USE O F T H E M E R C U R Y CYCLING M O D E L (MCM) TO P R E D I C T T H E FATE OF M E R C U R Y IN T H E GREAT LAKES

D. L E O N A R D , ~ R. R E A S H , 2 D. P O R C E L L A ? A. P A R A L K A R , 4 K. S U M M E R S 4 and S. G H E R I N r ~

1 Detroit Edison Company, 2000 Second Avenue, Detroit, Michigan, 48226,2 American Electric Power, 1 Riverside Plaza, Columbus, Ohio 43215, 3Electric Power Research Institute, P.O. Box 10412, Pale Alto, California 94303, 4Tetra Tech, Inc., 3746 Mr. Diablo Blvd., Lafayette, California 94549

Abstract. In response to U.S. EPA's proposed Great Lakes water quality criteria for mercury (Hg), a field- validated Hg cycling model (MCM) was used to predict Hg levels in the ahiotic and biotic components of Lake Superior and Lake Erie. The U.S. EPA criteria are based on water column Hg concentrations and simple trophic level transfer and, thus, do not consider sediment interactions and water chemistry factors. The model, using data from published reports, was run to simulate a 25 year steady state period. For these simulations, methylmercury (MeHg) represented 5% of total Hg in Lake Erie and 8% of total Hg in Lake Superior. These proportions are roughly 3-5 times lower than U.S. EPA's estimate that MeHg contributes about 25% of total Hg in the water column of the Great Lakes. The predicted median concentrations of total Hg in top-carnivore fish were 0.13 mglkg in Lake Superior and 0.16 mg/kg in Lake Erie. Predicted median MeHg concentrations in Lake Superior and Lake Erie (water column) were 0.019 and 0.075 ng/L, respectively. For both lakes, most (>55%) of the Hg was partitioned to sediments. Although the MCM simulation does have practical limitations (e.g., lakes are treated as fidly-mixed open systems), the results demonstrate that generic assumptions of Hg behavior in all Great Lakes waterbodies are too simplistic.

1. Introduction

Mercury (Hg) has long been recognized as a pollutant exhibiting toxic effects at levels slightly or only modestly higher than background levels. Because Hg and its compounds have no known biological function and less toxic forms can be transformed into more toxic forms through natural processes (Eisler, 1987), the presence o f Hg in environmental components is often regarded as a potential risk for effects in higher trophic levels (fish and piscivorous birds and mammals) . A crucial determinant o f potential H g effects in freshwater systems is the cycling o f various inorganic and organic forms. It has become apparent that site-specific factors affect Hg speciation, bioconcentrat ion, and bioaccumulation in each waterbody (Jackson, 1991; Lange et al., 1993). Recent studies o f Hg cycling in freshwater rivers and lakes are given in Watras et el. (1994), Parks (1988) and Driscoll et al. (1994). Zillioux et al. (1993) provide a comprehensive review of Hg cycling in freshwater wetlands, some of which are c o m m o n along the coastline o f the Great Lakes (e.g., the south shore o f Lake Erie).

In the Great Lakes region of North America Hg has been identified as a polhltant o f concern since the early 1970s, when elevated levels in sport fish resulted

Water, Air, and Soil Pollution 80:519-528, 1995. �9 1995 KluwerAcademicPublishers. Printed in theNetherlands.

520 D. LEONARD ET AL.

in consumption advisories or bans. Though substantial reductions of Hg in wastewater has occurred throughout the Great Lakes region, various components of the Great Lakes fauna continue to exhibit elevated, but some cases declining, body burdens of Hg (B~land, et al., 1993; Ropek and Neely, 1993). Long-term contaminant monitoring of Great Lakes fishes has shown that, presently, Hg does not pose a widespread fish consumption concern (Ontario Ministry of Environment and Energy, 1993). Mercury levels in Lake Erie walleye have actually decreased since the early 1970s (Armstrong and Sloan, 1980). Mercury concentrations in recently collected Lake Erie walleye, in fact, are similar to levels in pre-1900 museum specimens (personal communication, Ontario Ministry of Environment and Energy).

