Presentation to the Multi-Agency Critical Loads Workshop,
Charlottesville, VA May 23-24, 2006
Reducing Acidic Deposition:
the Canadian Experience
Using Critical Loads (CLs)
Dean S. Jeffries*
Environment Canada
National Water Research Institute
P.O. Box 5050, Burlington, Ontario L7R4A6
Tel: 905-336-4969
E-mail: dean.jeffries@ec.gc.ca
*
Collaborators: J. Ahearne, P.A. Arp, T.G. Brydges, V. Ballard, I. DeMerchant,
J. Dupont, J. Franklyn, D.C.L. Lam, F. Norouzian, R. Ouimet, C.-H. Ro, K.
Timoffee, R.J. Vet, S.A. Watmough, and I. Wong
Outline
• History (evolution) of Canadian acid rain policy
• Current state of CL science in Canada
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Definition
Models and methods
Aquatic, forest soil and combined CL maps
Current and potential exceedance maps
• Conclusions
• Recommendations
Developing Canadian Policy
to Reduce Acidifying Emissions
• A step-wise process using target and critical loads
• Involves both federal and provincial governments
• Driven by public concern and underpinned by
scientific information
History
(of Acid Rain Science-based Policy in Canada)
1950s & 60s
Scientific Milestones
Policy Actions
• Great Lakes eutrophication
assessed
• Acidic lakes identified in
Nova Scotia & near the
Sudbury smelters in Ontario
(Killarney)
• Vegetation damage by
ground level SO2 fumigation
• GLWQA limiting PO4 inputs
(a successful application of a
CL policy initiative)
• First SO2 control orders to
improve local AQ around
Sudbury (led to the
“superstack”)
History
(of Acid Rain Science-based Policy in Canada)
1970s
Scientific Milestones
Policy Actions
• 1972 – UN Conference on
the Human Environment
• Earliest evaluations of
transboundary transport
and the regional
occurrence of “acid rain”
and aquatic effects
• Sudbury Environmental
Study
• 1978 – Canada and the US
agree to the formation of a
joint taskforce to assess
the situation (led to the
MOI)
• 1979 – the UNECE
LRTAP Convention
History
(of Acid Rain Science-based Policy in Canada)
1980s
Scientific Milestones
• Memorandum of Intent
between the Government of Canada
and the Government of the United
States of America concerning
Transboundary Air Pollution
• 20 kg wet SO4/ha/yr target
(a Canada-only conclusion)
Policy Actions
• 1985 – Eastern Canada Acid
Rain Program (requiring a 50%
SO2 emission reduction from the 7
eastern-most provinces by 1994)
• 1985 – first SO2 Protocol
(under the UNECE LRTAP
Convention; established a national cap)
• 1988 – the NOx Protocol
History
(of Acid Rain Science-based Policy in Canada)
1990s
Scientific Milestones
• The 1990 and 1997 Acid
Rain Assessments (refining
eastern, aquatic CL values, often
defining values <<20 kg/ha/yr
target)
Policy Actions
• 1991 – Canada-US Air
Quality Agreement (AQA)
• 1994 – second SO2 Protocol
• 1998 – NEG-ECP Acid
Rain Action Plan
• 1998 – Acid Rain Strategy
for Post-2000 (long term goal to
reduce SO2 emissions to meet CLs;
www.ec.gc.ca/acidrain/strat/strat_e.htm)
History
(of Acid Rain Science-based Policy in Canada)
2000s
Scientific Milestones
• The 2004 Acid Rain
Assessment (providing both
Policy Actions
• Ozone Annex to the AQA
(focused on smog reduction)
aquatic and forest soil CLs expressed
in terms of total S+N deposition;
extended evaluation to western
Canada)
• Federal-provincial
agreements to further reduce
emissions (cf. Strategy goal)
• Possible PM Annex to the
AQA
see http://www.msc-smc.ec.gc.ca/saib/acid/acid_e.html
Current Status
of Canadian Critical Loads
• The following slides are based largely on the 2004 Acid
Deposition Science Assessment (available on the web or on cd)
• For more detail, see:
– Jeffries, D. and Ouimet, R. (eds). 2004 Critical loads: are they being
exceeded? In: The 2004 Canadian Acid Deposition Science Assessment,
Chapter 8. Environment Canada, Ottawa, Ontario, 341-370.
– Ouimet, R., Arp, P.A., Watmough, S.A., Aherne, J. and Demerchant, I.
2006. Determination and mapping critical loads of acdity and
exceedances for upland forest soils in eastern Canada. Water Air Soil
Pollut. 172:57-66.
