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Adjacent to Southeast Side of Town



The Milltown Reservoir Operable Unit of the Milltown Reservoir Sediments NPL site is located 2 to 3 miles east and upstream of the city limits of Missoula, Montana, at the confluence of the Clark Fork and Blackfoot Rivers. Historic mining activity in Butte and Anaconda, Montana, contaminated river sediments along the Clark Fork River with heavy metals and arsenic. Contaminated sediments collected behind the Milltown Dam and released arsenic to the underlying groundwater aquifer. A replacement drinking water supply has been provided to the people of Milltown, but two area churches with wells have not been provided with an alternate water source. Also, the sediments and groundwater remain contaminated and cause risk to downstream aquatic life.

The US Environmental Protection Agency (EPA) published a cleanup proposal for the Milltown Reservoir Sediments Operable Unit in April 2003 and accepted public comments until July 21, 2003 [1]. The plan includes removing a portion of arsenic-contaminated sediments from behind the dam as well as removal of the dam, to permanently address the source of groundwater contamination and allow the groundwater to recover naturally, and also to minimize the possibility of downstream releases of sediment that may impact aquatic life.

A private citizen living near the site contacted the Agency for Toxic Substances and Disease Registry (ATSDR) with concerns about possible exposure to arsenic-contaminated dusts potentially created by drying sediments during implementation of EPA's plan. In this document, ATSDR evaluates the public health significance of this potential exposure.


Beginning in the 1860s and continuing into the 1970s, mining and milling wastes from Anaconda and Butte, Montana, were released in an uncontrolled fashion into the upper Clark Fork River basin. Milltown Dam was constructed approximately 140 miles downstream near Missoula, Montana in 1906 and 1907 to provide hydroelectric power. Periodic flooding and storm events caused wastes and sediments contaminated with arsenic and heavy metals to be deposited in the Milltown Reservoir. In the 1980s, after large scale upstream mining and mineral processing operations ceased, arsenic exceeding federal drinking water standards was found in Milltown drinking water wells. In 1985, an alternate drinking water source was provided, but contaminated sediments are a source for continued release of contaminants into the groundwater. In addition, sediments could pose a threat to aquatic life if they are released downstream.

According to US Census 2000 information, 143 people live within the Milltown ZIP code area (59851) [2]. However, the potentially exposed population includes not only Milltown but communities including Bonner, Piltzville, West Riverside, Marshall Grade and Pinegrove. The Bonner School District, which includes all of these communities and surrounding areas, has an estimated population of about 1,400, according to the Bureau of Business and Economic Research at the University of Montana (personal communication, Peter Nielsen, Missoula City-County Health Department, August 2003). Milltown is about 2 miles outside of the city limits of Missoula, Montana, which had a population of just over 57,000 in 2000 [2].

In a 1993 analysis of the site, ATSDR found drinking water exposure to arsenic to be the primary public health concern [3]. No actions to address this risk were considered necessary since people had been provided an alternate drinking water source. Implementation of the proposed plan may change the possible exposures so that a re-analysis of the potential public health risks posed by contaminated sediments is warranted.

Overview of the Proposed Plan

ATSDR reviewed the April 2003 proposed cleanup plan for the Milltown Reservoir Sediments and other available site documents in producing this report. The final cleanup plan is likely to change appreciably from the proposed plan on the basis of public comments received (personal communication, Russ Forba, US Environmental Protection Agency, June 2003). ATSDR may modify the conclusions of this report if necessary based on the actual cleanup plan. The major features of the plan are summarized below:

  • Removal of 2.6 million cubic yards of sediments from "Area I" in the lower reservoir (the highest contaminant levels closest to the dam)
  • Removal of the Milltown Dam
  • Groundwater institutional controls and natural attenuation within the aquifer plume

The sediments that are to be removed have an average arsenic concentration of 320 milligrams of arsenic per kilogram of sediment (mg/kg) and an average copper concentration of 2,300 mg/kg [1]. The physical properties and size distribution of the sediment particles have been characterized; the majority of the test specimens were classified as silt or silt with sand [4].

