Recovery of ACIDIFIED European Surface Waters RICH ARD F. W RIGHT THORJØRN L ARSSEN NORWEGIA N INSTITUTE FOR WATER RESEARCH LLUIS CA M ARERO CENTRE OF ADVA NCED STUDIES (SPAIN) BERNARD J. COSBY UNIV ERSIT Y OF V IRGINIA ROBERT C. FERRIER R ACHEL HELLIW ELL MACAUL AY INSTITUTE (U.K.)
ˇ JIRI KOPÁ CEK HY DROBIOLOGICAL INSTITUTE (CZECH REPUBLIC) V L A DIMIR M AJER CZECH GEOLOGICAL SURV EY FILIP MOLDA N SWEDISH ENV IRONMENTAL RESEARCH INSTITUTE M A X IMILI A N POSCH DUTCH NATIONAL INSTITUTE FOR PUBLIC HEALTH A ND THE ENV IRONMENT
M ARTIN FORSIUS FINNISH ENV IRONMENT INSTITUTE
MICHEL A ROGOR A CNR INSTITUTE OF ECOSYSTEM STUDY (ITALY )
AL A N JENKINS CENTRE FOR ECOLOGY A ND HY DROLOGY (U.K.)
WOLFGA NG SCHÖPP INTERNATIONAL INSTITUTE FOR APPLIED SYSTEMS A NALYSIS (AUSTRI A)
To abate acid rain, policy makers can use results of acidification models to predict future recovery.
A
cid rain! Dead fish! Forest dieback! In the 1980s and 1990s, these headlines appeared frequently in environmental news coverage in Europe and North America. Air pollutants from highly industrialized regions had caused widespread damage to pristine ecosystems far downwind. The victims—people living in regions such
as eastern Canada and Norway—pressured the polluters. In Europe, 30 countries engaged in tough negotiations that finally resulted in international treaties to reduce the emissions of sulfur and nitrogen oxides. Acid deposition has now declined by ~60%, and some lakes and streams have begun to recover. Will all waters recover, or must emissions be reduced even more? And how long will recovery take? In this article, we try to answer these questions by using models to predict future acidification of surface waters in 12 acid-sensitive regions in Europe.
A short history In the 1970s, the situation was bleak indeed. Acidified lakes and streams were reported from many regions in Europe, including the uplands of the United Kingdom, southern eas in central and southern Europe. Norway and Sweden were especially hard hit, with trout populations lost in thousands of lakes and salmon stocks eradicated from many major rivers (1). And the situation was getting worse; every year brought reports of new areas affected.
© 2005 American Chemical Society
RICHARD F. WRIGHT
Fennoscandia, the low mountain ranges in Germany and the Czech Republic, and alpine ar-
FEBRUARY 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 65A
Many years of hard scientific detective work were required to explain why fish were dying in remote areas, such as southern Norway. The breakthrough came when scientists realized that the key to understanding surface-water acidification lay in the amount and source of the accompanying anion rather than in the acid itself. Two factors proved necessary to explain surface-water acidification: The water must be acid-sensitive, and the area must receive sufficient amounts of acid deposition (2). Acid-sensitive lakes and streams are found throughout the world in catchments with weathering-resistant bedrock, such as granite and quartzite, and young, often poorly developed, podzolic, and organic-rich soils (3). In these waters, the dominant anion is usually the weak-acid anion bicarbonate (HCO3–), whose source is CO2 from decomposition of organic matter, respiration by plant roots, and dissolution in soil water. The base cations calcium and magnesium generally accompany HCO3–. The concentrations of all three ions depend on how easily weathering breaks down the soil minerals. Acid-sensitive waters have low concentrations of all these ions. FIGURE 1
Sulfur and nitrogen emissions in Europe (1880–2030) The rise and fall of emissions in Europe over the period 1880–2030 as estimated by Schöpp et al. (12 ). Units are megatonne/yr of SO 2 (black), NO 2 (green), and NH 3 (pink). 70 60
Mt/yr
50 40 30 20 10 1880
1910
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Year
The second factor is acid deposition. The most sensitive waters are affected when the rain is more acidic than pH ~4.7 and sulfate (SO42–) concentrations exceed ~20 microequivalents per liter (µeq/L). In acidified waters, the strong-acid anion SO42– is usually the dominant ion and replaces HCO3–. Acidified, SO42–rich waters often have pH 50% and nitrogen by ~20% (12). Further decreases are predicted for the next 20 years if the Gothenburg Protocol and other national legislation are implemented (Figure 2). The waters are becoming less toxic for fish and other aquatic organisms, but complete recovery is a long way off.
