershed. The next three chapters evaluate in-lake data relevant to the committee’s charge to assess trends over time, including physical data (Chapter 4), dissolved oxygen and nutrient data (Chapter 5), and metals data (Chapter 6). 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. Given the analyses of the previous four chapters, Chapter 8 describes gaps in long-term monitoring that should be filled in order for water quality in CDA Lake to be adequately and continuously assessed into the future. Chapter 9 reviews the human health and ecological risks posed by metals in CDA Lake. Finally, Chapter 10 discusses the future of water quality in CDA Lake, including climate change and how it may alter the trends observed in Chapters 3 through 7. Each chapter ends with conclusions and (where appropriate) recommendations that synthesize more technical and specific statements found within the body of each chapter. Because of the extensive data analyses in this report, not every conclusion leads to a recommendation. Several key conclusions and recommendations are compiled in this summary.
ANALYSIS OF INPUTS TO COEUR D’ALENE LAKE
Inputs of lead, cadmium, and zinc to CDA Lake reflect the century-long legacy of mine waste deposition in the Lake’s watershed. The frequent floods that transport mining wastes downstream and the minimal dilution that occurs between the source of the primary contamination and the Lake contribute to ongoing metal loading to the Lake. In Chapter 3, the committee analyzes inputs to the Lake, focusing on the two major river systems that contribute 84 percent of the inflow—the CDA River and the St. Joe River. The trends observed in the river monitoring network data over the past 30 years provide clues about the effectiveness of the Superfund cleanup efforts in the upper basin and can help prioritize and plan for future remediation efforts. Although the Superfund remediation has reduced metal inputs from the upper basin via stabilization of the landscape, capping, and sequestration activities,
the lower basin comprises an immense stockpile of metal-enriched particulates poised for transport to CDA Lake. The following conclusions about trends in inputs to CDA Lake are found in Chapter 3.
Cadmium, lead, and zinc concentrations and loads into the mainstem CDA River from the South Fork have declined over the past 30 years, and Superfund activities have likely contributed to this decline. For the South Fork of the CDA River at Elizabeth Park, fluxes of the three metals have declined since the early 1990s, with 2020 values being 40 to 50 percent of their maximum. Similarly, at the South Fork near Pinehurst, just downstream of the Box,2 fluxes of zinc and cadmium have declined more than 60 percent since 1992, while the decline in lead flux has been about 80 percent. Stabilization of the landscape, capping, and sequestration activities have likely been effective at reducing fluxes of particle-associated lead. 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. Overall, lead flux to the Lake at Harrison was still 1.3 times higher in 2020 compared to the 1990s because of the increase in fluxes between 2000 and 2010. In 2020, lead fluxes into the lower basin at Pinehurst were only 2.6 percent of lead flux to CDA Lake at Harrison, demonstrating that there are large reservoirs of metals in the river sediments and floodplains of the lower basin. Future decreases in lead fluxes into the Lake will be determined more by evolving storage and release mechanisms in the lower basin than by further efforts to reduce lead flux from the South Fork watershed. The committee’s analysis of total lead in high-flow discharges at Harrison shows that lead concentrations in these flows have decreased over time, suggesting that remediation is having a beneficial effect. Remediation of the lower basin will require careful planning so as not to remobilize metals and increase their transport to the Lake.
There has been a downward trend in both zinc and cadmium concentrations and fluxes throughout the CDA basin, and fluxes of both metals to the Lake (measured at Harrison) were lower in 2020 than in 1992 (by 63 and 45 percent, respectively). At the CDA River at Harrison, the cadmium and zinc fluxes leveled off during 2000–2010 but are declining again in the most recent decade. Unlike total lead, as of 2020 the inputs of total cadmium and total zinc from the South Fork of the CDA River to the lower CDA River were 43 and 44 percent (respectively) of the outputs of the lower CDA River to the Lake. This suggests that further reductions in cadmium and zinc coming from the South Fork are likely to be important to reducing inputs of these metals to the Lake. Targeted studies and trend data show that the primary sources of cadmium and zinc are now base flow, presumably coming from the groundwater system. The Central Treatment Plant is now a minor source of zinc.
