2010 Progress Report - International Joint Commission

Progress Report 2010
Critical Loads and Exceedances
A region’s critical load is the amount of
acid deposition it can tolerate without
being adversely affected according to
present knowledge. When fi rst utilized in
Canada, 6 the critical load was expressed
in terms of sulphate deposition only and reported as
kg/ha/yr. In order to simultaneously account for both
sulphur (S) and nitrogen (N) acidifying inputs (S and
N have different atomic weights), the critical load is
currently expressed in terms of charge equivalents as
eq/ha/yr. The 20 kg/ha/yr wet sulphate target load that
was used to guide implementation of a SO 2
emission
reduction program in eastern Canada during the 1980s
equals 416 eq/ha/yr.
CANADA
In the 1980s and 1990s, management of the emissions
that produce acid deposition in Canada focused on the
east where sensitive terrain was coincident with high
levels of deposition. Limited survey and monitoring
data supported an assumption that acid deposition was
not a problem in western Canada. This was in keeping
with the fact that western sources were generally
distant from sensitive terrain. However, expansion of
existing and development of new emission sources
in western Canada required that this assumption be
re-evaluated. Recent regional surveys of lakes located
on the Canadian Shield conducted by federal and
provincial authorities 7 have allowed determination of
representative aquatic critical loads for northern parts
of Manitoba and Saskatchewan. Steady-state critical
loads were calculated on a lake-by-lake basis using
the Steady-State Water Chemistry (SSWC) model and
an acid neutralizing capacity threshold (ANC limit
) that
considered the infl uence of the high level of dissolved
organic carbon (DOC) which is prevalent throughout
the region. The critical load for a regional data set was
estimated by the 5 th percentile value so as to protect
95% of the lake ecosystems. Regional aquatic critical
loads ranged from 1.9 to 52.7 eq/ha/yr indicating that
very acid-sensitive lakes exist throughout northern
Manitoba and Saskatchewan. The lowest regional
critical loads occurred in that part of west-central
Saskatchewan that is downwind of the rapidlyexpanding
oil sand industry (as illustrated in Figure 37).
Regional critical load exceedances ranged from
54.5 to 909 eq/ha/yr, with the largest (positive) values
occurring close to base metal smelters in Manitoba
or downwind of the oil sands operations in western
Alberta (Figure 38). The exceedances were almost
entirely due to sulphate deposition. Nitrogen inputs
to the lakes, while signifi cant, were virtually entirely
6
Canada-United.States. 1983. Memorandum of intent on transboundary air pollution. Report of the Impact Assessment Working Group I,
Section 3-Aquatic Effects. 259 p.
7
Jeffries, DS, Semkin, RG, Gibson, JJ, Wong, I. 2010. Recently surveyed lakes in northern Manitoba and Saskatchewan, Canada:
characteristics and critical loads of acidity. J Limnol 69(Suppl. 1):45-55.
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