• Supplemented with some more recent analyses
Critical Load Definition
“The highest deposition of acidifying compounds that
will not cause chemical changes leading to long term
harmful effects on ecosystem structure and function
according to present knowledge” (UN ECE)
Implications:
• need an indicator and threshold value to define the onset of “harmful effects”
• must choose degree of ecosystem protection desired
• steady-state vs temporally specified indicator threshold
• determined values are necessarily evolutionary (cf. “present knowledge”)
• difference between “critical” and “target” loads
Region of Concern
Acidic Deposition + Sensitive Terrain → Ecosystem Effects
+
Annual wet SO4 Deposition (kg/ha/yr)
Source: National Atlas of Canada (1995)
• Southeastern Canada is the present region of concern
• Sensitive areas of western and northern Canada may be of future concern
A note about CL units
• Prior to the 2004 Assessment, CLs (and the policy “target
load”) were expressed as kg/ha/yr wet SO4 deposition
• To include both S and N, Cls are now expressed as eq/ha/yr
– 1 kg SO4/ha/yr = 20.8 eq/ha/yr
– 20 kg SO4/ha/yr = 416 eq/ha/yr
• CLs are now expressed in terms of total deposition, i.e., they
include both wet and dry deposition
Models and Methods (1)
• Aquatic CLs were estimated on a lake-by-lake basis using:
– the Expert Model (a component of the IAM; threshold was pH 6)
– the Steady-State Water Chemistry Model (SSWC; threshold was ANC
40 eq/L)
– for a given lake, the lesser of the 2 values was taken as the CL
– results were mapped on a grid basis (5th percentile value = cell CL)
• Upland forest soil CLs were estimated for polygon map units
using:
– the Simple Mass Balance Model (SMB; threshold was soil water Cb:Al
= 10 and gibbsite dissolution constant of 109)
– forest harvesting or fire were not considered
– soil polygons were mapped (southeastern Canada only)
Models and Methods (2)
• Combined aquatic-terrestrial CL maps were developed on a
grid basis:
– had to combine point-based aquatic and polygon-based soil CL values
and easiest compromise was to grid the soil map
– the soil polygon CL map was “re-sampled” within a grid overlay to
determine the 5th percentile value for each grid cell
– lower of the aquatic and soil 5th percentile values was taken as the cell
CL for the combined maps
– there were many grid cells in eastern Canada where only soils values
were available. Only aquatic values were available for western Canada.
Example
Index map shows which model produced the grid
value (yellow = SSWC, red = Expert, green = SMB)
Models and Methods (3)
• CL exceedances were calculated using estimates of total (wet
and dry) S and N deposition from the mid-90s:
– the current or “N-leaching” exceedance used total S deposition plus
measured or estimated NO3 export as the estimate of acidifying
deposition
– the steady-state or “N-saturated” exceedance used total S and N
deposition (available for southeastern Canada only)
• 95th percentile exceedance value mapped for each grid cell
Lake Chemistry Data
• Data compiled from multiple sources (eastern data 1997 or later)
• Data typically clustered; not a statistically-based lake survey
• Data from 2054 lakes that charge balanced within ±15% were used for CL
analysis.
• There were several spatial gaps and some sensitive terrain (particularly in the
west) was unrepresented or under-represented.
Aquatic CLs
• Policy-based target load (20 kg/ha/yr) covered by the four lowest classes
• 21% of eastern grid cells are in the lowest CL class; most of them occur in the
Atlantic provinces
• Provincial CLs range from “background” (~60 eq/ha/yr in Newfoundland,
Nova Scotia and New Brunswick) to 1620 eq/ha/yr (Manitoba)
• There is a data distribution effect
Forest Soil CLs
• Lowest CL classes reflect shallow, coarse-textured upland soils derived from
felsic or granitic bedrock (Canadian Shield plus other areas)
• Highest CL classes have calcareous soils
• Provincial forest soil CLs were generally <400 eq/ha/yr
Draft map: Upland forest CLs of acidity
for Manitoba and Saskatchewan
Aherne et al. (in press) Calculating critical loads of acid
deposition for forest soils in Manitoba and Saskatchewan:
data sources, critical load, exceedance and limitations.
Final Report. Environmental and Resource Studies, Trent
University, Peterborough, ON, 54pp.
Combined CL Estimates
• The lowest CL cells were usually contributed by the aquatic analysis
• There were many cells where the soil value was lower than the aquatic value
(terrain in such cases probably have deeper soils)
• Grid cells with CLs less than the old policy target of 20 kg/ha/yr (<416 eq/ha/yr,
lowest four classes) occur throughout south eastern Canada (also in northern
Saskatchewan and Alberta)
Combined Current Exceedances
• ~0.5 million km2 (21% of the mapped area) currently receive acidic deposition
in excess of forest soil or lake CLs (yellow to orange grid cells)
• 14% of the mapped area is exceeded by >100 eq/ha/yr (>4.8 kg SO4/ha/yr)
• 15% of the mapped area has only slightly negative exceedances (-100 to 0
eq/ha/yr); even a small increase in runoff NO3 could greatly increase the size of
the exceeded area
Combined Steady-State Exceedances
• Positive steady-state (or potential future) exceedances occur over ~1.8 million
km2 (75%) of the mapped area
• Expansion of the steady-state exceedances relative to the N-leaching
exceedances occurs principally in Ontario, southwest Quebec and
Newfoundland (not Labrador)
Conclusions
• Development of Canadian SO2 emission reduction policy has
depended on critical/target loads from the beginning.
• Aquatic and forest soil CLs have been determined and combined
into a single map.
• Extremely low regional CLs were predominantly defined by lakes
whose catchments have very thin soils. CLs in regions with
thicker soils were predominantly defined by forest soil estimates.
• ~0.5 million km2 of the mapped area currently experience CL
exceedance, most of it in southern Nova Scotia, southwestern
Quebec and south-central Ontario. Should ecosystem N-saturation
develop, the exceedance area could expand to 1.8 million km2.
• Either way, further reductions in acidic deposition are needed to
reduce the exceedances.
Some Recommendations
• Improve spatial coverage and representativeness of the lake
and soil chemistry databases, particularly in sensitive terrain
• Expand CL analyses in western and northern Canada,
particularly northern MB and SK, and the Georgia Basin of BC
• Further refine CL and exceedance analyses with:
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better deposition estimates (particularly in the west and north)
better runoff estimates
spatially variable chemical thresholds
explicit consideration of changing base cations (and DOC?)
more comprehensive/realistic handling of N exceedance component
• Improve/extend uncertainty analyses
• Expand analysis of time-dependent CLs (dynamic modelling)