Missoula County, MT

A.45 Missoula County, MT

Milltown Reservoir Sediments Operable Unit, Missoula County, MT (CERCLIS ID #MTD980717565)

A.45.1 Contacts

Diana Hammer, USEPA (Region 8, Lead Agency), 406-457-5040, [email protected]

Keith Large, MT DEQ (Supporting Agency), 406-841-5039, [email protected]

A.45.2 Summary

Environment:	Freshwater reservoir
Scale:	Full
Contaminants of Concern:	Groundwater: arsenic, cadmium, copper, lead, zinc, and mercury Surface water: arsenic, cadmium, copper, lead, and zinc
Source Control Achieved Prior to Remedy Selection?	No
Final Remedy:	Sediment removal (approximately 2.2 million yd³) and MNR (approximately 540 acres)
Expected Recovery Time:	4-10 years beginning in 2011
MNR viewed as a success?	Yet to be determined

A.45.3 Site Description

Primary source(s): For nearly one-hundred years, mine wastes were discharged into the headwaters of the Clark Fork River and after 1908, came to rest behind the Milltown Dam. In the same year, the largest flood on record in the area washed the mining waste into the Milltown Reservoir, creating approximately 6.6 million yd³ of contaminated reservoir sediments.

Location:The Milltown Reservoir Sediments Operable Unit (MRSOU) is one of three operable units, which also includes the Clark Fork River and Milltown Water Supply, near Milltown, Montana. The MRSOU covers approximately 540 acres (as defined by the area inundated by the maximum pool elevation of 3,263.5 ft).

Beginning in the 1860s, mine wastes were discharged to the headwaters of the Clark Fork River. In 1908, the Milltown Dam was constructed. In the same year, mine waste that had accumulated behind the dam was flushed into the Milltown Reservoir by a large flood. From here, the mine waste contaminated the local drinking water source.

In September 1983, the Milltown Reservoir was added to the National Priorities List (NPL). A year later, the city of Milltown installed a new water system. Remediation studies and investigations dominated activity at the site into the 2000s. The EPA issued a ROD in December 2004. It stressed a 3-R approach: remediationThe act or process of abating, cleaning up, containing, or removing a substance (usually hazardous or infectious) from an environment., restoration, and redevelopment. In August 2005, a Consent Decree identifying Atlantic Richfield Company and NorthWestern Corporation as the site’s responsible parties, was signed.

In the fall of 2006, remediation of the Superfund site began. Concurrent restoration of the site began in 2008. In 2009, sediment removal from the Milltown Reservoir was completed. By February 2010, the Milltown dam had been removed. Remediation action construction activities are ongoing and are expected to be completed in the near future (approximately a year).

CSM summary: The primary source of COCs in the Milltown Reservoir is contaminated sediment. Secondary sources of COCs are exposed aquatic flora and fauna, surface water, and suspended sediment transported from the Clark Fork River.

A.45.4 Remedial Objectives

The MRSOU poses risks to human health via ingestion of contaminated potable groundwater and ingestion of aquatic life. Risks exist for flora and fauna directly and indirectly exposed to COCs.

RAOs/Project objectives: Remediation of the MRSOU is twofold: (1) reduction or elimination of the groundwater arsenic plume and (2) risk reduction to aquatic life. RAO’s exist for groundwater and surface water at the site.

The groundwater RAOs, as reported in the site’s ROD, are as follows:

Return contaminated groundwater in the Milltown alluvial aquifer to its beneficial use within a reasonable time frame.
Comply with State and Federal groundwater standards, including nondegradation standards, for arsenic, cadmium, copper, lead, mercury, and zinc.
Prevent groundwater discharge containing arsenic and metals that would degrade surface waters.

Temporary and long-term surface water RAOs were developed for the MRSOU. The temporary RAOs waive ambient surface water standards for cadmium, copper, zinc, lead, arsenic, iron, and total suspended solids during the construction phase. The long-term surface water RAOs, as reported in the site’s ROD, are as follows:

Achieve compliance with surface water standards, unless a waiver is justified.
Prevent ingestion of or direct contact with water posing an unacceptable human health risk.
Achieve acute and chronic Federal Ambient Water Quality Criteria for arsenic, cadmium, copper, lead and zinc.

