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RESEARCH PROJECTS
H E A V Y M E T A L B I O G E O C H E M I S T R Y
Lake Coeur d'Alene (CDA) is a natural lake of glacial origin located in Northern Idaho, USA. The two main rivers feeding the lake are the St. Joe River which lies within a relative pristine watershed and the CDA River which includes the watershed of the South Fork of the CDA River. It is within the South Fork drainage where the world-class mining district known as the Silver Valley is located. During the first 70 years of the 20th century, mining in this district produced over 1 billion troy ounces of silver and 8 million tons of lead. As a result of this activity, mine tailings and mill slurries contaminated with Pb, As, Cd, Zn and other metal(loid)s have accumulated throughout the flood plain of the CDA River and in the sediments of Lake CDA. Annual and episodic flood events continue to resuspend, transport, and redeposit these sediments both in the river and in the lake. We are characterizing the processes responsible for metal(loid) mobilization and transport within Lake CDA itself and also in mine tailings located adjacent to the CDA River. Ultimately our results will be used to predict the effect redox changes have on the release of metal contaminants from the sediments to the overlying water column. This is especially important given that increased use of local water resources and population growth in the watershed will likely augment nutrient loading to the Lake.
Peepers
Our latest research on Lake CDA sediments indicates that a gradient from oxic
conditions at the sediment-water interface to anoxic conditions below 15 cm. This
dynamic redox environment at the sediment-water interface stimulates the dissolution
of reduced minerals, precipitation of oxides, and subsequent scavenging of metal(loids).
Fluvial depositions from frequent seasonal flood events bury these oxides and
reductive dissolution takes place as they transition to the anoxic zone, consequently
releasing associated metal(loids) to the porewater. Insufficient sulfur limits
the formation of sulfide minerals, but high carbonate content of this mining region
buffers pH and promotes diagenetic formation of siderite. Diagenetic reactions
create chemical gradients encouraging the diffusion of metal(loids) toward the
sediment-water interface, thereby increasing the potential for flux into the overlying
water.
Dialysis samplers or "peepers" were used to obtain sediment porewater in both Lake CDA and ponds along the CDA River.
We are collaborating with research groups at
Stanford and
Dartmouth in conducting synchrotron-based x-ray absorption studies (XAS) on sediments.
Here is our most recent publications on the topic.
Toevs, G., M.J. Morra, L. Winowiecki, D. Strawn,
M.L. Polizzotto, and S. Fendorf. 2008. Depositional influences on porewater
arsenic in sediments of a mining-contaminated freshwater lake. Environ. Sci.
Technol. (in press).
Toevs, G., M.J. Morra, M.L. Polizzotto, D.G. Strawn, B.C. Bostick, and S. Fendorf. 2006. Metal(loid) diagenesis in mine-impacted sediments of Lake Coeur d'Alene, Idaho. Environ. Sci. Technol. 40:2537-2543.
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Matthew J. Morra
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