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It's Sedimentary, My Dear Watson
Remediation - Clark Fork River Remediation
Written by Gary Matson   
Recent Missoulian articles called attention to the issue of sediment movement downstream from the Milltown Superfund site, during and following the breach of the dam. Concerns about possible effects of the downstream sediments were addressed at an October 15th EPA public meeting in Bonner. Here are some highlights:
* The Milltown reservoir was mostly filled with contaminated sediment transported downstream from Butte/Anaconda. During low flow years, sediment accumulated. During high flow years, it was scoured out and washed downstream in big, biologically damaging slugs. On balance, the reservoir in recent years has released all the sediments it received.
* Downstream sites have been accumulating contaminated sediment from the Clark Fork Basin for more than a hundred years. The Milltown Dam was only a temporary settling place for the Butte/Anaconda sediments before they were scoured out by floods in 1948, 1964, and 1981 and by ice jams in 1974, 1986, and 1996. In comparison to the much larger amount of downstream sedimentation that has been ongoing since the early 1900’s, the percentage of the present accumulation in the Thompson Falls reservoir that can be attributed to the Milltown Superfund project is estimated by EPA to be 1-2%.

Scouring graph from EPA presentation Oct 15, 2009


* Less than expected sediment release from the Milltown remediation area occurred in 2006 and 2007 before the breaching of the Milltown dam, and greater than expected sediment release occurred in 2008 during and after the breaching of the dam. The release of 300,000 tons of sediment, was expected to be scoured out of the Milltown site, suspended in the river flow, and transported downstream during the dam breaching. The actual amount was calculated to be 371,000 tons. Envirocon’s contractor, the engineering firm EMC2, predicted that 603,000 tons of sediment would be released downstream during the life of the remediation project. The firm’s calculations were peer reviewed by a panel of experienced specialists and found to be valid. To date, 87% of the predicted sediment release has occurred, leaving 76,000 tons more to be released during 2009 without exceeding expectations. EPA expects that the final total of the amount released will be very close to the 603,000 tons forecast, although unusually high flows could cause the estimate to be exceeded.
* The greatest downstream concentration of suspended sediments in recent years occurred during the 1996 ice scour event. Ice and debris pushed by higher than normal river flows in the Blackfoot and in the Clark Fork above the dam scoured out contaminated sediments which were then washed downstream.
The amount of sediment in the river is measured, in milligrams per liter (mg/l), as “Total Suspended Solids” (TSS). During the 1996 ice jam, TSS downstream reached a peak of 820 mg/l, measured 1.6 miles downstream of the dam. On March 28, the day of the Milltown dam breaching, peak TSS was measured at 700 mg/l. By August 2008, the number was 10 mg/l. The EPA construction standard for the Milltown Project was 550 mg/l and this was exceeded only on the day of the breaching.
* The greatest concentrations of total arsenic in the river downstream of the dam occurred during the 1996 ice scour. A peak of 96 micrograms per liter (µg/l) was measured in 1996. In comparison, the peak level measured March 28, 2008 on the day of the dam breaching was 42µg/l, and in August 2008 it was 4µg/l.
* Using data collected by the U.S. Geological Survey, EPA estimates that the Milltown site contributed approximately half of the 697,000 tons of sediment transported to Thompson Falls during the period from the March 28 dam breaching through the high water period to June 30, 2008. The other half came from the Clark Fork above and below the dam, and from the Bitterroot, Flathead, and Blackfoot rivers.
* The Milltown site contributed approximately half of the cumulative arsenic load reaching Thompson Falls during the period March 28 – June 30, 2008. Arsenic is also transported by the Clark Fork above and below Milltown, and in smaller amounts (10% of the total) by the Bitterroot, Flathead, and Blackfoot rivers.
* Each average year, 10 tons of arsenic is transported downriver from the Upper Clark Fork and Blackfoot Rivers upstream of Milltown. The amount is much higher during above average flow years, such as 1996 and 1997. It has occurred historically, and will continue for the indefinite future until there is success in limiting erosion of contaminated areas upstream from Milltown. In comparison, the amount of additional arsenic released downstream because of activity at the Milltown remediation site is estimated to be 21 tons.
* Human health risks downstream from Milltown do not exist, as shown by water sampling and measurements of dissolved arsenic. Except for the day after the March 28, 2008 breaching of the dam, dissolved arsenic in river water has remained well below the 10 µg/l drinking water standard. No downstream wells have shown increases in arsenic as a result of the Milltown project. Arsenic concentrations in wells around the reservoir have dropped considerably. The low arsenic concentration means there is no human health risk from direct contact with the river water, either by skin contact or by mouth.