Over the past decade, total phosphorus fluxes and concentrations at monitoring sites in the CDA River, the St. Joe River, and the Spokane River below the Lake outlet have all been declining (typically 20–30 percent reductions during the 2010–2020 decade). In the case of the CDA River, this is a reversal of the trend observed over the prior decade. Like lead, total phosphorus flux to the Lake in 2020 was higher (by 2.3 times) than in the early 1990s. Projecting future trends of phosphorus in Lake CDA will require a sustained effort at monitoring and regular data synthesis for phosphorus across the whole watershed, with monitoring efforts closely connected to research aimed at understanding the reasons for this current decline. Without a better understanding of the history of phosphorus transport in the whole watershed, there is no basis for projecting future phosphorus transport or the potential for future increases in phosphorus loading to the Lake.
IN-LAKE PROCESSES: HYDRODYNAMICS
Seasonal and long-term water quality trends within a lake are the result of the interactions among the key physical, chemical, and biological processes and the associated process drivers that alter inputs, outputs, and internal dynamics within the lake. The main high-level processes relevant within CDA Lake include the lake hydrodynamics, sediment transport, biogeochemical processes, and primary production—with the first two being covered in Chapter 4. The CDA River provides the main source of particulate and dissolved metals, while the cleaner St. Joe River dilutes metal concentrations in the southern reach of the Lake during the spring. Internal lake processes, including in-lake hydrodynamics, biogenic recycling, benthic flux from the Lake sediment to bottom waters, and physical processes
___________________
2 The Box is an area of the Superfund site 3 miles (4.8 km) wide and 7 miles (11.2 km) long, from Kellogg on the eastern end to Pinehurst on the western end, that had the most intense mining activity.
such as sediment transport all may contribute to elevated concentrations later in the year. Internal hydrodynamics have redistributed particulate metal contamination through the northern and southern Lake, as evidenced by widespread sediment contamination. In Chapter 4, the committee analyzed physical data from the watershed and CDA Lake, along with special studies on modeling and sediment transport in CDA Lake, and makes the following conclusions.
The limited data available on specific conductivity in the river inflows and CDA Lake showed that water from the St. Joe River entered CDA Lake as an interflow, meaning that, because of its density relative to the stratification of the Lake, it intrudes below the thermocline but above the Lake bottom. The CDA River inflow, on the other hand, is classified as an overflow, intruding into the epilimnion above the thermocline. Hence, the two major river inflows are not likely to play a major role in resuspending sediment within CDA Lake. Sediment resuspension could, however, occur in the littoral zones of the Lake due to other factors. The identification of those areas most subject to sediment resuspension, together with the relevant water quality impacts, can only be ascertained with a nearshore monitoring program, something that currently does not exist.
The committee’s analysis of conductivity data found that a St. Joe River inflow of 1,000 cubic feet per second (cfs) is a threshold below which thermal stratification commences and internal lake processes can predominate. At river inflows above 1,000 cfs, which occur during winter and early spring, river discharge controls water quality (dissolved oxygen and pH) in CDA Lake, and the Lake behaves like a run-of-the-river system with little opportunity for biogeochemical processes to become established. At St. Joe River inflows below 1,000 cfs, which generally occur in June or later, internal dynamics and thermal stratification become important for CDA Lake, especially at site C5. It is during this period that water column processes such as nutrient uptake, phytoplankton proliferation, and decomposition can happen.
There is a critical lack of deterministic model usage for CDA Lake—for both heuristic and predictive purposes—despite having invested in the first stages of developing such a model (ELCOM–CAEDYM).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. The chapter also specifically addresses the item in the statement of task that asks whether reduced levels of zinc are removing an important control on algal growth. The following conclusions about trends in nutrient and dissolved oxygen concentrations in CDA Lake are found in Chapter 5.
