Lead Isotopic Compositions of Bed Sediments Suggest Clark Fork River Sources
Bouse, Robin M., Hornberger, Michelle I., Luoma, Samuel N.,
US Geological Survey, 345 Middlefield Road, Mail Stop 465, Menlo Park, CA 94025

Lead (Pb) isotopic compositions of sediments can provide constraints on the contribution of different sources of metal contamination.  Sieved bed sediments (<64µ) were sampled in the Clark Fork River, Montana, in 1998 at 12 sites between Butte and Missoula.  Pb isotopic compositions in Clark Fork River bed sediments (Opportunity-Alberton) range from 206Pb/204Pb=17.86-18.315, 207Pb /204Pb=15.530-15.561, 208Pb/204Pb=38.149-38.339, and 206Pb/207Pb=1.1505-1.1796.  Pb concentrations range from 797 µg/g at Opportunity to 41µg/g at Alberton.  The tributaries Rock Creek and Blackfoot River have Pb concentrations between 10-13µg and Pb isotopic compositions around 206Pb/204Pb=19.2, 207Pb /204Pb=15.64, 208Pb/204Pb=39.0, and 206Pb/207Pb=1.23.  On a plot of 206Pb/207Pb versus 1/Pb, a linear regression through Rock Creek sediment, two slickens, and eight bed sediment samples between Pond Outfall and Goldcreek, has R2 = 0.9980.  Bed sediment at Turah and Alberton plot slightly off this regression line towards the isotopic composition of Flint Creek. 

The sources contributing Pb to Clark Fork River bed sediments were estimated, using Rock Creek as the background Pb isotopic composition and concentration.  From Opportunity to Goldcreek, the contaminant in bed sediments has the same isotopic composition as slickens near Dempsy and Opportunity.  At Turah, the Pb contribution from Flint Creek is approximately 30% and at Alberton approximately 20%.  If the Blackfoot River is used as the background value, the percent Pb contribution from Flint Creek is decreased to approximately 10% at Alberton.    

The data suggest that: 1) the dominant source of Pb and other metals in the river from Butte to Missoula are the slickens of Silver Bow Creek; 2) Pb contamination from slickens near Opportunity are progressively diluted downstream by cleaner sediments resembling those of the Rock Creek watershed; and 3) Flint Creek contributes only a small portion of the Pb downstream from its confluence with the Clark Fork River.