quantifying the role of ecological uncertainty in a public health policy decision annual meeting...
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Quantifying the Role of Ecological Uncertainty in a Public Health Policy Decision
Annual MeetingAmerican Association for the Advancement of Science
Seattle, WashingtonFebruary 14, 2004
Bruce HopeOregon Department of Environmental Quality
Portland, Oregon
AAAS Seattle (2/04)
Overview
Humans are exposed to methylmercury primarily through fish consumption.
Fish consumption advisories for mercury have been in effect within the Willamette River Basin (Basin) since 1979.
U.S. EPA’s Total Maximum Daily Load (TMDL) process will establish a target level for mercury in surface water.
Meeting the target level will, eventually, permit unrestricted fish consumption.
The Basin is located in northwest Oregon and drains into the Columbia River near Portland, Oregon. It holds the 13th largest river in the continental U.S. and all of Oregon’s major urban areas are located within it.
Willamette Basin
AAAS Seattle (2/04)
Project objectives
Use a Basin-specific food web biomagnification model to estimate a methylmercury (MeHg) biomagnification factor (BMF) for the region.
The model should Focus on fish (and other aquatic) species resident in
the Basin and of which are of concern to stakeholders, and
Be calibrated with fish tissue data collected during 25+ years of water quality monitoring in the Basin.
Use this BMF and probabilistic techniques to calculate a range of Basin-specific target levels.
AAAS Seattle (2/04)
Target level is based on…
A tissue criterion or the acceptable concentration of methylmercury in fish tissue.
80-100% of the Hg in fish tissue is in the methylated form.
A biomagnification factor (BMF) which relates mercury surface water concentrations to tissue levels in higher trophic level species, and
A distribution ratio of methylmercury to total mercury in surface waters in which edible fish reside.
AAAS Seattle (2/04)
Tissue criterion
U.S. EPA’s acceptable level for methylmercury in fish tissue assumes:
A reference dose for methylmercury of 0.0001 mg/kg-d, intended to protect all sensitive human sub-populations in the general population,
A fish ingestion rate of 17.5 g/d, every day, for 70 years, and
A body weight of 70 kg, the adult average.
Current criterion is 0.30 mg MeHg / kg fish, ww.
This value is controversial but was a given for this project.
AAAS Seattle (2/04)
Biomagnification factor
A BMF describes the increase in concentration across two or more trophic levels.
Encompasses bioaccumulation - uptake into an organism at a given trophic level.
U.S. EPA national BMF values exist but these: May not adequately reflect local conditions, and Cannot be linked directly to specific fish species of
concern to specific stakeholders.
A four-trophic level food web model was used to estimate Basin-specific BMF values.
AAAS Seattle (2/04)
Basin-specific food web model Includes 8 fish species commonly found in the
Basin, selected based on measured Hg levels in tissue, feeding guild, and trophic level.
Biomagnification is estimated with Thomann algorithms and Fordham-Regan pathways analysis.
Monte Carlo methods are used to show how ecological uncertain influences tissue concentration and target level estimates.
SURFACEWATER
AQUATICMACROPHYTES
PHYTO-PLANKTION
DETRITUS
ZOO-PLANKTON
AQUATICINSECTLARVAE
HUMANCONSUMERS
PERIPHYTON
LEVEL 2(1° Consumers)
AQUATICCRUSTACEANS
OMNIVOROUSFISH
Juvenile Adult
PISCIVOROUSFISH
INVERTIVOROUSFISH
Juvenile Adult
LEVEL 3(2° Consumers)
LEVEL 4(3° Consumers)
AQUATICINSECTS
LEVEL 1(1° Producers)LEVEL 0
(Exposure Media)
Northern pikeminnow (A)Largemouth bass (A)Smallmouth bass (A)
Rainbow trout (J,A)Cutthroat trout (J,A)
Bluegill (J,A)Largescale sucker(J,A)
Common carp (J,A)Northern pikeminnow (J)
Largemouth bass (J)Smallmouth bass (J)
AQUATICMOLLUSKS
AQUATICWORMS
AAAS Seattle (2/04)
Thomann model equations
WB CCBCF 1
1222 BCFfBCFBAF
1232333 BCFffBCFfBCFBAF
12342343444 BCFfffBCFffBCFfBCFBAF
2k
NDFNIRAEf IJK
Trophic Level 1
Trophic Level 2
Trophic Level 3
Trophic Level 4
Food Term
AAAS Seattle (2/04)
Pathways analysis equations
EfBCFfBCFBMF 1222
EfBMFfBCFBMF 2333
EfBMFfBCFBMF 3444
365exp1 2 tkfE
Trophic Level 2
Trophic Level 3
Trophic Level 4
Lifespan Term
AAAS Seattle (2/04)
Model calibration
Takes advantage of 1000+ fish tissue samples collected over 25+ years.