For the most recent decade, the data indicate declines in total phosphorus concentration in CDA Lake, although this decline was not statistically significant in the northern lake. Declines in total phosphorus in the Lake are consistent with the declines in total phosphorus from the two major rivers entering the Lake. The ability to project future changes in phosphorus inputs and concentrations in the Lake will depend on research aimed at better understanding the causative mechanisms of these recent trends in phosphorus delivery from the watershed to the Lake. Trends in total nitrogen concentration in the Lake are more complex than those for total phosphorus. Gaining a better understanding of the nitrogen cycle, and analyzing trends of nitrogen concentration in the Lake and rivers, is an important topic that needs to be included in the research agenda supporting adaptive management of lake trophic state.
The evidence that dissolved oxygen concentrations are worsening in the bottom waters of the C1, C4, C5, and C6 stations is equivocal at best. Rather, there is an increasing dissolved oxygen concentration trend particularly evident at C5.4Coupled with the recent trends in phosphorus concentrations in the Lake and phosphorus loading to the Lake, low dissolved oxygen is not a current problem in the main body of the Lake,5 nor is it expected to become a problem if current trends continue. However, if climate change were to strengthen
___________________
3 ELCOM-CAEDYM is the Estuary, Lake and Coastal Ocean Model–Computational Aquatic Ecosystem DYnamics Model.
4 This conclusion was edited after report release to reflect only the dissolved oxygen trend analyses shown in the report, which were for bottom waters.
5 As discussed in Chapter 1, the main body of the Lake is characterized by the primary long-term monitoring stations C1 through C5; this footnote was added after report release.
thermal stratification substantially in the future, dissolved oxygen concentrations at the sediment–water interface would likely decrease.
The available field evidence does not support the concept that the current high zinc concentrations in the Lake suppress chlorophyll a. CDA Lake supports lower amounts of chlorophyll a per unit phosphorus than generally observed for lakes worldwide. Although this is consistent with zinc suppression of phytoplankton biomass, beyond this observation there is little evidence from more detailed analysis of field data that the current high zinc concentrations suppress chlorophyll a. For example, chlorophyll a levels are disproportionately lower than expected in the southern part of the Lake in both zinc-enriched and zinc-poor locations. Consistent with this, multiple regression analysis relating chlorophyll to total phosphorus and total zinc finds a strongly positive and statistically significant association for total phosphorus but a highly nonsignificant association for total zinc. Further research involving field experimentation would help develop greater confidence in predicting the response of Lake chlorophyll concentrations and particular taxa of phytoplankton to potential reductions in legacy metal contamination in the Lake.
IN-LAKE PROCESSES: METALS
Chapter 6 focuses on the concentration of heavy metals of concern, particularly lead, zinc, and cadmium, in CDA Lake. In addition to revealing trends in metals concentrations over the past 30 years, the committee attempted to elucidate seasonal trends that are indicative of various processes occurring in the water column, such as hydrodynamics, benthic flux, and biogenic cycling. The following conclusions about trends in metals concentrations in CDA Lake are found in Chapter 6.
Downward trends in dissolved zinc concentration in CDA Lake at sites C1 and C4 over the past 30 years are highly significant. Furthermore, zinc concentrations at these sites are decreasing at a similar rate to declines in zinc inputs from the CDA River. Differences in dissolved zinc concentrations between surface waters and bottom waters during stratification suggest inputs to bottom waters from internal sources are occurring. But internal cycling of zinc has not detectably slowed the response of the northern Lake to changes in zinc input driven by remediation in the CDA basin. The dynamics of zinc at C5 (where zinc concentrations are declining more slowly) are more complex than at C1 and C4 and reflect spring dilution from the St. Joe River, summer/fall hydrodynamic redistribution of metals from north to south, and other internal processes.
