Lake Michigan Mass Balance

Contaminant Transport and Fate Model

The mass balance for toxic chemicals in the lake will be computed in a contaminant transport and fate (CTF) model, which describes contaminant transport, intermedia exchange, phase distribution, and biogeochemical transformations, in both the water column and sediments. The CTF model will be calibrated and confirmed for each of the priority toxics: atrazine, mercury, selected individual and sum-of-PCB congeners, and trans-nonachlor. Mass balance analyses will be performed for each contaminant to evaluate the significant source, transport, and loss pathways. Effectiveness of alternative load reduction scenarios upon reducing toxic chemical concentrations will also be forecast. Although calibration and confirmation will be limited to the period of available enhanced monitoring program (EMP) data, the CTF model will be required to forecast contaminant concentrations for substantially longer periods: on the order of 20-50 years. Long simulations are necessary because of the substantial lag time associated with the chemical concentration response in the lake to changing loads. The lag time is associated with the residence time of contaminants in the surficial sediments, which is constrained by confirmation of CTF model hindcasts for cesium-137 and/or plutonium-239/240. These particle-associated radionuclides have been demonstrated as important tracers for the long-term transport of sediments and contaminants in Lake Michigan and the Great Lakes. Because their loading histories are known with relative certainty, available water and sediment data for these contaminants are directly useful for model confirmation. Such data are critical to development of a model intended to make long-term forecasts, especially since EMP monitoring will only be 2 years in duration. Intensive sediment trap data collected in 1982-83 (Robbins and Eadie, 1993) and water column measurements from the same period, will provide further measurements for confirmation of particle transport fluxes.

Chemical fluxes between model compartments are computed from advective and dispersive transport of aqueous and particulate contaminant fractions. The model will describe chemical partitioning between dissolved and particulate sorbent compartments, including multiple particle types, using an organic carbon-based equilibrium assumption. Both logical equilibrium and first-order kinetic partitioning process descriptions will be tested. Chemical transformations such as hydrolysis and biodegradation are modeled as first-order or pseudo first-order reactions with daughter chemicals retained in the mass balance as additional state variables (for atrazine, these include desethylatrazine and deisoproplylatrazine). For mercury, a two-state (organic and inorganic) multiple-sorbent class framework proposed by Thomann (1993) will be applied.

The CTF model will be linked to hydrodynamic and sediment transport simulations by appropriate filtering and averaging of transport fields (Hamrick, 1987; Hall, 1989; Dortch et al., 1992). Total suspended solids (TSS) and SPCB (the sum of the PCB congeners) simulations will be reproduced in both SEDZL and CTF models, providing computational "tracers" to validate the transport linkages.

The CTF model will be applied at an intermediate (Level 2) scale. In the water column, segment resolution is defined at a scale compatible with the definition of food web zones (approximately 20x40 km.), with 2-5 vertical layers. In sediments, segmentation will be based upon deposition regime and contaminant distribution, with 1-cm vertical resolution. Fine-scale simulations are necessary for accurate predictions of hydrodyamic and cohesive particle transport as well as accurate simulation of short-duration event processes. However, the computational cost of fine-scale models is high and makes long-term (20-30 years) simulations infeasible, especially with the significant number of state variable s required for multiple contaminants, sorbent phases, etc. Resolution at the scale of POM and SEDZL is also not appropriate for the mass balance objectives of this project. Intermediate scale models have substantially lower computational cost and have been demonstrated for contaminant transport and transformation over temporal and spatial scales appropriate for toxics exposure prediction and linkage to bioaccumulation models (DePinto et al., 1993; Connolly et al., 1992).

Although CTF model compartments are generally well-defined, no single framework presently available has the capacity to accurately predict all components of CTF while retaining the aggregate behavior of hydrodynamic and sediment transport simulations. To develop an appropriate framework for the LMMB study and future lake-wide analysis and management projects, existing and developmental mass balance water quality modeling frameworks such as those used for Chesapeake Bay (Cerco and Cole, 1993), Green Bay (Bierman et al., 1992; Velleux et al., 1994), and other projects (Richards et al., 1993; Katopodes, 1994) will be reviewed. Appropriate features of these models will be synthesized into a single framework and extended to meet the requirements of the LMMB study.

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