Calibration was done at two levels: Measured tissue EDFs were aligned with modeled
tissue CDFs. Modeled body length~tissue concentration
regressions were aligned with those observed in measured data.
Methylmercury assimilation factors and elimination rates were variables most frequently modified during calibration.
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TISSUE TOTAL Hg CONCENTRATION (mg/kg, wet weight)
PE
RC
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Measured TC EDF
Modeled TC CDF, pre-calibration
Modeled TC CDF, post-calibration
CONCENTRATION CALIBRATION - NORTHERN PIKEMINNOW
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LENGTH (mm)
log1
0[TI
SS
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CO
NC
] (m
g/kg
)
Measured data
Log-linear regression, measured data
90% CI, measured data
Log-linear regression, modeled data
CONCENTRATION-LENGTH CALIBRATION - NORTHERN PIKEMINNOW
AAAS Seattle (2/04)
Monte Carlo analysis
Provides decision makers with a credible range of target levels and the probability of any given target level achieving the tissue criterion.
For 1-D analysis… Initial values for distributions of physicochemical and
species life history variables from the literature. All variables influenced by stochastic variability and
lack of knowledge combined. For 2-D analyses, 11 measurable variables were
considered driven by lack of knowledge, rather than stochastic variability.
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TISSUE TOTAL Hg CONCENTRATION (mg/kg, wet weight)
PE
RC
EN
TILE
OF
DIS
TRIB
UTI
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Measured TC EDF
Modeled TC CDF, post-calibration
5th / 95th percentile 2-D MC
2-DIMENSIONAL MONTE CARLO RESULTS - NORTHERN PIKEMINNOW
AAAS Seattle (2/04)
Distribution ratio
The ratio of dissolved MeHg to total mercury in surface water is highly variable.
Lakes v. streams/rivers (flow) Epilimnion v. hypolimnion (depth) Total v. dissolved (sample processing) Nationally, it is reported as ranging from <1% to
>30%.
Values measured (n = 64) in the Basin have a mean of 6% (median = 3%), with a 90% confidence interval of 1% to 18%.
AAAS Seattle (2/04)
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DISSOLVED MeHg / TOTAL (unfiltered) Hg RATIO
PE
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TILE
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TRIB
UTI
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WRB D-MeHg / THg
USEPA D-MeHg / THg (lakes & reservoirs)
USEPA D-MeHg / THg (rivers & streams)
COMPARISON OF WILLAMETTE BASIN EMPIRICAL RATIO VALUES TO USEPA GENERIC VALUES
AAAS Seattle (2/04)
Target level calculation
Given a tissue criterion, a Basin-specific BMF estimate, and an estimate of distribution ratio in surface water, a target level can be calculated as:
where: CTL= Mercury target level; TC = Tissue criterion; BMF = Biomagnification factor; = Distribution ratio; CF = Conversion factor
CFBMF
TCCTL
AAAS Seattle (2/04)
Target level estimates (ng/L)
Fish Species 5th-%tile 50th-%tile 95th-%tile
Northern pikeminnow 10.03 0.92 0.07
Largemouth bass 15.16 1.27 0.11
Smallmouth bass 38.42 2.82 0.20
Rainbow trout 54.72 4.78 0.31
Bluegill 37.56 3.65 0.40
Largescale sucker 28.97 2.75 0.22
Carp 34.96 3.25 0.21
Cutthroat trout 73.40 6.02 0.50
AAAS Seattle (2/04)
Choosing a target level
Target tissue levels were calculated for each fish species using model estimated BMF values.
There are a number of choices. Largemouth bass, for example:
Upper bound 15.16 ng/L (5% of individuals) Lower bound 0.11 ng/L (95% of individuals) Median 1.27 ng/L (50% of individuals) Mean 0.39 ng/L
Actual target level selection is a public policy matter requiring stakeholder input.
Even with uncertainty quantified, the challenge is still in making the choice .