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A mercury transport and fate model (LM2-mercury) for mass budget assessment of mercury cycling in Lake Michigan
Citation:
Zhang, X., K. Rygwelski, R. Kreis, AND R. Rossmann. A mercury transport and fate model (LM2-mercury) for mass budget assessment of mercury cycling in Lake Michigan. JOURNAL OF GREAT LAKES RESEARCH. International Association for Great Lakes Research, Ann Arbor, MI, 40(2):347-359, (2014).
Impact/Purpose:
LM2_Mercury, a mass balance model, was developed to investigate and evaluate the transport, fate, and biogeochemical transformations of mercury in Lake Michigan; to forecast mercury concentrations in the lake; and to provide insights into long-term responses of the lake to various mercury loading scenarios. This manuscript includes a detailed description of the model development, model construct, model inputs and parameterization, model calibrations and confirmation, and mercury mass budget assessment. The model simulates total mercury (tHg), and the three mercury species – elemental mercury (Hg0), divalent mercury (Hg2+), and methylmercury (MeHg), and their interaction with total suspended solids (TSS) and dissolved organic carbon (DOC). The model was designed to have enough sophistication in model conceptualization to capture key processes while maintaining simplicity of speciation processes. The model was thoroughly calibrated to intensive and comprehensive field data collected during 1994 and 1995. A mercury model hind-cast was conducted. The results of the model hind-cast provide further confirmation to the representativeness of model parameterization and capability of the model used for long-term prediction. A mass budget assessment conducted based on the model results showed that loads from atmosphere (wet and dry deposition, and absorption) are the largest sources of mercury to the lake water column; interaction between water and the surficial sediment, and air-water exchange are the other key processes controlling tHg cycling and concentrations in the lake. In-situ production of Hg0 from Hg2+ reduction is the dominant internal source of Hg0 for Lake Michigan. The internal source for MeHg contributed from Hg2+ methylation is a significant source of MeHg to the lake. This work supports the Clean Water Act, Clean Air Act, Critical Programs Act (Lake Michigan LaMP), Great Lakes Legacy Act, Binational Toxics Strategy, and the US-Canada Great Lakes Water Quality Agreement.
Description:
LM2-Mercury, a mercury mass balance model, was developed to simulate and evaluate the transport, fate, and biogeochemical transformations of mercury in Lake Michigan. The model simulates total suspended solids (TSS), disolved organic carbon (DOC), and total, elemental, divalent, and methyl mercury as state variables. The transport and fate of mercury are linked to the movement of water, TSS, and DOC in the model. Simplified speciation processes among the mercury species including reduction of divalent mercury, methylation, and demethylation are incorporated in the model. LM2-Mercury is the only comprehensive, multi-compartment model to date that is capable of simultaneously simulating the dynamics of suspended solids, total mercury, and the three most important mercury species in Lake Michigan. The results of the model calibration and confirmation demonstrate the representativeness of the model construct and parameterization for mercury cycling in Lake Michigan. Using the model, a mass budget assessment of mercury cycling in the lake was conducted. The results of the assessment indicate that the atmospheric loading, including wet and dry deposition and absorption of reactive gaseous mercury (RGM), was dominate direct external contributor of total mercury to Lake Michigan, while water-sediment interaction (settling and resuspension) and air-water exchange were the key processes controlling the fate of total mercury in the lake. The dominate internal sources for elemental and methyl mercury in the water column were from divalent mercury reduction and divalent mercury methylation, respectively. The greatest loss of methyl mercury from the surficial sediments was due to demethylation
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A mercury transport and fate model (LM2-mercury) for mass budget assessment of mercury cycling in Lake Michigan