THE LAKE COEUR D'ALENE STORY: A PHYSICAL AND BIOGEOCHEMICAL MODEL TO SIMULATE ZN AND NUTRIENT CYCLING AND TO ASSIST MANAGEMENT DECISIONS

Vast quantities of metal-enriched mine waste were produced during historical milling, concentrating, and smelting of Ag/Pb/Zn-rich ores in the Coeur d’Alene mining district in northern Idaho. Prior to environmental regulation in 1968, the waste was deposited into the South Fork Coeur d’Alene River and its tributaries. Dynamic physical, chemical, and biological processes now influence the distributions and concentrations of particulate and dissolved metals in the river basin (Balistrieri and others, 2002). Stream transport, especially during major floods, carries metals from their sources in the district to downstream repositories, including Lake Coeur d’Alene. The health of humans and other biota in the basin is affected by this legacy of contaminated water, soil, and sediment. In particular, dissolved Zn concentrations govern water quality in the region because of their adverse impact on the health of fish.

The Centre for Water Research at the University of Western Australia in collaboration with the U.S. Geological Survey developed a 3-D numerical hydrodynamics-biogeochemical model (ELCOM-CAEDYM) to understand the interplay among physical transport processes, primary production and nutrient cycling, and dynamics and toxicity of Zn (Dallimore and others, 2007; Hipsey and others, 2007). The goals of the work were to 1) describe the cycling of Zn and nutrients in the lake, 2) improve the understanding of feedbacks between Zn toxicity and eutrophication, 3) provide information on data gaps and sampling strategies, and 4) evaluate alternative remediation scenarios. Data collected in the lake were used to validate the model and identify processes currently controlling Zn concentrations in the lake, and long-term scenarios were run to evaluate how the lake may respond to different remedial actions.

Of importance to lake managers and decision makers, the simulations indicated that 1) a combination of low phosphate and Zn toxicity keeps the biomass low, 2) the lake is more sensitive to phosphate than to Zn concentrations; specifically eutrophication pressure due to development may be more important than upstream remedial action to reduce Zn loading, and 3) a strategic monitoring plan is needed to improve model use and prediction.

Balistrieri, L.S., Box, S.E., Bookstrom, A.A., Hopper, R.L., and Mahoney, J.B., 2002, Chapter 6 - Impacts of historical mining in the Coeur d'Alene River Basin, in Balistrieri, L.S., and Stillings, L.L., eds., Pathways of Metal Transfer from Mineralized Sources to Bioreceptors: A Synthesis of the Mineral Resources Program's Past Environmental Studies in the Western United States and Future Research Directions: U.S. Geological Survey Bulletin 2191, p. 1-34.

Dallimore, C.J., Hipsey, M.R., Alexander, R., and Morillo, S., 2007, Simulation model to evaluate Coeur d'Alene Lake's response to watershed remediation: Volume 1: Hydrodynamic modeling using ELCOM: Centre for Water Research, The University of Western Australia Report WP 2133 CD, 51 p.

Hipsey, M.R., Alexander, R., and Dallimore, C.J., 2007, Simulation model to evaluate Coeur d'Alene Lake's response to watershed remediation: Volume 2: Water quality modeling using ELCOM-CAEDYM: Centre for Water Research, The University of Western Australia Report WP 2132 MH, 83 p.