Dissolved cadmium concentrations at C1 and C4 declined from 2004 to 2020, with virtually all of the decline occurring after 2014. Dissolved cadmium concentrations in deeper water (> 20 m) are greater than those in shallower water, by about 15 percent, suggesting that internal processes affect dissolved cadmium concentrations (as they do for zinc). Lake cadmium trends are highly responsive to changes in cadmium inputs from the CDA River.
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. Despite thermal stratification in summer, there were only small differences between total lead concentrations in surface and bottom waters at C1 and C4. This suggests minimal flux of lead from internal sources and is consistent with strong lead binding to particulates and the lower bioavailability of lead to phytoplankton compared to zinc and cadmium.
LAKE BED PROCESSES
Chapter 7 examines the (bio)geochemical reactions that control the partitioning of lead, cadmium, zinc, and arsenic between Lake sediments and porewater, ultimately leading to the specific conditions of CDA Lake. Although data available to analyze were sparse, the committee examined metal deposition and migration within the Lake sediments and the operative processes controlling dissolved porewater metal(loid) concentrations and possible entry into Lake waters. It also assessed the potential for eutrophication and/or pH changes to release metals from Lake sediments into the water column. The following conclusions about processes in Lake sediments, including possible metals release to the overlying water column, are found in Chapter 7.
Iron(III) (hydr)oxides are an abundant and dominant control on metal and arsenic concentrations in the Lake sediments (at the sediment–water interface). With the pH conditions of the sediments, the Fe(III) oxides serve as principal adsorbents of arsenic, cadmium, lead, and zinc that regulate dissolved concentrations as shown through measurement and modeling. Due to the abnormally high metal concentrations (particularly iron, which makes up more than 10 percent of the solids) within the sediments, sulfur occurs in quantities insufficient to control the full suite of heavy metal concentrations and may exert local and selective controls, but not universal control, on metal retention in the sediments.
The greatest threat of enhanced anoxia, if it were to occur, is release of arsenic into the Lake water column; however, there is no evidence that anoxia is getting worse. With As(V) adsorption on Fe(III) 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. Although not redox active itself, phosphate is largely bound to Fe(III) oxides, and anoxia leads to the reductive dissolution of the Fe(III) phases and thus the concomitant release of phosphate. If the Lake is phosphorus limited, it is possible that such release of phosphorus from the Fe(III) oxides could create a feedback loop that further promotes biological productivity, anoxia, and arsenic release from the sediment.
Because the adsorption edge for zinc on iron oxides occurs in the pH range of 6.0–7.0 (typical of CDA Lake), local lowering of pH can cause release of dissolved zinc from the sediment. A decline in pH to less than 7 occurs in bottom waters of CDA Lake in some locations and some years, with the onset of stratification. Of the metal(loid) contaminants, zinc has the highest dissolved concentration within the sediment porewater, while cadmium is present at much lower concentrations than zinc. Upward diffusion of zinc from porewaters and desorption from surficial layers of the sediment is possible at pH less than 7. Less of a concern for zinc release would be an alteration in oxygen concentrations within the bottom waters and top few centimeters of the sediment. Lead partitions strongly to Lake sediments both under oxygenated and anoxic conditions and is less likely than zinc or cadmium to be released from sediments into the water column.
GAPS IN WATERSHED MONITORING
Chapter 8 considers improvements in several aspects of the Lake and river monitoring programs, such as where monitoring occurs, which analytes should be monitored (including questions of detection and precision), how samples should be collected and how many, and when samples should be collected. The program would benefit by adding monitoring for physical and ecological parameters and taking additional sediment cores (including porewater) as well as by improving methods for sampling of phosphorus and metals. The following conclusions and recommendations about long-term monitoring are found in Chapter 8.
