Modeling the Biogeochemical Cycle of Mercury in Lakes: The Mercury Cycling Model (MCM) and Its Application to the MTL Study Lakes

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1 ~- -.,, OLO ('''"' ~...,;;,, FEB Chapter V.1 Modeling the Biogeochemical Cycle of Mercury in Lakes: The Mercury Cycling Model (MCM) and Its Application to the MTL Study Lakes CONTENTS Robert J. M. Hudson, Steven A. Gherlnl, Can J. Watm~ and Donald B. Porcella Abstract I. Introduction II." Theory of Mercury Biogeochemical Cycling... : A. Mercury Speciation in Lakes B. Speciation and Biologically Mediated Processes C. Diffusion Limitation of Biotic Mercury Uptake D. Speciation and Abiotic Processes ill. Modeling the Mercury in Temperate Lakes (MTL) Study Data A. Seston-Water Partitioning and Aqueous Speciation of Mercury B. Steady-State Model of the Mercury Cycle in Seepage Lakes C. Accumulation of Methylmercury in Fish D. Summary and Synthesis N. The Mercury Cycling Model (MCM) A. Description B. Application V. Conclusions Acknowledgments Appendix 1: Seston-Water Partitioning of Mercury Appendix 2: Steady-State MCM Equations... ; Appendix 3: Gas Exchange of HgO and CH~g References ABSTRACT: As a part of the Mercury in Temperate Lakes (MTL) study in Wisconsin, U.S., we have developed a mechanistic model of the biogeochemical cycle of mercury in lakes. The Mercury Cycling Model (MCM) is a deterministic simulation model that incorporates the major processes that transport mercury across lake boundaries--atmospheric deposition, gas exchange, inflow and outflow of water, and burial in sediments; chemically transform it-reduction, methylation, and demethylation; and lead to its accumulation in aquatic biota-uptake~ depuration, and trophic level transfer. In this chapter, we discuss the theory of mercury biogeochemical cycling, apply a simplified, steady-state version of the MCM to analyze field data from the MTL study, and examine mechanistic issues using the dynamic MCM model. Theories of mercury biogeochemistry are based on knowledge of the aqueous speciation of mercury and the mechanisms of its reactions-considerable gaps in /94/$0.00+ $ Lewis Publishers 473

2 474 MERCURY POLLUTION: INTEGRATION AND SYNTHESIS our knowledge of both remain, however. Our approach, therefore, is to use what is known about mercury biogeochemistry to formulate hypothetical rate and equilibrium expressions. Using a simplified, steady-state version of the MCM model, we then examine the ability of these expressions to describe the field data of the M1L study. For example, although it is known that Hg 2 and CH3Hg' ions are complexed by the hydroxide, chloride, sulfide, and humic acids present in lakewater, uncertainties in the strength of organic complexation and the concentrations of competing metals make equilibrium calculations speculative. Applying our model of seston-water partitioning for Hg 0 and CH 3 Hg 11 to the MTL data, we examine potential mechanisms of mercury uptake by phytoplankton and the strength of organic complexation in these lakes. Our analysis suggests greater than 70% organic complexation of both Hg 0 and CH 3 Hg 0 and significant accumulation of CH 3 Hg 0 by plankton. To model the in-lake cycling of mercury, we derive theoretical rate laws, corresponding to a variety of hypothesized mechanisms for the reactions of the lacustrine mercury cycle and test their ability. to predict the observed concentrations of HgO, Hg 0, and CH 3 Hg 0 in lakewater. Only a limited number of mechanisms were consistent with the data. In modeling bioconcentration factors (BCFAsH) for CH~g 0 in fish, we found that the binding of CH 3 Hg 0 by DOC explains the observed dependence of BCFAsH on DOC, and that calcium inhibits bioaccumulation, likely at trophic levels above phytoplankton. Our results suggest observed correlations between lake ph and fish mercury content arise from generally higher CH~gn concentrations at low ph and lower bioconcentration factors at high ph. To investigate issues relating to food chain accumulation and the generation of CH~g 0 in anoxic waters in greater detail, we employed the dynamic MCM model. We hypothesize that passive uptake of the neutral Hg(SH)~ species by methylating bacteria is an explanation for the apparently high methylation rate of Hg 0 in suband anoxic waters. I. INTRODUCTION Reports of elevated levels of mercury in fish from remotely located, low ph lakes have generated public concern about the scope of environmental mercury contamination. 1-4 Because of mercury's toxicity to humans and wildlife, these findings may have significant public policy implications if the elevated levels of mercury in fish are caused by lake acidifications or recent increases in atmospheric mercury deposition. 6 7 Consequently, the quantity and sources of mercury in atmospheric deposition and the environmental factors that govern its biogeochemical cycling and bioaccumulation in lake ecosystems are the subjects of much scientific inquiry. These questions have proved difficult to answer in the past. The primary reason is that methods of sampling and analysis that are sufficiently sensitive and contamination-free to obtain accurate measurements of environmental mercury levels have been developed only recently. Using these new methods, limnologists and oceanographers have discovered that mercury exists at extremely low concentrations in surface waters-as low as 1 ng/l total with 5 to 20% methylmercury-and in precipitation-} 0 ng/l total with about 1% methylmercury.s-13 These concentrations are 2 to 3 orders of magnitude lower than the levels of mercury typically employed in experimental studies, raising doubts about the relevance of such studies for understanding mercury biogeochemistry. A knowledge of mercury chemistry and its microbial transformations has, however, enabled scientists to predict some features of the mercury cycle in natural waters that are consistent with recent observations Along with the basic reactions of the mercury cycle, the chemical principles which govern the behavior of mercury in the environment-the

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