In 1993, U.S. EPA proposed new water quality criteria for the Great Lakes drainage basin (U.S. EPA, 1993). Three water quality criteria were proposed for Hg based on protection of aquatic life, human health, and wildlife. The most stringent of these was the wildlife criterion (0.18 ng/L), which is between 1-2 orders of magnitude lower than measured Hg in rainfall (Kelly et al., 1991). For calculation of the Hg human health and wildlife water quality criteria, EPA established a set of generic assumptions to apply throughout the entire Great Lakes drainage basin. These assumptions dictate that Hg levels in fish are determined largely by water column concentrations and that a generic bioconcentration factor (52,000), a constant methylmercury (MeHg) to total Hg ratio (25%), and a fixed bioaccumulation factor (144,000) can be valid defaults for deriving protective criteria. The purpose of this research was to simulate the cycling of Hg in two differing lakes and address the generic assumptions established by EPA.

2. Materials and Methods

The MCM Lake Mercury Model (Hudson et al., 1994) dynamically simulates the biogeochemical cycling of Hg in lakes. It was developed as part of the Mercury in Temperate Lakes Program sponsored by the Electric Power Research Institute and the Wisconsin Department of Natural Resources. The model simulates Hg in elemental, methyl, and divalent (mercuric and inert) forms - each of which may comprise several species - in the epilimnion, hypolimnion, and sediments of lakes. Four trophic levels of biota are included: phytoplankton, zooplankton, planktivorous fish, and piscivorous fish. The major processes included in the model are given in Hudson et al. (1994) and Figure 4 of Zillioux et al. (1993). The primary input components are listed in Table I. The MCM model dynamically simulates the transport and transformation reactions in each of the main lake compartments based on principles of mass conservation, chemical equilibria and kinetics, and ecosystem bioenergetics. The model routes Hg up the food chain using four trophic levels. Required input parameters include lake physical characteristics, water quality parameters, atmospheric and other inputs, and biomass characteristics.

MERCURY IN THE GREAT LAKES 521

The MCM model was set up for a simplified two layer representation of Lake Superior and Lake Erie. The input data for deposition, climate, lake water quality, sediment chemistry, temperature profiles, and stage-flow data were obtained primarily from published articles. A recent accurate measurement of Hg in Lake Superior was also obtained from Wisconsin DNR, which showed a total Hg concentration (unfiltered) in the water of 0.9 ng/1 and a MeHg concentration of 0.035 ng/1 at a depth of 10 m (personal communication, Doug Knauer, Wisconsin DNR). Gill and Bruland (1990) reported a total dissolved Hg concentration of 1.8 ng/L in Lake Erie water samples.

First, the hydrologic budget and hydraulic calibration for the lakes were achieved. Then, the ecosystem was calibrated with phytoplankton biomass profiles and specific growth rates for the lakes so that a steady state biomass was achieved for the zooplankton and two fish types. The characteristics for the planktivorous fish were based on those for yellow perch (Perca flavescens), while the higher trophic level was based on walleye (Stizostedion vitreum). Adjustments to the various rate coefficients were made to obtain a good match between simulated and observed lake water Hg levels and fish tissue levels for the given input loadings. The model was run to achieve steady state Hg concentrations in the lake water and fish populations. A 25-year time interval was chosen as this time; mercury levels in the various ecosystem compartments did not vary appreciably after this time. The model results for Hg water column and fish concentrations were then used to calculate fish bioaccumulation factors. Lastly, a sensitivity analysis was conducted using the model to determine how variations in selected input parameters affected resulting bioaccumulation factors.

3. Results and Discussion

3.1 MERCURY PARTITIONING AMONG COMPONENTS

The MCM model simulated the partitioning of Hg among three forms (elemental, divalent, methyl) between all abiotic and biotic components. This analysis provided estimates of total Hg inputs and sinks at steady state. Results of the modeling for Lake Superior and Lake Erie are given in Tables II and III, respectively. For Lake Superior, almost all Hg inputs (93%) originate from deposition. Once Hg enters the lake, most of this (60%) is buried in the sediments. The difference between deposition input and sediment burial is mostly accounted for in porewater and outflow. The percent of total Hg partitioned in fish biomass is very small (2x10 -2 ); of the total Hg in fish, about 99% is in the methylated form (Table IV). The annual Hg accumulation in fish is estimated to be 0.47 mg/kg carbon of fish.

