Reducing Acidic Deposition:the Canadian Experience

Using Critical Loads (CLs)

Dean S. Jeffries*

Environment CanadaNational Water Research Institute

P.O. Box 5050, Burlington, Ontario L7R4A6Tel: 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

Presentation to the Multi-Agency Critical Loads Workshop,Charlottesville, VA May 23-24, 2006

Outline

• History (evolution) of Canadian acid rain policy

• Current state of CL science in Canada– Definition

– Models and methods

– Aquatic, forest soil and combined CL maps

– Current and potential exceedance maps

• Conclusions

• Recommendations

Developing Canadian Policyto 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)

Scientific Milestones• Great Lakes eutrophication

assessed

• Acidic lakes identified in Nova Scotia & near the Sudbury smelters in Ontario (Killarney)

• Vegetation damage by ground level SO2 fumigation

Policy Actions• 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”)

1950s & 60s

History(of Acid Rain Science-based Policy in Canada)

Scientific Milestones• 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

Policy Actions• 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

1970s

History(of Acid Rain Science-based Policy in Canada)

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)

1980sPolicy 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)

Scientific Milestones• The 1990 and 1997 Acid

Rain Assessments (refining eastern, aquatic CL values, often defining values <<20 kg/ha/yr target)

1990sPolicy 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)

Scientific Milestones• The 2004 Acid Rain

Assessment (providing both aquatic and forest soil CLs expressed in terms of total S+N deposition; extended evaluation to western Canada)

2000sPolicy Actions

• Ozone Annex to the AQA (focused on smog reduction)

• 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 Statusof 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 ConcernRegion of ConcernAcidic Deposition + Sensitive Terrain → Ecosystem Effects

• Southeastern Canada is the present region of concern

• Sensitive areas of western and northern Canada may be of future concern

+

Source: National Atlas of Canada (1995)Annual wet SO4 Deposition (kg/ha/yr)

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.

Index map shows which model produced the grid value (yellow = SSWC, red = Expert, green = SMB)

Example

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 aciddeposition for forest soils in Manitoba and Saskatchewan:data sources, critical load, exceedance and limitations.Final Report. Environmental and Resource Studies, TrentUniversity, 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:– 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)