Modeling and mechanisms If the Gothenburg Protocol is implemented, will acidified waters recover or are additional reductions required? And how long will the recovery take? Models for acidification of soil and water are required to address these questions (12). Two such models are Model for Acidification of Groundwater in Catchments (MAGIC; 13) and Simulation Model for Acidification’s Regional Trends (SMART; 14). These models are consistent in structure and output (15) and have been extensively tested and evaluated (16). Researchers involved with two large EU projects, European Mountain Lake Ecosystems: Regionalisation, Diagnostic and Socio-economic
FIGURE 2
An example of acidification and recovery The MAGIC model (solid line) was used to reconstruct the acidification history and to predict the future recovery given implementation of the Gothenburg Protocol as it applies to Birkenes, a small acidsensitive stream in southern Norway. Measurements (points) carried out since 1972 confirm significant recovery as of the mid-1980s in (a) sulfate deposition, (b) streamwater sulfate, (c) streamwater acid neutralizing capacity, and (d) soil percent base saturation. (a) Sulfate (eq/m2 yr)
250 200 150 100 50 0 1850 (b)
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2100
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2050
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Sulfate (eq/L)
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50 0 –50 –100 –150 –200 –250 1850 (d) 18 16 14 12 10 8 6 4 2 0 1850
%BS
Evaluation (EMERGE; 17) and Predicting Recovery in Acidified Freshwaters by the Year 2010, and Beyond (RECOVER:2010; 18) have recently used these models to predict the future status of acid-sensitive surface waters in 12 European regions (Table 1; see next page). The models were first calibrated to present-day deposition and water and soil chemistry at each site. Deposition sequences for sulfur and nitrogen compounds from 1860 to 2020 were used to scale through time. For predictions through 2016, the reductions in deposition of sulfur, oxidized nitrogen species (NOx), and reduced nitrogen species (NHy ) were calculated using the RAINS model (19) on the 150 × 150 km Cooperative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe (EMEP) grid (Table 1; 12). The predictions assumed that the Gothenburg Protocol and other current legislation will be implemented by 2010, as agreed. For predictions beyond 2010, modelers assumed that deposition would remain constant. As expected, the model simulations showed that in 1860, prior to the onset of acid deposition, very few waters were acidic and most waters had ANC concentrations >20 µeq/L (Figure 3; see p 69A). Extensive historical and paleolimnological records for fish and other acid-sensitive aquatic organisms also indicate that acidification was not an environmental problem before the early 1900s—even in extremely sensitive regions, such as southern Norway (1). In the regions that received acid deposition exceeding the threshold pH 4.7, model simulations show that waters became increasingly acidified during the 1900s. By 1980, >25% of the waters in 8 of the 12 regions were acidified to ANC < 20 µeq/L and in 5 of these 8 regions to ANC < 0 µeq/L. Conditions were clearly harmful for fish; again, this agrees well with the widespread reports at the time of damage to fish and other organisms. In contrast, northern Sweden, northern Scotland, and the Pyrenees had relatively low levels of acid deposition, with ANC < 20 µeq/L at only a few sites (Figure 3). For the period 1980–2000, extensive measurements document the state and geographical extent of freshwater acidification and the onset of chemical recovery as sulfur deposition began to decrease (10, 11). By the year 2000, only a few of the sites modeled here still had a large fraction of waters with ANC < 20 µeq/L. The model simulations predict that recovery will continue and that by 2016 most waters, except those in southernmost Norway, will have ANC > 20 µeq/L. However, these reductions do not mean that the waters will have returned to their pre-acidification state. After 2010, small but significant deposition of sulfur and nitrogen will remain. In addition, decades of acid deposition have reduced the soils’ ability to neutralize acid deposition. Two mechanisms change surface-water ANC during the acidification and recovery phases. The first mechanism is a concentration effect; changes in cation concentrations are required to balance the changes in concentrations of strong-acid anions.
The more acidic the soil (i.e., the lower the percent base saturation; %BS), the greater will be the fraction of acid cations (4, 20). The second mechanism is related to the size of the pool of exchangeable base cations. Base cation inputs from weathering and atmospheric deposition add to this pool, and net uptake by vegetation and loss to runoff deplete the pool. An increase in the flux of SO42– in runoff will boost the flux of base cations out of the soil. If these base ions are not replenished, the soil pool of FEBRUARY 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 67A
TA B L E 1
Modeling acid-sensitive surface waters in Europe Regional mean nonmarine sulfur deposition in 1980 is given, and percent reductions in 2000 and 2016 (relative to 1980) used in the model simulations are also shown (12 ). Vegetation types: F, forest; H, heathland; M, moorland; A, alpine.