Understanding the water quality of CDA Lake would be improved by increasing the spatial and temporal intensity of sampling in the Lake and the rivers. With respect to Lake assessments, the Lake program would benefit by expanding to encompass littoral areas, where the greatest interactions with the public occur and watershed impacts manifest. A strategy to increase temporal resolution in Lake sampling can be achieved by implementation of carefully chosen sensors targeting physical (temperature), chemical (oxygen), and biological (chlorophyll, fluorescence) parameters of interest. For the rivers, the sampling strategy needs increased frequency, particularly at the upstream and downstream ends of the lower basin, with continued attention to sampling high-flow events. Use of new continuous monitoring strategies, particularly for turbidity, can be of great value to the estimation of transport into and out of the lower CDA River reach.
An efficient sampling strategy designed to better understand inputs of nutrients from the lakeshore tributaries would benefit Lake management as the population in nearshore areas grows. Since the time this study began, the IDEQ has initiated a strategy to fill this information gap. It will be crucial after a few years of data have been collected to undertake and publish a synthesis of what can be learned from this monitoring of the Lake’s tributaries, and to use that to improve the monitoring network and ongoing analyses. The goal would be to use these new data and geospatial landscape and development data to produce estimates of nutrient fluxes across the majority of the previously unmonitored lakeshore watersheds.
Important ecological components of the Lake are understudied. Targeted expansion of ecological monitoring beyond the phytoplankton community could help identify how ecological processes in the Lake are responding to changing metal and nutrient concentrations. Obvious questions relate to the effects of the legacy of metal enrichment on the pelagic and benthic food webs as well as the status of food webs in parts of the Lake with different exposures to metals.
The agencies involved in data collection are encouraged to provide a mechanism to make the relevant data available to the wider community of stakeholders, agencies, and scientists. The river data are already available through such a system, but the Lake data are not. Furthermore, to succeed at adaptively managing the Lake for decades into the future, a scientific and institutional structure for carrying out data synthesis, coordinated among jurisdictions and interest groups, is needed. The required synthesis tasks include regularly evaluating mass balances, relating concentration trends to Lake processes and inputs, generating hypotheses about system drivers, and periodically evaluating the ecological and human health implications of the findings.
RISKS OF METALS CONTAMINATION IN COEUR D’ALENE LAKE
Human exposures and ecological risks associated with CDA Lake itself have not been the subject of comprehensive study compared to the systematic evaluation of risks in the basin upstream of the Lake. Trends in the basin indicate that human exposures to metals like lead have declined, but further assessment of human health risks from occupational, recreational, and subsistence-living exposure to lead and arsenic in the Lake could benefit the region. To better assess ecological risk of metals in CDA Lake, it is important to advance the body of knowledge across multiple levels of biological organization. Identifying present-day ecological implications of the legacy of metal contamination in CDA Lake is a first step toward addressing the future viability of ecosystem services, such as biodiversity, ecological functions, fisheries, wildlife, and support for activities ranging from recreation to a subsistence life style. The following conclusions about better understanding human health and ecological risk from CDA Lake metals are found in Chapter 9.
Expansion of existing monitoring to include a few sensitive nearshore environments could provide an early warning system for the onset of harmful algal blooms and expansion of nuisance-attached algae and of invasive plants. Although on a lake-wide basis the Lake remains oligotrophic with some mesotrophy in the south, experience elsewhere suggests the first signs of changes in trophic status can occur in nearshore, local waters in the form of blooms of attached algae. Expanded lakeshore monitoring could aid in detecting those changes before they become widespread.
Systematically developing a body of knowledge on how CDA Lake food webs are influenced by the legacy of mineral extraction will inform decisions about remediation and efforts to maximize ecosystem services. High priorities for better understanding ecological processes in the Lake include (1) expanded characterization of benthic and pelagic food webs; (2) evaluation of metal exposures in key components of the food web, and (3) experiments with benthic and water column mesocosms to identify thresholds below which the Lake ecosystem will improve.
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) in order to provide a context for the progress that has been made to date in reducing inputs.
