Copper Mine Remediation Project

  • Railroad Creek and Copper Creek (shown) were both impacted by Phase 1 of the Remedial Action and required compensatory mitigation.
  • Confluence has conducted annual biomonitoring (sampling fish, macroinvertebrates, and habitat and water quality parameters) in Railroad Creek since 2019 to assess the success of the Remedial Action.
  • Preparing to sample the trout population in one of seven stations in Railroad Creek with backpack electrofishers.
  • Confluence has conducted vegetation monitoring since 2020 to evaluate reforestation planting associated with the Record of Decree.
  • Confluence evaluated the 2,100-acre Bear Mountain property as a mitigation option to compensate for impacts from the Remedial Action.

Holden Mine was a copper mine active from 1938 to 1957. Through acquisition of former operators, Rio Tinto took over remediation of the relic copper mine as required by the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Phase 1 of the Remedial Action (RA) encompassed 170 acres and caused both temporary and permanent impacts to “waters of the United States” (WOTUS). Confluence is supporting Rio Tinto to meet its regulatory requirements by assessing impacts of the RA on wetlands, streams, fish, benthic macroinvertebrates, water quality, and physical habitat, and by developing a compensatory mitigation plan.

Wetland and Stream Impact Assessment and Mitigation Planning

Confluence conducted a WOTUS impact assessment involving 24 wetlands and 3 streams to characterize the extent and functional effect of impacts that occurred to WOTUS from the RA. During the site assessment, Confluence scientists delineated and rated wetlands to establish the post-impact conditions for comparison with pre-impact baseline conditions done by others. Mitigation ratios were developed to determine the total compensatory mitigation need according to impact type and wetland category. Confluence determined that 4.8 acres of wetland impact and 1 acre of aquatic resources impact resulted from the RA and that 19.4 acres of wetland establishment (or equivalent) was appropriate to offset the ecological losses sustained.

Confluence used a watershed approach to identify candidate mitigation sites, then assessed them to determine the potential ecological lift of each. Due to the relatively undisturbed nature of the upper Chelan watershed and the semi-arid environment of the lower Chelan watershed, wetland mitigation opportunities were limited, and a creative solution was required. Confluence identified a 2,100-acre property near Chelan, the Bear Mountain property, that was available for preservation through site acquisition. A field effort evaluating this property revealed several locally rare perennial streams and wetlands that would likely be degraded if the site were to be developed. Confluence recommended acquisition of this entire property as the best option for mitigating impacts to both streams and wetlands as an out-of-kind resource trade-off. Following review of the mitigation site options, the lead agencies (U.S. Forest Service and Environmental Protection Agency in cooperation with the Washington State Department of Ecology and the Confederated Tribes and Bands of the Yakama Nation) agreed the Bear Mountain preservation site (now Chelan Coulee Preserve) was the most viable and ecologically beneficial mitigation option for Phase 1 impacts. The Chelan Coulee Preserve is now under the stewardship of the Chelan Douglas Land Trust and will continue to provide valuable habitat and ecological functions in a rapidly growing area for generations to come.


Confluence is conducting annual biomonitoring, including surveys of fish population, benthic macroinvertebrates, water quality, and physical habitat, in Railroad Creek to assess the performance of the Phase 1 RA relative to baseline conditions prior to RA implementation in 2013. Confluence has been conducting the annual biomonitoring since 2019 (which was year 2 of monitoring). Work involves sampling fish and benthic invertebrates and collecting related data at 7 locations in Railroad Creek downstream, within, and upstream of the Holden Mine site. These data are then analyzed with respect to biological metrics used to interpret the health of the biological community. Findings are presented in annual biomonitoring reports. Thus far, biomonitoring is indicating improvement of both the benthic macroinvertebrate and fish communities since implementation of Phase 1 of the RA.

Project Location
Chelan County, WA

Rio Tinto

Project Timeline
Ongoing since 2019

“Confluence’s expertise, creativity, and ability to collaborate with a variety of stakeholders was key to our success in finding a solution for mitigating our impacts from Phase 1 while also providing Rio Tinto an opportunity to help preserve and protect key habitat in perpetuity.”
—Craig Stevens, Rio Tinto Project Manager