Milltown Dam Sediment Release Tracking Study

Principal Researchers

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 

Background

For 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 Blackfoot Rivers to flow free again. This is one of the largest dam removal and river restoration projects ever attempted and has a price tag of $120 million. Before the start of the restoration, Milltown Reservoir held more than 6 million cubic yards of highly contaminated sediment transported from the mining and smelting operations upstream. Arsenic, Cadmium, Copper, Lead and Zinc are highly enriched in the reservoir sediment and were transported downstream in the past during high-flow years and ice-jam floods. Arsenic was also mobilized from the reservoir sediment, contaminating the groundwater, including adjacent drinking water wells. The goal of the restoration of the river is to remove this source of groundwater and river contamination and then remove the dam to restore upstream connectivity to allow fish passage at the confluence of the Blackfoot and Clark Fork Rivers. (See: EPA Milltown Superfund Site). As of mid June 2008, nearly 900,000 cubic yards of sediment have been excavated from the reservoir and transported upstream to a repository site. The final objective is to remove about 2.2 million cubic yards of the most contaminated sediment out of the more than 6 million cubic yards residing in the reservoir. The dam was breached on March 28, 2008 (Milltown Breach YouTube Video) allowing these two rivers to flow free for the first time since the dam was closed in 1907.

Objectives

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.

Methods

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.

Preliminary Results

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.

In 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 Blackfoot River above dam and Bitterroot River below dam, showing background concentrations at the same sampling time. The Green squares with a dot are data from the USGS (various open file reports) measured on samples below and above Milltown Dam in years 2004, 2005 and 2006. These data show the values downstream of the dam before breaching at the dam site. The post-breaching data show that metals concentrations below the dam have been elevated more than 5 times for arsenic and about 3-4 times for copper. Similar trends are seen for other metals of concern (cadmium, lead and zinc) found in the CFR mining wastes.