Region
Finland Northern Sweden Southern Sweden Central Norway Southern Norway Northern Scotland Southwestern Scotland Wales Southern Pennines, England Tatra Mountains, Slovakia Southern Alps, Italy Pyrenees, Spain
No. of sites
Sulfate deposition Vegetation
1980 (µeq/m2 yr)
2000 (% reduction)
2016 (% reduction)
Ref.
36 30 35 19 60 30
F F, H F, H F, H F, H H
66 52 78 54 105 52
73 55 55 66 64 42
77 66 73 77 77 65
48 49 49 50 51 52
54 95
F, H F, M
228 73
68 70
82 85
53 53
59
M
328
68
86
53
31 13 79
A F, A A
146 95 66
57 34 49
65 70 65
54 55 56
exchangeable base cations will decrease over the long term. Depleting and rebuilding of the base cation pool takes decades and occurs only when the rate of base cation weathering and deposition input exceeds the rate of depletion of the base cation pool due to the leaching of strong acids. The model simulations indicate that, as expected, the %BS decreased during the long period of acidification of 1860–1980. Between 1980 and 2000, the large reductions in sulfur deposition appeared in most cases to be sufficient to stop the decrease in %BS but still insufficient to allow %BS to recover. And the prognosis for the future is also bleak: Little or no recovery of base saturation in the soil is expected, and in the Tatra Mountains in Slovakia the soil continues to acidify (Figure 4; see p 70A).
Other factors Of course the picture is not that simple, and several important caveats accompany these general conclusions. First, although sulfur deposition has caused most surface-water acidification across Europe, nitrogen is also an important component. On a molar basis, nitrogen deposition in Europe is nearly as large as that of sulfur and has become more important because sulfur emissions have decreased much more rapidly than nitrogen emissions (12). In contrast to sulfur, nitrogen is usually strongly retained in terrestrial ecosystems; typically 20, good population. Data from the acidification model MAGIC (SMART in Finland). Four key years are shown: 1860, pre-acidification (no simulations for Finland, because the SMART model was initiated in 1960); 1980, maximum acidification; 2000, present; and 2016, complete implementation of emission reduction protocols. 1860
1980
2000
2016
hind improvements in water quality. Organisms require time to migrate back into restored habitats, and populations must adjust to new arrivals. Therefore, several generations may be necessary before the population stabilizes. Characteristic lag times will probably differ widely between groups; for example, algae can be expected to react within a few years, whereas fish populations may require more than a decade (26). False starts may also occur, because episodes of acidic water may wipe out newly re-established pop-
ulations. Thus far in Europe, reports of biological recovery are scattered (27).
Some success in Europe For our analysis, we intentionally chose the most sensitive waters rather than a statistically representative sample of all European freshwaters. The spirit of the work follows CLRTAP: Efforts are being made to reduce the total ecosystem area in Europe in which the critical load of acidity is exceeded. FEBRUARY 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 69A
FIGURE 4
Simulated base cation pools Incremental changes [(yearn + 1 − yearn )/yearn ] of exchangeable base cation pools in catchment soils in each of 12 regions are estimated by application of the models. Red: depletion by >0.1%/yr; yellow: negligible change (0.1%/yr. 1980
2000
2016
Given the emission ceilings of the Gothenburg Protocol, ~8% of Europe will still exceed the critical load of acidity in 2010 (28). Our sensitive freshwaters generally lie within this area. Should further reductions in emissions be sought such that recovery of these acidified ecosystems extends over a greater area more quickly? The prediction of future recovery of these sensitive waters is key scientific infor70A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / FEBRUARY 1, 2005
mation for political decisions regarding revisions and potential new protocols to CLRTAP. The model results indicate that even after complete implementation of the Gothenburg Protocol and other current legislation, acidification with commensurate adverse biological effects will continue to be a significant problem in southern Norway, southern Sweden, the Tatras, the Italian Alps, and