Additionally, performance standards exist for 1) the protection of waste left in place and local repositories, 2) the new channel, and 3) re-vegetation of river banks and the flood plain. Notably, the site will be redeveloped as a state park.

A.45.5 Remedial Approach

Final selected remedy: Remedy alternative 7A2, modified: partial dam removal with partial sediment removal of the Lower Reservoir plus Groundwater ICs and natural attenuation within the aquifer plume.

Remedy Selected alternative 7A2, modified consists of many phases. The major remedial elements, as reported in the ROD, are as follows:

Water in the Milltown Reservoir was drained and a bypass channel for the Clark Fork River was constructed.
Approximately 2.2 million yd³ of contaminated sediment (with the greatest pore waterWater located in the interstitial compartment (between solid-phase particles) of bulk sediment. contaminant concentrations and significant potential to cause future surface water degradation) was removed from the Reservoir.
A railroad was built specifically to transport this sediment to a lined solid waste disposal facility less than one mile away.
The Milltown Dam was removed.
The replacement water supply program and implementation of temporary groundwater institutional controlsNon-engineered instruments, such as administrative and legal controls, that help minimize the potential for human exposure to contamination and/or protect the integrity of the remedy. will continue until the Milltown aquifer recovers using monitored natural recovery.
Long-term operation and maintenance will be conducted.

Advantages of the selected remedy:

Remedy alternative 7A2, modified was selected for the following reasons:

Permanent, long-term protection of public health and the environment
Recovery of the Milltown drinking water supply
Use of existing waste management areas for waste disposal
Substantial elimination of contaminant release from ice-scouring and catastrophic events
Return of the Clark Fork to a free flowing state, enabling unrestricted fish passage
Redevelopment possibilities, including a recreational fishery

Disadvantages of the selected remedy:

The following limitations or disadvantages exist for remedy alternative 7A2, modified:

Reservoir drawdown and remedy construction could negatively impact downstream aquatic life in the short-term
The remedy is complex
The remedy is expensive

Expected recovery time: The selected remedy will allow recovery of the Milltown/Bonner aquifer within 4-10 years.

A.45.6 Monitoring

Monitoring elements: In addition to monitoring that occurred during the remedial action, O&M activities will take place for at least five years after construction has been completed in order to ensure that performance standards are being met.

The Statement of Work for Milltown Reservoir requires that a long-term monitoring plan be developed at least 60 days prior to the completion of remediation action construction activities. Such a plan has not yet been written.

RAOs/project objectives achieved: The success of MNR at the MRSOU is yet to be determined.

A.45.7 Costs

Projected monitoring costs: N/A

Net present value for project costs: $106,000,000 (discounted by 3% per year for the estimated life of the project)

A.45.8 References

USEPA. Integrating the “3 Rs”: Remediation, Restoration and Redevelopment, the Milltown Reservoir Sediment Site and Missoula County, Montana. April 2011. Available at www.epa.gov/region8/superfund/mt/milltown/2011ReadyForReuseFactSheet.pdf.

USEPA. Record of Decision: Milltown Reservoir Sediments Operable Unit of the Milltown Reservoir/Clark Fork River Superfund Site. December 2004. Available at www.epa.gov/region8/superfund/mt/milltown/mrsrod.html.

USEPA. Superfund Program Record of Decision Factsheet: Milltown Reservoir Sediments Operable Unit of the Milltown Reservoir/Clark Fork River Superfund Site. No date. Available at www.epa.gov/region8/superfund/mt/milltown/pdf/mrsRODfs.pdf..

Envirocon. Repository Operation and Monitoring Plan, Milltown Reservoir Sediments Operable Unit, Final. February 3, 2010.

Publication Date: August 2014

Permission is granted to refer to or quote from this publication with the customary acknowledgment of the source (see suggested citation and disclaimer).