Project Name: Moon Gulch Mine, a.k.a. Silver Crescent Mine or Moon Creek Project Cost: $1.9 Million Moon Gulch Mine Before Reclamation

Location: County: Shoshone State: Idaho Nearest town and distance: Approximately 5 miles northeast of Kellogg, Idaho Location of Moon Gulch Mine

Project Sponsor(s): U.S. Department of Agriculture, Forest Service (USFS) under provisions of the U.S. Comprehensive Environmental Response Compensation and Liability Act (CERCLA) as a non-time critical removal action. The Forest Service is the lead federal agency addressing sites with uncontrolled hazardous substances located on National Forest Lands.

Project Partners:

• Characterization - U.S. Bureau of Mines (USBM)
• Design / Consulting - Ridolfi Engineers
• Construction Contractor - Environmental Reclamation, Inc.

Project information supplied by:

Site Problems:

• Release of lead, zinc, cadmium, copper, mercury, and arsenic from the site
  • Dissolved and suspended load of Moon Creek
• Human Health Hazard
  • Near a residential area
  • Popular recreation site

Reasons for Action: The potential for human, fish, and wildlife exposure to elevated metals concentrations from materials remaining on-site. Lead and zinc were the primary contaminants of concern for reclamation planning.

Goal and Objectives: The goal of the project was to reduce the release and threat of release of hazardous substances from the site, thereby reducing risks to human health and the environment. The objectives of the reclamation plan were to: (1) achieve the CERCLA non-critical removal action; (2) develop an area that is safe for human health; (3) rehabilitate the stream and adjacent riparian area to support Westslope cutthroat trout.

History of Site: The site is located on the East Fork of Moon Creek on federally owned lands within the Idaho Panhandle National Forest in Northern Idaho. The project site is an abandoned mine and mill complex that was operated as part of the Coeur d’Alene Mining District. The complex includes the Charles Dickens Mine, which operated from 1902 until 1930, and the Silver Crescent Mine and Mill that operated from 1911 to 1954. The mine facility includes at least four adits, two shafts, and several miles of underground workings. These mines together produced 4,606 tones of ore. Metals production from the Moon Creek mine complex was primarily lead and zinc, with limited silver, copper and gold.

The Silver Crescent Mill was initially operated as a jig-type mill, but flotation circuits were added in the early 1920s. The resulting tailing deposits include the coarser-grained, higher-concentration jig tailings and the finer textured flotation tailings. The Silver Crescent Mill was a custom mill that produced ore from several off-site mines in the Coeur d’Alene Mining District. Therefore, the volume of tailings on site is proportionally greater than would be expected given the production from the mines on site. The volumes of tailings and other mine waste material remaining on site are summarized below.

Flotation Tailings 69,000 CY

Mine Waste Dumps

• Main Dump 21,900 CY
• North Dump 5,800 CY
• South Dump 400 CY

Mixed Soils and Tailings 30,500 CY

TOTAL 128,100 CY

Site Conditions:

• Mine and Mill site approximately 20 acres
• Elevation range on the site 2,600ft to 2,800ft
• Temperature ranges from –2F to 100F
• Rainfall approximately 40 in/yr
• Watershed size is 3.28 square miles with 2.73 square miles above the site
• Base flow of East Fork Moon Creek is approximately 0.5cfs (8.8cfs peak 1993)

Geology:

• Precambrian Prichard Formation of the Belt Supergroup
• Consists of thick bedded quartzose argillite and argillite
• Pb, Zn, Ag Ore came from the Middle Prichard argillites – pyrite, sphalerite, galena

Site Characterization: The initial site characterization was conducted by a multi-discipline, multi-agency teaming that included engineers, scientists, and forest managers from the USFS, the U.S. Bureau of Mines (USBM), and Ridolfi Engineers. Concurrent environmental sampling, survey, and riparian inventories were obtained to allow an understanding of not only the contaminant pathways, but also the environmental conditions with respect to vegetation, water and soils. This collaboration allowed early concept development to integrate ecological principles into the engineering design for stabilizing and rehabilitate the site. In this way, the site environmental conditions, multiple methods for removing and isolating the contaminants, and methods for stabilizing the site that would enhance recovery could be evaluated considering the goals for the mine reclamation.

