Milltown Dam Sediment Release Tracking Study
Johnnie N. Moore, Professor, Geosciences Department and Director of the Center for Riverine Science and Stream Renaturalization
Katrina Garcia, Graduate Student, Chemistry Department
Heiko Langner, Assistant Research Professor, Geosciences Department and Director of the Environmental Biogeochemistry Laboratory
over 100 years, Milltown Reservoir, just east of Missoula, MT, accumulated
the waste from the mining and smelting operations over 180 Km upstream
at Butte and Anaconda, MT. The Dam and the contaminated sediment
behind the dam is now being removed under a “Superfund” agreement
to allow the Clark Fork and
Along with the construction and deconstruction at the Milltown site, monitoring of water quality of the river water and adjacent groundwater are being done periodically (Downlaod from this ftp site: ftp://milltown.envirocon.com/). This data shows that during much of the time that the reservoir was drawn down for sediment removal and since the dam has been breached a large amount of sediment and metals have been mobilized from the reservoir. For example, data collected on May 19, 2008 by consultants for the project showed that approximately 800 lbs Cu/day came into the reservoir but nearly 20,000 lbs Cu/day were released (see data in Master1013_Surface_Water_Monitoring_Data_2008.xls at the ftp site listed above). More detailed sampling by the U.S. Geological Survey (see ftp site above for individual dates) showed that a month earlier, about 2100 lbs Cu/day was being released from the reservoir with almost 900 lbs Cu/day coming from the sediment within the reservoir upstream of the removal area. It is estimated that approximately 200,000 cubic yards of sediment, and their associated metals, had been released from the reservoir by June 2008. There is approximately another 3 million cubic yards of sediment available for erosion and transport downstream. The objective of the Milltown Dam Sediment Release Tracking Study is to determine the fate of the sediment-bound metals released to the river below the reservoir site.
Sediment collection sites were first established below the reservoir for approximately 70 Km to determine the distribution of metals released from the reservoir and above the reservoir to determine concentrations of metals entering the site (see table below). However, after the first sampling event showed highly elevated metals concentrations in sediment at the farthest downstream site (70Km), the downstream sampling was extended to 250 Km below the reservoir. Fine-grained bed sediment (less than about 63 micrometers, i.e., mud) is collected at these sites using standard methods for collecting fine-grained bed material (for detailed sampling methods see: USGS Open File Report 2007-1301, p11 at http://www.usgs.gov/pubprod, and/or Nagorski et al., Mine Water and the Environment (2002) 21:121-136). Sediment samples were digested using USEPA Method 3050B and analyzed on ICPOES using USEPA Method 200.7 and for mercury on CVAFS by USEPA Method 245.7.
Samples have been collected on 3-5 May, 2008, 21-25 May, 2008 and 9-10 June, 2008. Below are the preliminary plots of the geochemical data from the 3-5 May sampling event.
the two figures below, all data are from the chemical analyses of mud
deposited on the bed of the river (sediment less than 63 micrometers in
size). Red squares are Clark Fork River (CFR) sediment below Milltown
Dam (to the right of the vertical line) and above the dam (left of the
vertical line). Blue circles are for samples from tributaries, the