About 15% of the sediments (dry sediments and the vegetative mat) will be mechanically removed (i.e., excavated) and either mulched and transported as a slurry or hauled by truck or rail to a repository outside the 100-year floodplain. The remaining 85% will be dredged hydraulically, pumped as a slurry to the dredge pond/ repository, and then dewatered. The repository will be lined, and a leachate collection system will be used to prevent mobilization of contaminants from the repository. The repository will be capped according to State solid waste standards at the completion of the project [1]. The sediment removal portion of the cleanup is expected to last for three years and take place in years 3-6 of the project [5].

Russ Forba, the EPA remedial project manager, stated that EPA would follow a "no visible dust" policy during the cleanup activities. This will be achieved by spraying water, applying the dust suppressant magnesium chloride or other appropriate dust suppressants, and, if necessary, covering piles if they begin to dry out. Also, EPA is required to follow applicable and relevant requirements including particulate matter standards and OSHA requirements for arsenic. Mr. Forba also stated that the dredged material (85% of the total to be removed) will be wet, with the main challenge being to remove enough water to handle the material effectively. Dust should not be an issue with this material (personal communication, Russ Forba, US Environmental Protection Agency, June 2003).

Concerns of Private Citizen

The concerns expressed by the private citizen focused on the potential formation of dust during the cleanup operations. The proposed repository is less than one mile upwind of homes, and the citizen felt that because the repository will not be capped for several years, dust could blow towards these homes, aggravating asthma or exposing people to harmful levels of arsenic in the dust. Likewise, the proposed staging area, where mechanically-removed dry sediments will be prepared for transport, is close to a school, a Little League baseball field, and homes and churches. The citizen was concerned that dust from these sediments could be carried in the air to these locations, exposing people to harmful levels of arsenic. The citizen questioned whether the cleanup was necessary since safe drinking water is already available to the community.


Exposure to arsenic by breathing (inhalation) increases the risk of lung cancer, and respiratory irritation, nausea, and skin effects may also occur. Noncancer effects are unlikely below a concentration of 100-1,000 micrograms of arsenic per cubic meter of air (µg/m3) [6]. The Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit for exposure of workers to arsenic in air. OSHA's limit is a time weighted average over a 8-hour shift, 40-hour workweek of 10 µg/m3 [7]. The National Institute of Occupational Safety and Health (NIOSH) recommends a ceiling arsenic concentration of 2 µg/m3 averaged over a 15-minute period [7]. Arsenic is a known carcinogen; inhalation exposures are primarily associated with lung cancer [6]. EPA has calculated an inhalation unit risk of cancer of 0.0043 per µg/m3. For an increased risk of cancer of 1 in one million, this would correspond to a lifetime of breathing a concentration of 0.0002 µg/m3 of arsenic in air [8].

A person's exposure to arsenic can be measured in samples of blood, urine, hair, or fingernails. The urine test is the most reliable test for recent arsenic exposure (within the last few days). Results from this test can be affected by recent consumption of seafood, which contains "fish arsenic," an essentially nontoxic form of arsenic [6]. Tests on hair and fingernails can measure exposure to high levels of arsenic over the past 6-12 months. Care must be taken to wash the test specimens carefully to remove arsenic deposited on the exterior surface. The minimum exposure levels that produce measurable increases in arsenic levels in hair and nails have not been precisely defined. Inhalation exposure of workers to about 0.6 µg/m3 of arsenic in air significantly increased average levels in nails, although there was wide variation between individuals [6].

