Zinc…essential for
life
Correction for natural
background in risk assessment
Example: Zinc
Frank Van Assche
International Zinc Association
168 Avenue de Tervueren
1150 Brussels
Zinc…essential for
life
Regulatory context
• A risk assessment on zinc and 5 zinc compounds
was made under EU regulation 793/93/EC
– Rapporteur country: The Netherlands
– Published: ECB 2008
• Zinc case was first on essential element with
significant background
–  Development of RA methodology needed
• Effects assessments concluded at SIAM 20 (2005)
Zinc is a natural element
AIR: 40 - 100 ng Zn/m3
SOIL: 25-200
mg Zn/kg DW
WATER: 3 - 40 µg
Zn/l
BIOMASS: 25200 µg Zn/g
DW
3
Zinc Background (µg Zn (total)/l):
variability in surface waters
–
–
–
–
EU - lowland range:
EU rivers in mineralised areas:
N-American mountain streams:
N-American Great lakes:
(dissolved)
– Coastal waters:
– Open ocean:
– FOREGS : EU « stream waters »
(dissolved)
• 10P
• 50P
• 90P
3 – 40
40 - >200
< 10
< 0.3
0.5 – 1
0.001-0.06
0.7
2.7
18
4
The Optimal Concentration range for
Essential Elements
deficiency
Optimal
dose
toxicity
5
The Optimal Concentration range for
Essential Elements depends on BG
Shift to
higher
sensitivity
Shift to
lower
sensitivity
deficiency
Optimal
dose
toxicity
6
Zinc…essential for
life
Background may be
significant in metals RA
• For some metals natural background can
be significant
– as compared to derived PECs and PNECs
– because of conditioning/acclimatisation
•  background needs to be integrated in
quantitative risk assessment.
Zinc…essential for
life
Definition of natural background range
for RA purposes (zinc case, µg total Zn/l)
• Zuurdeeg (1999):
– 4 µg/l and 35 µg/l (EU lowland brooks in unpolluted areas in
Northern Europe)
– geometric mean: 12 µg/l - taken as the “best estimate” for the
natural BG concentration in fresh surface waters
• EU countries: lower end of observed ranges: 3 µg Zn/l
•  EU RA
– 12µg total Zn/l as “best-estimate” mean value (4.5 µg dissolved)
– 3 µg total Zn/l as “lower limit” conservative value (1.1 µg/l dissolved)
– Confirmed by FOREGS (2006)
• BG can be defined based on measured data in
“unpolluted” areas relevant for the assessment
Zinc…essential for
life
Integration of background
in the EU zinc RA
• To integrate natural BG in the EU RA, 2 specific
approaches were developed:
– selection of ecotoxicity data on (BG-)
relevancy criteria
– Application of the “added risk” approach
Zinc…essential for
life
1) Selection of ecotoxicity data
on (BG-) relevancy criteria
• “Zinc background” criterion to select
ecotoxicity data for relevancy for EU waters
– to account for conditioning of test organisms
to natural (EU-relevant) Zn BG
• a minimum value for relevant background zinc
concentration (dissolved) was set at “around 1
µg/l” (cfr EU relevant range)
• Maximum value: ~25 µg Zn/l
•  ensure test data (for PNEC setting) are
relevant for EU aquatic ecosystems.
Sensitivity to zinc as function of BG
Culture conditions
species
< 2 µg Zn/l
2 < [Zn] < 50 µg Zn/l >50 µg Zn/l
Ephydatia : 3.3
Daphnia magna :
59
Selenastrum c. : 8
Ceriodaphnia: 31
N-American Great
Lakes
8
Ephydatia : 34
Daphnia magna :
365
Selenastrum c. : 37
Chroococcus: 200
Synecoccus: 390
Dreissenia: 400
Potamopyrgus: 75
Hyallella: 42
Phoxinus: 50
Salmo: 210
European lowland
rivers
10
36 +/- 5
2.5
100 +/- 3
17
Corbicula: 25
Salvenilus: 530
Jordanella: 51
Pimephales : 78
corresponding
ecosystem
Number of species
NOECs
Mean NOEC (µg Zn/l)
HC5 (µg Zn/l)
Hormidium : 1000
Scenedesmus:
1400
Mineralised areas
2
1175 +/- 1
/
Zinc…essential for
life
2) The “added risk” approach
(after Struijs et al 1997)
• Only the antropogenic part of PEC and PNEC is
considered for RA
– “PECadd” and “PNECadd”
– Background is assumed not to contribute to
toxicity (essentiality!)
• The added risk concept was applied at 2 levels of
the zinc RA:
– Conversion of NOEC values to “NOECadd” values
– Conversion of monitored PEC values into PECadd.
Zinc…essential for
life
Conversion of NOEC values to
“NOECadd” values
• if NOEC based on measured zinc concentration
•  subtraction of (estimated) background
•  NOECtotal (measured) – BG = NOECadd.
– correction not required in case the test medium was
reconstituted water (without zinc background), or, in case of
natural waters, if the (estimated) background was very low.
• If NOEC based on a nominally added concentration
•  corresponds directly with a NOECadd.
• HC5/PNEC derived from NOECadd values: HC5/PNECadd
Zinc…essential for
life
Conversion of monitored
PEC values into PECadd
• If measured concentrations: correction for
(estimated) background: PEC – BG = PECadd .
– Often significant for zinc (BG vs total).
– 12µg/l and 3µg/l (Total Zn)) applied as a mean and
worst case estimate, resp.
Water
Measured
concentration
Background
PECadd
(µg Zn total/l)
(µg Zn total/l)
3
12
6 (specified)
30
21
22
(µg Zn total/l)
River A
33
River B
28
• If modelled emissions: directly PECadd
Zinc…essential for
life
conclusions
• For some metals, background can be
significant and needs integration in RA
– At level of data selection: setting relevancy
criterion corresponding to the area
– At level of PEC/PNEC: e.g. by applying the
added risk approach
• Corrections at level of PNECadd, PECadd
– Background can be measured in (relatively)
unpolluted waters
Integrating background in RA:
case zinc
Effect assessment
Selection of ecotoxicity values
for relevancy to EU
environment, criterion:
natural zinc background
Exposure assessment
Measured zinc
concentrations in EU
waters
NOEC values relevant for
the EU environment
Conversion to NOECadd
values
Subtraction of (estimated)
natural background
Species sensitivity
distribution
PNECadd
PECadd
Zinc…essential for
life
drawbacks
• Main difficulty is defining a realistic natural background
at the local/regional scale.
– information is often lacking
– generalisations may often fail to capture variability at
the local and temporal scale.
• Especially in areas where water quality is influenced by
specific metal-enriched geology, the natural background
should be assessed in a more precise way, since it can as
such result in significant exceedance of the PNEC.