the Southern Pennines in the United Kingdom. More than 5% of the ecosystems in each of the regions evaluated here will not meet the ANC criterion to protect sensitive aquatic organisms. Additional mitigation measures will be required in these regions to meet long-term European policy objectives. Further reduction of emissions of sulfur and nitrogen is one possible measure. The Gothenburg Protocol will be up for revision one year after it enters into force. The models used to establish the protocol were steady-state, and the time for recovery was not considered. The dynamic models used here (MAGIC and SMART) add time as a variable. Work under way to advance the next revision of CLRTAP is now taking time into account. The new concept is the target-load function: What are satisfactory levels of sulfur and nitrogen, and when must deposition be further decreased to obtain a satisfactory water quality (i.e., ANC limit) at a given time in the future? (29). In Europe, this work is organized by the International Cooperative Programme (ICP) for Waters (30) and the ICP for Modelling and Mapping (31). The first European data set on targetload functions for waters and soils is currently being collated under the auspices of the CLRTAP. These results are relevant in a wider context for other EU policy. The EU Water Framework Directive (WFD; 32) calls for member states to develop plans for remedial measures to achieve “good ecological status” by 2016. Acidification is one of many pollution factors currently causing degradation of water quality and thus nonachievement of the WFD. Further, international conventions need the results to protect the sea, because a portion of the nitrogen deposited on land and lost to surface waters as NO 3– ultimately reaches the sea. Nitrogen losses predicted by acidification models can be used to estimate future loading of the nutrient nitrogen to coastal marine ecosystems. Relevant EU policy includes management of coastal zones (33), the convention to protect the North Sea (Oslo and Paris Commissions; 34), and the Baltic Sea (Helsinki Commission; 35).
Progress in North America? This saga of the rise and fall of acid deposition in Europe has been running in parallel to similar events in eastern North America. Surface-water acidification became widespread in the 1970s in large regions of southeastern and eastern Canada and the northeastern United States, because of acid deposition on acid-sensitive terrain. Since the mid-1980s, sulfur deposition has decreased by ~40% as a result of legislation implemented in Canada (Eastern Canada Acid Rain Programme; 36) and the United States (Clean Air Act Amendments; 37) and the 1991 Canada–U.S. Air Quality Agreement (38). Surface-water chemistry has begun to recover (11, 39–42). Additional reductions in sulfur deposition may occur in the future. If implemented, the Canada-Wide Acid Rain Strategy for Post-2000 (43) and the proposed U.S. Clear Skies Act (44) would reduce sulfur emissions in 2010 by 50% from 2000 levels. Predictions for future recovery of surface waters— given this scenario—have been made using MAGIC
and other models for southeastern Ontario (45), the Atlantic Provinces (46), and the Adirondack region of New York (47). These predictions point to continued recovery in response to the reductions in sulfur deposition, but a significant percentage of sites will still be acidic in the future. In many regions of eastern North America, as in Europe, further measures appear to be necessary to help acidified surface waters fully recover. Richard F. Wright is a senior research scientist and Thorjørn Larssen is a research scientist at the Norwegian Institute for Water Research. Lluis Camarero is a research scientist at the Centre of Advanced Studies (Spain). Bernard J. Cosby is a research professor at the University of Virginia. Robert C. Ferrier is head of catchment management and Rachel Helliwell is a research scientist at the Macaulay Institute (U.K.). Martin Forsius is head of the Research Programme for Global Change at the Finnish Environment Institute. Alan Jenkins is the science director for the Water Programme at the Centre for Ecology and Hydrology (U.K.). Jiri Kopáˇcek is a research scientist at the Hydrobiological Institute (Czech Republic). Vladimir Majer is a research scientist at the Czech Geological Survey. Filip Moldan is a research scientist at the IVL Swedish Environmental Research Institute. Maximilian Posch is a senior researcher at the Coordination Center for Effects at the Dutch National Institute for Public Health and the Environment (RIVM). Michela Rogora is a research scientist at the Italian National Research Council’s Institute of Ecosystem Study. Wolfgang Schöpp is a research scientist at the International Institute for Applied Systems Analysis (Austria). Address correspondence regarding this article to Wright at
[email protected].
Acknowledgments This work was funded in part by the European Commission under the EMERGE (EVK1-CT-1999-00032) and RECOVER:2010 (EVK1-CT-1999-00018) projects; the Natural Environment Research Council (U.K.); the U.K. Department for Environment, Food and Rural Affairs (Contract No. EPG 1/3/194); the Scottish Executive Environment and Rural Affairs Department; the Academy of Finland; the Spanish Interministerial Committee on Science and Technology (REN2000-0889/GLO); and the Norwegian Institute for Water Research.
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