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

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g/kg

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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

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TILE

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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

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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 .