522 D. LEONARD ET AL.

TABLE I

INPUT PARAMETERS FOR LAKE ERIE A N D LAKE SUPERIOR

Parameters Lake Erie Lake Superior

River Input Flow(m3/d) 4.95x10 8 [1] 1.33x10 8 [2] Hg (reel/L) 7.1x10-11 [3] 1.5x10-12

Mercury Rain (raM) 6.5x10-11 [3] 6.6x10-11 Dry Dep.(mol/ha/yr) 5.6x10-4 [3] 2.5x10-4

Epilimnion pH 6.8-8.0 * 7.8-8.2 [4] CI (/aM) 563 * 33.8 DOC (rag/L) 3 * 0.3 Particles (mg/L) 19 * 4 SO 4 (/aM) 437.0 * 66.6

Oxic Sediments pH 6.5 7.3 CI (/aM) 563 33.8 D e c (mg/L) 10 10 Particles (kg/L) 0.05 0.05 SO 4(/a'Vl) 3O 30 0 2 (/aM) 300 300

Monthly Inputs [ 1 ] , [ 5 ] , [ 6 ] , [ 7 ] , [ 8 ] , [ 9 ] * *

Phytoplankton Biomass (g/cm) Epilimnion depth (m), Epilimnion Temp. (~ Hypolimnion Temp. (~ Rainfall (cm) Evaporation (cm) Hypolimnion D.O. (mg/L)

0.05-1.2 0.05-0.2 15-25.7 10-145 0.1-15 1-13 0.1-15 3 ~, 2.8-30 2.8-10.3 0.5-18 0-10.5

0-9 13

Sources: * Ohio Edison Co., Edgewater Plant, Lake Erie 1DiToro, D., Connolly, J.F.:1977, Mathematical Models of Water Quality in Large Lakes. Part

2: Lake Erie. Manhattan College, Bronx, N.Y. 2 Thompson, M.E.:I978, J. Great Lakes Res. 4, 361-369. 3 Kelly, J.T., Czuezwa, LM., Sticksel, P.R., Sverdrup, G.M., Koval, P.J., Hodanbosi, R.F.:I991,

J. Great Lakes Res. 17, 504-516. 4 Matheson, D.H., Munnawar, M.:1978, J. Great Lakes Res. 4, 249-263. 5 j. Great Lakes Res. 4 (1978). 6.I. Great Lakes Res. 13 (1987). 7 Collins, C.D., Wlosinski, J.H.:I983, "Coefficients for Use in the U.S. Army Corps of Engineers

Reservoir Model, CE-QUAL-RI". Technical Report E-83-15. U.S. Army Engineer Waterways Experiment Station. Vicksburg, Miss.

8 Leidy, G.R., Jenkins, R.M.:1983, "The Development of Fishery Compartments and Population Rate Coefficients for Use in Reservoir Ecosystem Modeling". Final Report. U.S. Fish and Wildlife Service. National Reservoir Research Program, Fayetteville, Arkansas.

MERCURY IN THE GREAT LAKES 523

9 Harris, R.C:1991, A Mechanistic Model to Examine Mercury in Aquatic Systems. Masters Thesis, McMaster University, Hamilton, Ontario, Canada.

** Values given are the minimum and maximum monthly value in a year.

TABLE H

MERCURY MASS-BALANCE FOR LAKE SUPERIOR (25-YEAR SIMULATION)

USING MCM MODEL

Category

Mercury Mass (Kg)

Hg(O) Hg(ED MeHg Total

Percent o f

Inpu t /Ou tpu t

Input

Output

Discharges 0 1,825 0 1,825 6.00

River 0 375 0 375 1.23

Wet deposition 0 17,833 146 17,979 59.08

Dry deposition 0 10,252 0 10~252 33.69

Total Input - - - 30,432 100.00

Gas exchange 783 12 155 641 2.10

Sediment burial 0 15,792 2,683 18,474 60.71

Porewater 1 597 473 1,071 3.52

Outflow 20 7,854 529 8r403 27.61

Total Outpu t - - - 28,589 93.94

In Lake Erie, atmospheric deposition accounts for about 20% of all Hg inputs (Table III). It is interesting to note that tributary inputs are relatively high for this lake (80%), reflecting the natural influence of large tributary flows and anthropogenic sources within an industrialized basin. Like Lake Superior, most of the Hg in Lake Erie is partitioned to the sediments (55%). Unlike Lake Superior, however, the mass of Hg accunmlated within fish biomass per year is relatively small (0.026 rag/kg carbon in Lake Erie). Like Lake Superior, almost all of the Hg in Lake Erie fish is the methylated form. However, the percent of total Hg in Lake Erie partitioned to fish is almost negligible (8.8x10 8 %).

