The Bureau of Land Management is exercising its delegated authority to act as lead agency for the completion of this Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) Engineering Evaluation/Cost Analysis (EE/CA) of the Kolmakof Mine Site (KMS) to address contaminants of concern in four areas of concern (Camp Area, Mill Area, Pit Area and Tailings Area) and characterization and disposal of abandoned site-related equipment, structures, and supplies. The Alaska Department of Environmental Conservation (ADEC) is being provided meaningful and substantial participation in the CERCLA process by providing oversight of the project to ensure the State of Alaska’s interests are adequately protected.
The KMS is an abandoned cinnabar mine on the North Bank of the Kuskokwim River near the village of Napaimute, Alaska and is scheduled to be conveyed to The Kuskokwim Corporation (TKC) (surface estate) and Calista Corporation (Calista) (subsurface estate) in accordance with the Alaska Native Claims Settlement Act. Based on a preliminary assessment/site investigation (PA/SI) conducted by the United States Environmental Protection Agency (USEPA) and two site inspections and removal actions conducted by the BLM in 2000 and 2006, a Removal Site Inspection (RSI) was performed by Ecology and Environment, Inc. (E&E) in 2007 to identify facility features, inventory abandoned mine-related equipment and debris, and identify contaminants requiring possible remediation and or removal prior to transfer of the land to TKC and Calista. The RSI identified mercury as the primary contaminant of concern and identified the need for further investigation to further characterize the site for metals, petroleum hydrocarbons, and explosives, as well as the need for characterization and removal of hazardous and non-hazardous site materials.
MACTEC Engineering and Consulting Inc. (MACTEC) conducted initial investigative removal activities in 2008, including metals background concentration investigation, geophysical survey for a potential monofill location, explosives residue sampling, mercury speciation analysis, and an asbestos and lead-based paint survey. Removal activities included removal and disposal of various hazardous and non-hazardous materials from abandoned waste containers and equipment. In 2011, AMEC Environment & Infrastructure (AMEC, formerly MACTEC) prepared an EE/CA Workplan to complete site characterization activities required to delineate the extent of remaining contamination in each of the AOCs in support of this EE/CA. Field work including investigation of metals, petroleum hydrocarbons, and other contaminants of concern as well as a biological survey was conducted between September 25 and October 1, 2011. The work completed in 2011 identified metals, including mercury, arsenic, chromium, and nickel as the primary contaminants of concern (present at levels either posing a potential risk to human health or above typical background concentrations) in both the Camp Area and the Mill Area, and localized petroleum hydrocarbon contamination in the Mill Area. The results of the 2011 site characterization are presented in Appendix A of this document.
As part of the EE/CA, a streamlined human health risk assessment (HHRA) was conducted using a two-tiered approach. Potential health risks were first evaluated by comparing chemicals of potential concern (COPCs) in environmental media at the Site to risk-based screening levels. If a COPC was found to be present at a concentration greater than the defined screening criteria, then the HHRA assessed whether or not the observed concentration is truly likely to constitute a risk to human health given the nature and extent of the constituent Site soils and sediments and other Site specific considerations.
The receptors evaluated in the HHRA included child and adult subsistence/recreational receptors. Mercury and arsenic were determined to be the primary non-cancer hazards for the child receptor from incidental ingestion of soil, and arsenic for the adult receptor from ingestion of above ground produce. Arsenic was also determined to be the primary cancer risk. The cumulative cancer risks in the Camp and Mill Areas were determined to be above ADEC’s target cancer risk of 10-5.
A streamlined ecological risk assessment was also completed as part of this EE/CA to identify contaminants of concern in the affected media, contaminant concentrations, and toxicity associated with the contaminants for ecological receptors. Benthic macroinvertebrates were determined to have limited available habitat on the site due to the intermittent nature of the Unnamed Creek in the Mill Area. Benthic macroinvertebrate communities in the Kuskokwim River are not likely to be at true risk of adverse effects to compounds originating in the Tailings Area.
Three animal species were evaluated in the streamlined ecological risk assessment: a semi-aquatic avian invertivore, a terrestrial avian invertivore, and a terrestrial mammalian herbivore. Avian receptors are exposed to the most risk of potential adverse effects from metals in the Camp and Mill areas based on COPCs in the top 6 inches of soil. Mammalian herbivores have less potential of adverse effects because much of their diet is vegetation where COPCs do not accumulate as much as in invertebrates. Arsenic was a COPC posing risk to all receptors in all areas investigated. While mercury is wide-spread, concentrations of the bioavailable fractions were very low and so mercury is thought to pose little or no actual risk to ecological receptors using the site. Total petroleum hydrocarbons as diesel-range organics (DRO), residual range organics (RRO), and other metals were detected in a sufficiently small area that it can be considered a negligible exposure likelihood for ecological receptors; constituent polycyclic aromatic hydrocarbons were present at levels that did not pose risk to terrestrial receptors.
Removal action cleanup levels (CULs) were compiled for Site COCs detected in soil and sediment from screening levels protective of human health, ecological receptors, and groundwater, or background values. Based on the results of the HHRA and ERA, there is no surface water or sediment beneath surface waters that contains COCs at concentrations above cleanup levels.
The findings of the EE/CA indicate that approximately 200 cubic yards (cy) of soil containing elevated metal concentrations in two of the four areas of concern (Camp Area and Mill Area) and a limited amount of soil containing DRO in the Mill Area should be addressed in a removal action. In addition, approximately 1,000 cy of debris from site-related equipment and structures remaining at the Site would be demolished and/or removed and disposed as part of site restoration.
The removal action objectives (RAOs) developed to prevent, minimize, or mitigate risks to public health or the environment for the KMS are to evaluate removal actions that will:
- Prevent unacceptable risks to recreational users and ecological receptors that may be exposed to chemicals in soil that contains COCs at concentrations that exceed cleanup levels.
- Remove and properly dispose of abandoned site-related equipment, structures, supplies, and associated materials.
The removal action alternatives identified and analyzed are considered proven remedies because they have been selected and successfully implemented in the past at similar sites and/or for similar contaminants. Remedial options and technologies were screened and assembled into the following four removal action alternatives:
- Alternative 1 - No Action
- Alternative 2 – Excavation and Onsite Consolidation of Soil and Debris in a Repository
- Alternative 3 – Excavation and Offsite Disposal of Soil and Onsite Consolidation of Debris in a Monofill
- Alternative 4 - Excavation and Offsite Disposal of Soil and Debris
Based on the alternatives evaluation using EE/CA guidance and a comparative analysis of the removal action alternatives, Alternative 4, which involves excavation and offsite disposal of soil and debris, is recommended for the KMS. Alternative 4 would provide the highest level of protection of human health and the environment and long term effectiveness, because it is the only alternative that completely removes all of the contamination and site structures and equipment from the site, and would not require any long term monitoring or operations and maintenance.