* Human health risks from recreational activities do not exist, as measured by exposure to total arsenic in river bank sediments that are much lower than health-based standards. The sediments in the river and its banks pose no health risk through skin absorption, inhalation, or incidental ingestion.
* Significant impacts to fish and aquatic life were expected for the Milltown project, and have occurred during the past 3 years. However, they are expected to be short term.
The fishery during 2008 is doing better than expected below the dam. Fish population density has increased at Turah, declined at Milltown, is unchanged at Huson, and has slightly increased in the Bitterroot. Fish mortality was very high in 2006 and 2007, due to the combined effects of the dam drawdown, other pollution sources along the river, and very high water temperatures that stress fish, causing mortality from bacterial infections.
Impacts on Fish and Invertebrates
Populations of aquatic invertebrates from the Milltown site 13 miles downstream to the confluence with the Bitterroot River have been significantly impacted during 2008. Populations were about 30% of normal this year, with the reduction being attributed to the physical smothering of stream habitat by sediments, including sand, eroded from the former reservoir.
In summary, there has been sediment released downstream. However, it is not significantly more than predicted and should in fact be lessened during 2009 by greater attention to bank stabilization in areas just upstream of the Milltown site. With the goal of reducing sediment transport in 2009 and the future, detailed plans for sediment removal and bank stabilization are now being developed by the State and EPA.
Arsenic levels in monitoring wells around the reservoir have dropped significantly. Missoula’s drinking water is not at risk. It is safe to recreate in the river. The project has impacted the Clark Fork fishery, but biologists expect the effects to be short term and to be far outweighed by the long term benefits. Aquatic macroinvertebrates (the water-dwelling worms and insects that are essential links in a river’s food chain) have been most significantly impacted, with a 70% reduction in populations downstream to the confluence with the Bitterroot River. However, populations are expected to recover within the next several years. Significant impacts to fish and macroinvertebrates did not extend downstream of the Bitterroot.
UM's & EPA's Testing Methods Differ
How about the warning by the University of Montana scientists that the dam breaching caused greater than predicted sediment release and more downstream contamination? To answer this uncertainty, I visited with Dr. Andrew Wilcox, UM geomorphologist who along with Dr. Johnnie Moore studies impacts upon the river downstream from Milltown. The UM scientists used different data than EPA to calculate sediment transport. They use the weekly measurements taken by USGS; EPA uses daily USGS measurements. The weekly measurements have the advantage of being somewhat more rigorous because they take readings across the breadth of the river channel. The daily measurements have the advantage of greater frequency, but take readings only at the center of the channel. The different numbers in the measurements explain the different calculations of the amount of sediment transport.
A second reason that the UM scientists obtained a larger sediment transport amount than EPA is because they included “bed load”. That’s the amount of material that moves along the bottom of the river rather than being suspended in the flow above. Under conditions of moderate flow, finer sediments that can contain metals contamination move along the bottom. During the more turbulent higher flow, finer sediments are suspended off the bottom. It is the finer sediments that carry the greater metals contamination downstream. Samples of this fine-grained sediment collected by University researchers in early May – after the breach and before the peak of runoff – contained higher than average arsenic and copper concentrations, five to six times higher than usual. But as the river rose throughout May, metal concentrations decreased substantially as cleaner material was mixed in. By early June, concentrations of arsenic and copper in fine-grained bed sediments were close to typical levels for this part of the river. The impact of this short-term spike of metals in the silts and clays of the river bed is unknown.
Long Term Benefits
Although UM and EPA scientists’ sediment transport numbers differ, each has confidence in its estimates. The public can expect the implications of these differences to be understood through open, careful comparison of methods and conclusions. The EPA evaluation suggests that the risks are proving to be less than the benefits. At the same time, the UM scientists’ evaluation suggests a need to remain wary. They expect their studies to yield useful information about how the river downstream of the dam reacts to the uniquely powerful influences of dam and contaminant removal. They recognize that downstream impacts deserve ongoing evaluation. No one denies that removing the dam and contaminated sediments will cause short term impacts but bring long term benefit to the Clark Fork River and to the people, fish, and wildlife within its influence.
Although there are surely unexpected events ahead, so far those that have occurred during the remediation project have been successfully dealt with and a successful cleanup is the likely outcome. Although some may rightfully choose to remain wary, the evidence is building that there are no culprits afoot to cause lasting harm and diminish the benefit of the Milltown clean-up for us and for our environment.
The author thanks Chris Brick, Judy Matson, and Mike Kustudia for reviewing the text and making suggestions. Peter Nielsen offered many substantive changes that greatly improved accuracy and content.