524 D. LEONARD ET AL.

TABLE IH

MERCURY MASS-BALANCE FOR LAKE ERIE (25-YEAR SIMULATION) USING MCM MODEL

Category

Mercury Mass (Kg) Percent of

Hg(O) Hg(II) MeHg Total Input/Output

Input Discharges 0 0 0 0 0.130

River 0 64,599 0 64,599 80.30

Wet deposition 0 8,503 74 8,577 10.66

Dry deposition 0 7275 0 7,275 9.04

Total Input - - - 80,451 100.00

Output Gas exchange 17 0 -48 -31 0.04

Sediment burial 0 42,547 1 ,725 44,272 55.03

Porewater 0 1,579 314 1,893 2.35

Outflow 34 31256 3 , 0 2 7 34,317 42.66

Total Output - - - 80,451 100.00

The relatively high outflow from Lake Erie (the annual discharge is 50% of lake volume) causes a considerable amount of the total Hg loading to be partitioned to its outflow. Although the large inflow and outflow rates tend to mask the importance of sedimentation, a comparison of the mass of Hg in the water column and biota (<10%), versus that in sediments validates the importance of sedimentation in Lake Erie.

MERCURY IN THE GREAT LAKES 525

T A B L E IV

P R E D I C T E D TOTAL AND M E T H Y L M E R C U R Y CONCENTRATIONS IN EPILIMNION (WATER COLUMN) AND WALLEYE FOR LAKES SUPERIOReAND ERIE.

P R E D I C T E D BIOACCUMULATION FACTORS AND SENSITIVITY RESULTS ALSO GIVEN.

( N O T E : C1 = CHLORIDE, DOC = DISSOLVED ORGANIC CARBON,

SS = S U S P E N D E D S O L I D S )

Lako Superior Lake Erie

Epil imnion Walleye Epilimnion Walleye Analysis (ned L) (me,/kg) BAF i (rig/L) (mg/kg) n A P

Base 2 0.63 0.13 204,000 1.03 0.16 158,000

MeHg 0.019 0.13 6.7 x 10 6 0.075 0.16 2.2 x 10 6

10 x CI 0.78 0.13 171,000 - - -

10 x DOC 4.20 0.08 19,500 - - -

0.1 x DOC - - - 0.11 0.17 1,550,000

0.1 x SS - - - 9.48 0.67 70,400

0.1 x Deposit ion 0.08 0.02 260,000 0.17 0.03 188,000

10 x Deposit ion 6.37 0.20 30,600 9.40 1.53 163,000

1 Bioaccumulation factor (walleye concentration + epilimnion total Hg concentration).

2 Base case represents model results of total mercury using input variables from Table I.

3.2 MERCURY IN WATER AND BIOCONCENTRATION FACTORS

The model-predicted total Hg concentrations in the water column and top- carnivore fish for both lakes are given in Table IV. For the base case (i.e., data from Table I), the predicted epilimnion (water column) concentrations were higher for Lake Erie. This reflects the much higher levels of Hg input (due mostly to river inflow contribution) to Lake Erie.

Predicted MeHg concentrations in the water column are also given in Table IV. For Lake Superior, the predicted MeHg concentration of 0.019 ng/L represents 3% of total Hg. In Lake Erie, the predicted MeHg concentration of 0.075 ng/L represents 7.3% of total Hg. These proportions are 3-5 times lower than U.S. EPA's default proportion for the Great Lakes drainage basin (25%). In walleye from both lakes, the MeHg concentration comprised the total Hg concentration. This pattern has been observed in several field studies (Grieb et al., 1990).

526 D. LEONARD ET AL.

For the base case, the predicted bioaccumulation factor (BAF) for Lake Superior walleye was 204,000. This value is approximately 23% higher than the predicted bioaccunmlation factor for Lake Erie walleye (BAF = 158,000). The relatively higher BAF for Lake Superior is not surprising as the standing crop biomass in this lake is expected to be substantially lower relative to Lake Erie. A lower biomass pool allows a greater concentration potential for each walleye.

While the MCM has the capability to simulate these processes, accurate input parameters should be available to def'me the system. For both Lake Superior and Lake Erie, the large areas and volumes result in heterogeneity of water quality, habitat, and hydrologic partitioning (e.g., epilimnion vs hypolimnion). This data limitation can only be corrected by using measured lake-specific input values. The model-predicted results can be compared to actual measurements of Hg in aqueous samples and fish. For Lake Superior, a BAF value of 422,000 is obtained from limited walleye tissue results (0.38 mg/kg; Ontario Ministry for the Environment) and an aqueous total Hg measurement of 0.9 ng/L (Wisconsin DNR). For Lake Erie, a BAF value of 95,000 is obtained from measured Hg concentrations in walleye (mean = 0.19 mg/kg; Ontario Ministry for the Environment) and an estimated total Hg aqueous concentration of 2 ng/L. Although the predicted BAF results agree with the calculated BAF values for actual samples from the lakes (at least on a relative scale), our modelling results must be considered preliminary and subject to in-situ validation. The relative importance of feeding ecology, trophic transfer, and water quality influences upon Hg bioaccumulation in Great Lakes biota probably varies both within and among lakes.