Particulate Matter

Particulate matter (PM) is the generic term used for air pollution consisting of varying mixtures of particles that may include fine solids such as dirt, soil dust, pollens, molds, ashes and soot as well as aerosols formed as combustion by-products. EPA regulates PM10, which consists of particles with diameters less than 10 micrometers (µm), and finer PM2.5 particles, a subset of PM10 consisting of particles less than 2.5 µm in diameter. Particles greater than 10 µm in diameter are too large to be inhaled and are not regulated.

In general, the coarser particles of PM10 come from sources such as windblown dust from deserts or unpaved roads. Coarse particles can accumulate in the respiratory system and aggravate health problems such as asthma. The finer particles of PM2.5 are emitted from activities such as combustion and vehicle exhaust or are formed in the atmosphere by chemical reactions of combustion byproducts. Fine particles penetrate deeply into the lungs and are more likely than coarse particles to contribute to health effects such as premature death, increased hospital admissions, and a variety of lung and respiratory problems [9].

EPA has set standards for PM10 of 50 µg/m3 as an annual average and 150 µg/m3 as a 24-hour average. PM2.5 standards are set at 15 µg/m3 as an annual average and 65 µg/m3 as a 24-hour average. The annual average standards correspond to "good to moderate" air quality, with no adverse health effects expected [10]. The 24-hour standards may be unhealthy for sensitive groups. Sensitive groups for PM10 include people with respiratory disease such as asthma, and for PM2.5 include people with respiratory or heart disease, the elderly, and children [10].


To evaluate the potential public health impacts of the proposed cleanup, ATSDR calculated a worst-case concentration of arsenic people might breathe in. A number of overly conservative assumptions were made to evaluate the maximum amount of arsenic exposure that could be expected and whether any adverse health effects would be possible. These include the following assumptions:

  • All the sediments at the surface are dry. In reality, a significant portion of the sediments will be wet and not easily suspended by wind.
  • All particles released are inhalable. Only particles less than about 10 micrometers (µm) are small enough to penetrate deeply into the lung where contaminants can be absorbed. Silt particles can be as large as 50 µm in diameter [11]. Characterization of sediments from the Milltown Reservoir indicated that only between 10% and 40% of the particles were actually small enough to be inhaled [4].
  • All arsenic is in a highly absorbable form. Studies have shown that the rate of absorption may be lower for insoluble forms of arsenic such as sulfides which make up a fraction of the sediments [6,12].
  • No dispersion of particles occurs before the exposure point. The particle concentration would actually decrease as dispersion occurred over even a few hundred feet.
  • No dust control measures are in effect. In practice, EPA will follow a "no visible dust" policy.
  • Dust exposure will occur 16 hours per day, 350 days per year, for 10 years. These are overly conservative assumptions as the sediment removal is only scheduled to take 3 years and dust would likely be generated only occasionally throughout the day.

To estimate the average amount of arsenic a person might breathe in from dust that might be created by drying piles of sediment, we assumed that all dust created would contain arsenic at the average concentration measured in the Area I sediments, 320 mg/kg. To estimate the amount of dust that people could breathe in, we used a mass loading factor of 2×10-7 kg of soil per cubic meter of air (kg/m3) [13]. This value is conservative for the situation of soil piles with some mechanical disturbance and is two to three orders of magnitude greater than the default for wind erosion of residential soils, equivalent to 7.6×10-10 kg/m3 [14].

The average arsenic concentration a person might breathe in, in µg/m3, is given by:

CAs, air = CAs, sed × MLF × CF × EF, where

CAs, sed = average concentration of arsenic in sediment in mg/kg, MLF = soil mass loading factor in kg/m3, CF = conversion factor (1000 µg/mg), and EF = exposure factor.

The exposure factor EF is a unitless fraction to account for noncontinuous exposure and is given by 16/24 for 16 hours of exposure per day multiplied by 350/365 for 350 days of exposure per year. Performing this calculation, the average arsenic concentration a person might breathe in is 0.041 µg/m3. This is much lower than occupational guidelines and not expected to result in adverse noncancer health effects. The increased risk of lung cancer from breathing in this concentration of arsenic in air over ten years is so low as to be negligible.

