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

9th April 2014

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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

9th April 2014

• 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

9th April 2014