9th april 2014kari austnes1 critical limits for acidification of surface waters vs boundary values...
TRANSCRIPT
Kari Austnes 19th April 2014
Critical limits for acidification of surface waters vs boundary values in the Water Framework Directive (WFD) – a Norwegian case study
24th CCE workshop and 30th task force meeting
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Objective
• Two sets of management regulations related to acidification of surface waters− LRTAP: Acid deposition below the critical load (CL) for
protection of the ecosystems− WFD: Ecological indicators (quality elements) above
certain boundary values for achieving good ecological status (with respect to acidification)
• Both use acid neutralising capacity (ANC) as chemical criterion linking water chemistry to biological effects− Do they set the same requirements?− Not directly comparable – CL calulated based on both
criteria for comparison
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Critical loads for surface waters
• SSWC and FAB the commonly used models• Both use ANClimit as link chemistry-biology
− The criterion we want to compare with the ANC criterion in the WFD
• SSWC: CL(A) = BC0 - ANClimit
− BC0: Flux of (non-marine) base cations from the catchment in pre-acidification times
− ANClimit • Minimum ANC to avoid harmful effects
on selected biota• Originally a fixed limit at 20 eq/l 95% probability of no damage
9th April 2014Lien et al. (1996)
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The variable ANClimit
• Less sensitive systems higher biodiversity higher ANClimit needed to keep the diversity intact
• The variable ANClimit varies with the original base cation concentration− [ANC]limit,var = k*CL = k*Q*[BC*]0/(1+k*Q)
− ANC-range: 0-50 eq/l
• Organic acid adjustment− Organic acids contribute to the strong acid anion
concentration larger buffer needed at higher organic acid concentration
− [ANC]limit,oaa,var = k*Q*([BC*]0-1/3*m*[TOC])/(1+k*Q) (m=10.2, k=0.2)
− CL=Q*([BC*]0- [ANC]limit,oaa,var -3.4*[TOC])9th April 2014
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The WFD and boundary values
• Ecological status of surface waters based on a set of quality elements− Biological and physicochemical
• Boundary values set for different quality elements (e.g. ANC) with respect to different pressures (e.g. acidification)− Five status classes from high to bad− Good status environmental target the
good/moderate (G/M) boundary essential− The boundary values represent deviation from a
reference status (natural conditions)• Different sets of boundary values for different types of
water bodies (e.g. small, lowland, humic lakes)9th April 2014
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WFD boundary values (Norway)
• Lake types for acidification classification defined by Ca- and TOC-concentration− Boundary values vary according to buffering capacity and
humic acid content as the ANClimit,oaa,var
− Discrete boundary values (ANClimit,oaa,var continuous)
• 1st classification manual (2009)− 6 types: 2 Ca classes (split at 1 mg/l), 3 TOC classes− Reference value median of reference lakes− G/M boundary
• Brown trout status vs ANC • Benthic invertebrate vs ANC (adj)• Expert judgement• ANC-range: 20-40 eq/l
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Hesthagen et al. (2008)
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WFD boundary values (Norway) cont.
• 2nd classification manual (2014)− Revised reference values
• Pre-industrial ANC from MAGIC many lakes pre-industrial ANC below G/M boundary large range within the same lake type
• Lower Ca-class (<1 mg/l) split into four
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• Median of MAGIC-results used
− Revised G/M boundary values• Re-analysis of brown trout data for new lake types• Boundaries adjusted downwards to take delayed biological
recovery into account (Wright, 2013)
• Expert judgement• ANC-range: 0-30 eq/l
Wright and Cosby (2012)
Ca<1, TOC<2
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Background – variability across Norway
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The G/M boundary
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• Markedly higher values with the 1st WFD manual
• Both reflect the Ca and TOC patterns
• Not directly comparable to ANClimit,oaa,var TOC incorporated in the ANClimit,oaa,var value
• CL calculation used for comparison
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Critical load: Compare ANClimit,oaa,var and G/M boundary
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• Markedly lower CL with G/M boundary based on the 1st manual
• Negative CL aiming for the unachievable
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Exceedance 2007-2011: Comparison
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• Fairly similar results
• More realistic G/M boundary values in the 2nd manual
• Drawing on experience from the LRTAP work
• WFD requirements slightly lower
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Differences CLA_oaa and CLA_WFD2
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• CLA_oaa mainly lower than CLA_WFD2
• Largest differences when CLA_oaa is lower than CLA_WFD2
• Largest relevant differences found on the west coast
Grid cells with CLA>90 meq/m2/yr removed from the analysis – not relevant - no risk of acidification
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Differences: Humic acid content
• For both criteria CL decreases with increasing TOC
• Large range in deviation at low TOC− Different TOC-approach
less important
• At TOC>8 mg/l CLA_oaa is always lower− G/M boundary no
differentiation at TOC>5 mg/l (with respect to TOC)
− Too low G/M at high TOC?
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• Marked changes at class boundaries− Larger differences at the
lower end
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Differences: Buffering capacity
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• G/M boundary increases with increasing Ca
• ANClimit,oaa,var increases with increasing BC0
• Fairly similar pattern reflect the same effect
• Lower CLA_WFD2 mainly at Ca0.75-1.25 mg/l
• Lower CLA_oaa along the whole range− Majority Ca<0.75 mg/l − Most grid cells with
Ca>0.75 mg/l high TOC
• Marked changes at class boundaries− Larger differences at the
lower end
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Differences: Summary
• Exceedance− CLA_oaa stricter at very low Ca− CLA_WFD2 stricter at Ca0.75-1 mg/l− Differences in TOC approach mainly relevant at high TOC
• Discrete G/M boundaries problematic− Artificial trend within classes− High uncertainty around the class boundaries
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TOC increases – then what?
• WFD− Boundaries dynamic for their actual purpose, i.e. to
assess ecological status with respect to acidification• Increase in TOC type changes stricter requirement
(higher ANC G/M boundary)• But - only three TOC classes: Marked changes
needed, and no effect of TOC increase above 5 mg/l
• LRTAP− In theory critical loads based on ANC limit constant
• In practice revised as calculations and data improve
− Using ANClimit,oaa,var introduces a non-constant factor (TOC) not taken into account• Increasing TOC lower CL higher exceedance 9th April 2014
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TOC increase: Effects on critical loads
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• Rough test− TOC increase in line
with long-term monitored lakes
− Increase from 1995-present
− Different factor in different regions: 1.0-1.9
• Small changes only− Most pronounced in
regions with higher TOC increase
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TOC increase: Effects on exceedances
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• Relatively small differences so far
• Larger impact in other countries?
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Conclusions
• WFD requirements far more in line with the LRTAP requirements in the 2nd manual for WFD classification
• Still some differences− WFD requirements somewhat lower overall− The upper TOC class should probably be split− Major differences at very low Ca – difficult to say which
criterion is more correct
• Discrete WFD boundaries give higher uncertainty• CLA_oaa decreases with increasing TOC
− Small changes so far, but may call for revision if continued TOC increase
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