Monarch Mine Site CERCLA Engineering Evaluation/Cost Analysis

US Forest Service

Girdwood, Alaska

Monarch Mine

Hart Crowser is conducting an Engineering evaluation/cost analysis (EE/CA) for the Forest Service at this abandoned gold mine and mill site along the Iditarod National Historic Trail in the Chugach National Forest. The site includes three underground lode gold mines, an aerial tram, tailings and waste rock piles, and a mill area with residual concentrate as well as outlying areas of contamination.

We prepared CERCLA-compliant investigation work plans, including Sampling and Analysis Plans and Quality Assurance Project Plans (SAP/QAPP) as well as a site-specific Health and Safety Plan (HASP).

We conducted a site investigation over two field seasons that included characterizing mine wastes (tailings and waste rock), mine-impacted soils, surface water, and aquatic sediments, including establishing site-specific background concentrations. We also conducted two rounds of benthic invertebrate sampling in creeks draining the site to assess potential risk to aquatic receptors and assessed impacts to terrestrial habitat (plants and wildlife).

We mobilized from our Seattle headquarters to access the site during the short summer season, establishing a logistical base and field lab in Girdwood. Field work utilized a combination of helicopter support and hiking for access.

We characterized the nature and extent of contamination (metals and acid-mine drainage) and conducted human health and ecological risk assessments to identify removal action objectives. We developed preliminary designs, prepared cost estimates, and compared several alternatives to address site risks, including removal and off-site disposal of mine wastes and impacted soils and management of wastes in a repository to be constructed on-site.

Our preliminary designs considered significant site drainage and slope stability issues, the need to regrade and revegetate disturbed areas, and post-removal action cleanup verification monitoring. Our comparative evaluation of alternatives included assessing the costs and environmental impacts to the watershed habitat of constructing an access road into this pristine site.