Sensitivity analysis results (Table IV) indicated that for Lake Superior, a ten- fold increase in dissolved organic carbon (DOC) had a much larger influence on predicted walleye levels and BAF values compared to a similar increase in chloride levels. DOC had an inverse relationship with predicted BAF values in both lakes; DOC complexes active Hg in the water column, apparently making Hg less bioavailable to higher trophic levels. This relationship was observed in Wisconsin seepage lakes (Grieb et al_._~. 1990). Gilmour and Henry (1991) reported that increased DOC levels decreased the methylation rate of Hg. For Lake Erie, a ten-fold decrease in suspended solids resulted in higher aqueous concentrations and higher walleye concentrations, but a lowered BAF value. When Hg deposition rates are decreased, a similar proportional decrease of aqueous Hg and walleye Hg levels are observed for both lakes. When deposition rates increase, however, the resulting increase in walleye Hg concentrations is more marked for Lake Erie. The sensitivity analysis results highlight the deficiencies of using bioconcentration factors as a predictive measure of Hg levels in fish at high trophic levels.

3.3 APPLICATION OF MODEL FOR REGULATORY PURPOSES

The chief advantage of the MCM model is that it accounts for sediment interactions in the cycling of Hg. Moreover, the model incorporates measured, lake- specific input variables to simulate the partitioning, speciation, and bioaccumulation potential of Hg in lakes. We feel that these features provide a much more realistic estimate of allowable, protective Hg loadings compared to U.S. EPA's generic

MERCURY IN THE GREAT LAKES 527

bioconcentration assumption. As indicated previously, these assumptions fail to distinguish even large-scale limnological and morphometric differences among the Great Lakes. The MCM model, in contrast, can utilize measured sediment characteristics as input variables. Because Hg methylation is often driven by sediment processes (both abiotic and biotic), the predictive power of the model is greater than simple calculations that do not account for sediment characteristics.

U.S. EPA's proposed water quality criteria for human health and wildlife protection are expressed as a total Hg concentration. Because MeHg is the form of concern for human health effects and bioaccunmlation, any regulatory criteria for Hg should have a reasonable ability to predict the concentration of MeHg at various trophic levels. Some research findings (e.g., Kelly et al., 1994) have demonstrated a lack of any predictable relationship between total Hg and MeHg concentrations in the water column. When using the MCM model, a calibration of the mercury cycle for Lakes Superior and Erie was accomplished by partitioning the incoming mercury (mostly in the Hg +2 form) among compartments based on observed water column and fish tissue (essentially all MeHg) concentrations. If the basic cycling processes known to occur in smaller lakes are valid for larger lakes such as those studied, then the MCM approach, wihich includes sediment processes, may be a more accurate paradigm compared to the water column-based, total Hg criteria advocated by U.S. EPA.

4. Conclusion

For two Laurentian Great Lakes differing in morphometric characteristics and biological productivity, the predicted water column Hg concentration and bioaccumulation factor (top carnivore fish) for Hg was different also. Most of the Hg entering the two lakes was predicted to be buried in sediments. These results have practical application for regulatory concerns of Hg in the Great Lakes. Because most of the mercury entering the Great Lakes is from atmospheric sources and the concentration of mercury in rainfall is about ten times greater than that in surface waters of the Great Lakes, mercury cycling and sediment burial are significant processes in these systems.

The application of the MCM to the Great Lakes does have limitations. Because the model treats the water column of a given lake as two layers (epilimnion and hypolimnion, each of which is completely mixed), site-specific habitats such as embayments, marshes, or other hydrologically-disjunct areas cannot be treated separately. Data limitations included the lack of fish biomass estimates and sparse data for aqueous Hg concentrations. Nonetheless, we feel that the MCM model can provide good predictions of Hg behavior, especially among lakes having marked physicochemical or biological differences. Further research on lake-specific processes will enhance the applicability of the model.

528 D. LEONARD ET AL.

Acknowledgements

We thank Gayle Pakrosnis for typing the manuscript. research was provided by the Utility Water Act Group.

Funding for this

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