The USFS, the U.S. Bureau of Mines (USBM), and Ridolfi Engineers conducted sampling and monitoring at the site. Surface water data collected at the monitoring station downstream of the site indicated that at low-flow conditions, mean concentrations of cadmium, copper, lead, and zinc exceeded background concentrations, Federal Water Quality Criteria or safe drinking water maximum contaminant levels. Ground water data collected from monitoring wells down gradient of the mill site showed arsenic, cadmium, copper, and lead in excess of federal drinking water standards. Seeps along the stream banks indicated a pathway for ground water to enter the creek.

Extensive soil sampling was conducted throughout the site including the waste dumps, upper mine area, lower mill area, and flotation tailings piles. Soil sample results from the mine waste dump areas indicated that arsenic, copper, lead, zinc, and mercury exceeded background or ecological effects levels by several orders of magnitude. Sample results from the upper mine area and the lower mill area showed contaminant concentrations that exceeded background and ecological effects. Samples taken in the flotation tailings areas showed mean concentrations of antimony, arsenic, copper, lead, zinc, and mercury that were greater than background and ecological effects levels.

Removal Action Goals:

• Reduce the release of lead, zinc, cadmium, copper, mercury, and arsenic to Moon Creek
• Rehabilitate the site, limiting exposure pathways
• Improve cutthroat habitat

Opportunities and Constraints: Through the site characterization effort, several opportunities and constraints were identified that helped shape the reclamation plan for this site. The valley geometry presented both an opportunity and a constraint in that the site is located within a fairly steep (3-5 percent slope) and narrow valley. The opportunities were presented by well-forested, healthy hillsides adjacent to the site that serve as shade for the stream and a source of colonization for vegetation. The constraints were posed by the need to find a suitable area on site to locate the waste containment.

An existing site road served as a source of sediment to the stream. Because the road was adjacent to the stream, and had existing vegetation, it provided an opportunity to remove the road, and re-locate the stream.

Westslope cutthroat trout (Onchrynchus clarki lewisii), a candidate species for listing under the Endangered Species Act were identified in upstream reaches above the mine and mill site. This provided opportunities with respect to shaping the final stream channel, and constraints with respect to construction scheduling around fry emergence.

Moon Creek passed through the mill site, physically bisecting the tailings impoundment. There were several seeps in the vicinity of the mill site. The floodplain in both the upper mill area, and the tailings impoundment did not support vegetation, and previous revegetation efforts had failed. However, metals concentrations do not increase through the reach passing through the tailings, and lysimeters in the tailings indicated low ground water flow rates to the stream.

From the identification of possible opportunities and constraints, considerations for the reclamation design were to: (1) use existing features to the greatest extent possible; (2) develop an innovative and effective waste containment design; (3) rehabilitate the steam to provide fish habitat, and (4) use bio-engineering methods for site stabilization.

Use Existing Features: Existing features that could be used in reclamation included the narrow valley geometry, forested hillsides, boulders, the narrow road through the site, and on-site vegetation. The new stream alignment was based upon maintaining part of the existing riparian area and providing cover for the stream. The narrow valley and heavily forested sides slopes contributed to this goal. Trees and boulders within the area to be disturbed outside the contaminated areas were identified for salvaging and re-use in stream habitat measures. The road embankment through the site contained a significant quantity of clean rock and soil that was used for fill in constructing the floodplain and waste containment. In the upstream reach, the stream was relocated to the previous road alignment. This allowed for better sequencing during reclamation. Removing the road also eliminated an existing source of sediment to the stream. On-site vegetation was used as a source of seed and cuttings for native planting. Vegetation from clearing and grubbing operations was composted on-site and used for soil amendment.

Innovative Waste Containment Design: Consolidating and isolating the mining and milling waste posed the greatest challenge for this project. The solution depended upon an innovative waste containment design. Based on synthetic leach testing of the on-site materials and modeling of the cover elements, it was determined that the on-site wastes were compatible, and that the potential impacts from leachate containing elevated metals concentrations were limited. Therefore, the waste rock, jig and flotation tailings, and soils containing elevated metals concentrations were consolidated into an unlined combined waste facility. The combined waste facility is long and narrow to fit into the valley section and allow a stream buffer; the combined waste facility is approximately 800 feet long by 150 feet wide, and 27 feet in height.

