Spatial distribution of trace elements in floodplain alluvium of the upper Blackfoot River,Montana |
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Authors: | Gregory S Vandeberg Charles W Martin Gary M Pierzynski |
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Institution: | (1) Department of Geography, University of North Dakota, 221 Centennial Drive Stop 9020, Grand Forks, ND 58202-9020, USA;(2) Department of Geography, Kansas State University, 118 Seaton Hall, Manhattan, KS 66506-2904, USA;(3) Department of Agronomy, Kansas State University, 2004 Throckmorton, Manhattan, KS 66506-6101, USA |
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Abstract: | The Mike Horse Mine tailings dam in western Montana was partially breached in 1975 due to heavy rainfall and a failed drainage
bypass. Approximately 90,000 tons of metal and arsenic-enriched tailings flowed into Beartrap Creek and the Blackfoot River.
The spatial distribution of trace elements As, Cd, Cu, Mn, Pb, and Zn in floodplain alluvium of the upper Blackfoot River
were examined along 20 transects in the upper 105 river kilometers downstream from the tailings dam. Trace element concentrations
decrease with distance from the failed dam, with As reaching background concentrations 15 km from the Mike Horse dam, Cd and
Pb at 21 km, Cu at 31 km, and Mn and Zn at 37 km. Distance from the Mike Horse tailings dam and mine area is the dominating
factor in explaining trace element levels, with R
2 values ranging from 0.67 to 0.89. Maximum floodplain trace element concentrations in the upper basin exceed US. EPA ecological
screening levels for plants, birds and other mammals, and reflect adverse hazard quotients for exposure to As and Mn for ATV/motorcycle
use. Trace element concentrations in channel bank and bed alluvium are similar to concentrations in floodplain alluvium, indicating
active transport of trace elements through the river and deposition on the floodplain. The fine fraction (<2 mm) of floodplain
alluvium is dominated by sand-sized particles (2.0–0.05 mm), with Cu and Mn significantly correlated with silt-sized (0.05–0.002 mm)
alluvium. Ongoing remediation in the headwaters area will not address metal contamination stored downstream in the channel
banks and on the floodplain. Additionally, some trace elements (Cu, Mn and Zn) were conveyed farther downstream than were
others (As, Cd, Pb). |
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