Project Overview

Dam Removal & Sediment Excavation

Site Overview

Envirocon was awarded the Remedial Design/Remedial Action contract for the Milltown Reservoir Sediments Superfund site. A 100-year flood washed tailings from a copper mining operation 100 miles downstream to the Milltown Reservoir. The dam was located where the Clark Fork and Blackfoot Rivers converge into a common reservoir. Reservoir backwater conditions resulted in aggradation of 6 million cubic yards of sediment, 2.6 million of which impacted groundwater quality in the adjacent area due to leaching arsenic. The Milltown Reservoir Sediments site is part of the larger Milltown Reservoir Sediments/Clark Fork River Superfund Site which stretches 120 miles.

Project Highlights

  • Managed the Remedial Design/Remedial Action (RD/RA) under CERCLA authority and EPA oversight
  • Constructed a 4,000-foot long Clark Fork River bypass channel and diverted the river
  • Dewatered sediments through reservoir drawdown and active dewatering practices
  • Excavated, dried, transported, placed, and graded of over 2.2 million cubic yards of impacted sediments and transported them 100 miles via rail
  • Constructed and operated a rail car unloading facility and transported sediments to a landfarm area as capping material
  • Demolished and removed the historic Milltown Dam powerhouse and spillway structures
  • Constructed the final Clark Fork River alignment, restored habitat, and re-routed the river to its permanent location

Value Delivered

The draft Record of Decision proposed hydraulic removal of sediments from the river. Envirocon developed an alternative approach and negotiated regulatory approval for river re-routing, early dam removal, and dewatering that allowed sediments to be “dry” excavated, increasing production and resulting in $2 million cost savings.