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Sediment-Water Interactions in the Metal-Contaminated Floodplain of the Clark Fork River, Montana, USA

Rebekah Brooks and Johnnie N. Moore
GeoJournal
Vol. 19, No. 1, Floodplain Environments (July 1989), pp. 27-36
Published by: Springer
Stable URL: http://www.jstor.org/stable/41144488
Page Count: 10
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Sediment-Water Interactions in the Metal-Contaminated Floodplain of the Clark Fork River, Montana, USA
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Abstract

The distribution and concentration of metals and metalloids in the floodplain of the Clark Fork River of western Montana, USA, are mainly controlled by postdepositional diagenetic mechanisms of metal fractionation. Due to the influx of wastes into the river's headwaters from mining processes around the turn of the century, extensive amounts of contaminated material were deposited onto the floodplain. Tailings were deposited as widespread overbank deposits and point bars adjacent to abandoned channels, and are characterized by orange and gray mottled sediment, which is devoid of vegetation and covered by a blue metal sulfate precipitate during dry periods. Examination of stratigraphic profiles of floodplain sediment indicates three periods of deposition: 1) pre-mining, represented by coarse sand and organic overbank deposits under reducing conditions; 2) syn-mining, characterized by transition sediments and tailings deposits under oxidizing conditions; and, 3) post-mining, distinguished by grass-bound topsoil. Sites were established where sediments and water throughout the stratigraphic section were collected and analyzed. Chemical analyses indicate enriched concentrations of cadmium, copper, manganese, and zinc in sediments and porewater, and arsenic in groundwater, in areas contaminated by tailings deposits. Vertical trends in concentrations of metals show that they are distributed based on apportionment of metal phases between reducingoxidizing environments and pH fluctuations.

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