If all the dust suspended were PM10, the assumed soil mass loading factor would correspond to a level of PM10 of 200 µg/m3, which is considered unhealthy for sensitive groups. Therefore, the dust control measures are warranted to minimize particulate matter releases. Because EPA will be following dust control measures and given the conservative assumptions made in performing the above calculations, the sediment removal from Milltown Reservoir is not expected to result in any increased risk of adverse health effects from inhalation exposure to arsenic or particulate matter.


  • Inhalation exposure to arsenic in dust potentially generated during the sediment removal will be too small to result in adverse health effects.

  • If EPA does not employ planned dust control measures, then particulates could be released at levels that would be unhealthy for sensitive groups.


  • Follow dust control measures to minimize the release of particulate matter from drying sediments.



Jill J. Dyken, Ph.D., P.E.
Environmental Health Scientist
Division of Health Assessment and Consultation

Regional Representative:

Dan Strausbaugh
Regional Representative
Region 8, Montana Office
Office of Regional Operations

Reviewed by:

John R. Crellin, Ph.D.
Senior Environmental Epidemiologist
Division of Health Assessment and Consultation


  1. US Environmental Protection Agency. Superfund program clean-up proposal, Milltown Reservoir Sediments operable unit of the Milltown Reservoir / Clark Fork River Superfund site. Helena: Environmental Protection Agency, Region 8, Montana Office; April 2003.

  2. US Census Bureau. American fact finder, data set from 2000 summary file 1. Washington, DC: US Department of Commerce. URL: . Accessed on July 28, 2003.

  3. Agency for Toxic Substances and Disease Registry. Site review and update, Milltown Reservoir Sediments, Milltown, Missoula County, Montana. Atlanta: US Department of Health and Human Services; September 1993.

  4. CH2MHill. Technical Memorandum TM2, Milltown reservoir - Area I sediments consolidation phase water estimate. Prepared for the US Environmental Protection Agency, Region 8, Montana Office; November 2002. Available at URL: Accessed on July 24, 2003.

  5. US Environmental Protection Agency. Ref: Response to Office of Emergency and Remedial Response (OERR) Sediment Team comments on the Milltown Reservoir Sediments Site proposed remedial action. US Environmental Protection Agency, Region 8. Denver: 2003. Available at URL: Accessed on July 24, 2003.

  6. Agency for Toxic Substances and Disease Registry. Toxicological profile for arsenic (update). Atlanta: US Department of Health and Human Services; September 2000.

  7. National Institute of Occupational Safety and Health. Online NIOSH pocket guide to chemical hazards. URL: Accessed on July 29, 2003.

  8. US Environmental Protection Agency. Integrated risk information system (for arsenic). Available at URL: Accessed July 28, 2003.

  9. US Environmental Protection Agency. Fact sheet on EPA's revised particulate matter standards, July 1997. Available at URL: Accessed July 30, 2003.

  10. US Environmental Protection Agency. Guideline for reporting of daily air quality - air quality index (AQI). Research Triangle Park: Environmental Protection Agency, Office of Air Quality Planning and Standards; July 1999.

  11. Bouwer H. 1978. Groundwater Hydrology. New York: McGraw Hill. p. 16.

  12. CH2MHill. Technical Memorandum, Acid-base-potential (ABP) for Area I Milltown Reservoir and Clark Fork Channel Sediments. Prepared for the US Environmental Protection Agency, Region 8, Montana Office; November 2002. Available at URL: Accessed on July 24, 2003.

  13. Healy, JW. Review of resuspension models. In: Hansen, WC, editor. Transuranic Elements in the Environment. Technical Information Center/ US Department of Energy; 1980. p. 209-35.

  14. US Environmental Protection Agency. Soil screening guidance: user's guide. Washington: US Environmental Protection Agency, Office of Research and Development; July 1996.

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