A major impact of climate change likely to affect water quality in the CDA region is air temperature warming as much as 2.5–3°C (4.5–5.4°F) by the year 2050, depending on the month. Data from the CDA region over the past 30 years show warming of about 0.4°F per decade. Increases in air temperature are expected to increase lake temperature and increase fire risk across the region.
Although there are no apparent precipitation trends in the CDA region over the past 30 years, studies in the greater Pacific Northwest suggest that extreme precipitation events will become 5–34 percent more intense by 2080. A shift is expected in the percentage of precipitation that falls as snow versus rain, such that by 2080 the peak snow water equivalent could decrease by an average of 73 percent. Finally, in the Pacific Northwest, the center of timing of the annual hydrograph is predicted to shift from April 15 in the 1980s to as early as March 4 by 2040, although there is not yet evidence of this shift over the past 30 years in the CDA region.
Lake water temperatures have been increasing in surface water at station C4 over the past 30 years (although not in bottom water). It is reasonable to assume that this warming is related to the general global trend toward rising air temperatures, and this can be expected to increase in the future based on ongoing greenhouse gas forcing of the climate. Responses in the Lake to such warming will likely include increased rates of ecosystem metabolism and lengthening of the duration of lake stratification, which may extend periods of dissolved oxygen consumption in the bottom waters and sediments of the Lake as well as lower pH in the bottom waters.
Future climate change may slow or reverse the trends in metals and phosphorus loading to CDA Lake (discussed in Chapter 3) 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). The changes in climate considered by the committee were (1) increased frequency and magnitude of large runoff events, (2) a forward shift in the timing of flow to the Lake, (3) warming of Lake water, and (4) increased frequency and size of fires, along with increases in lakeshore populations. Although they are predicted to occur for the Pacific Northwest, there is not yet evidence in the CDA region of climate effects (1), (2), and (4)—emphasizing the importance of monitoring, data analysis, and further process studies into the future.
Zinc concentrations in surface waters are at or approaching the Lake Management Plan (LMP) target of 36 μg/L in some months and at some locations. If trends from the past decade continue into the future, it will take bottom waters 10 to more than 100 years to reach that target. The slowest changes are occurring in the southern Lake, where the response to declining inputs appears to be buffered by internal inputs from bottom sediments and hydrodynamic inputs from the northern Lake. Dissolved lead concentrations in CDA Lake are already below the LMP target of 0.54 μg/L in the measured Lake locations, with the exception of Site C4 during the spring. Cadmium concentrations are also already below the LMP target of 0.25 μg/L in the three measured Lake locations (C1, C4, and C5).
***
In summary, CDA Lake is beginning to recover from the waste inputs of more than a century of mineral extraction and beneficiation in its watershed. Data from the past 25 years do not show evidence of increasing eutrophication in the main body of the Lake. However, conditions are undocumented in shallower waters of nearshore areas where early signs of deteriorating water quality might first appear. The evidence from available field data does not support the concept that declining zinc concentrations have enhanced eutrophication over the past 15 years; detailed monitoring and experiments could provide further clarification. Better understanding of how metals are affecting the Lake ecosystem is necessary to address the question of whether further reductions in concentrations of zinc and cadmium in the water column of the Lake are more likely to result in ecosystem benefits than in greater risks to the ecosystem.
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. The processes controlling metal and nutrient inputs, flux from sediments, and cycling within the Lake are generally understood, but they are complex, they differ among contaminants, and important details are missing. The Chapter 10 projections of future water quality in CDA Lake, which assume that trends from the recent past carry forward, suggest that recovery from the legacy of mining will be a decades-long project at best. The committee recognizes that such projections provide an invaluable perspective on the task that lies ahead, but are rarely completely correct. The best preparation for the future will involve fortification and expansion of monitoring to provide an early warning of deteriorating conditions, regular syntheses of data, and targeted studies—all coordinated among interest groups—followed by application of those results to managing the Lake as the future unfolds.