Design considerations and key components of the combined waste containment included physical location, a compacted gravel drain system, geocell mattress, subgrade scour protection berm, and water balance cap to prevent surface water percolation through the waste material into the containment. The containment is located within the middle flotation tailings area with one edge benched into the valley wall and a portion of the containment base located on top of the tailings. The benches have been meandered into the existing hillside to soften the appearance of the containment, and to maximize storage volume. Placing the containment on the tailings limits the material excavation costs; however, it presents several design challenges related to soft foundation conditions. Typically the geotechnical solutions for soft foundations include reduction of pore pressure, installation of lateral stability measures, and placement of a surcharge prior to loading.

The design for the combined waste containment base incorporated features that perform all three functions. A compacted gravel drain system is provided along the periphery of the containment with lateral headers on an 80-foot spacing. This drain provides a preferential drainage path for in-situ ground water. A common sewer cleanout is used at the upper ends of this drain system to allow future monitoring of the containment base. Limestone gravel was placed in the lower portion of this drain to attenuate any metals present in pore-water drainage.

The initial layer in the containment is comprised of a geocell mattress. This is an eight-inch thick, three-dimensional perforated polyethylene web (Geoweb™) system that is carefully backfilled with waste rock. A geocell mattress has been shown to provide lateral stability, and a surcharge prior to load placement for similar soft foundation conditions.

A five-foot deep impervious scour protection berm provides lateral stability during construction and support for the compacted gravel drain and in-situ tailings within the containment base. The external face of this berm is armored with riprap to provide protection from future stream erosion, should the stream meander toward the containment. This berm also limits ground water intrusion into the containment.

Within the containment, the waste will be mixed for homogeneity. A mid-height gravel drain is included to help de-water the in-situ waste within the upper lifts of the containment.

The Hydraulic Evaluation of Landfill Performance (HELP) computer model8 developed by the U.S. Environmental Protection Agency (EPA) was used to optimize the water balance cap design for the containment. This cap features two feet of growth media underlain by a gravel drainage layer. A combination of evapo-transpiration from the vegetation and a capillary break between the soils and the underlying gravel act as the primary means of preventing surface water percolation. A geosynthetic clay liner with underlying cushion material (sand) has been included to further reduce surface water infiltration. These innovative measures combine to allow an on-site waste containment design that effectively isolates the mine waste materials found on the Moon Creek site.

Stream Rehabilitation: Stream rehabilitation measures focused on providing suitable habitat to support Westslope cutthroat trout, a fish native to the Moon Creek watershed. The considerations and components of the stream rehabilitation for the Moon Creek project include stream location, a constructed wetlands buffer, microhabitat, and substrate placement.

The new channel for Moon Creek was relocated into the existing road area in the upper segment of the site, and into the base of the hillside in the lower segment. The middle segment passes through the existing mill area.

The new channel section was designed to accommodate the bank-full flow, which is approximately a 1.5 year frequency storm event. The channel was designed with input from USFS fish biologists, and using Idaho Fish and Game Habitat Curves. Because the valley slope of three to five percent is greater than the slopes presented in these curves as optimal for this species, microhabitat in the form of in-stream spawning substrate material, large woody debris, and boulder pools were included to enhance habitat for trout. Because of extensive removal in the existing valley, clean fill was imported to reconstruct a floodplain outside of the main channel. Meanders following existing patterns on site and based on reference areas immediately upstream of the site were used for the new stream layout. In general, the channel was designed as a "deformable boundary channel" to allow natural geomorphologic processes to perform the majority of the channel shaping. Bank protection using bio-engineering methods were be used at the two junctions with the existing stream section.

Bio-Engineering for Site Stability: Bio-engineering measures were included into the design to provide a natural self-sustaining means to stabilize the site. These measures included cutstakes with coir fabric for stream banks, willow wattles for flood erosion control, block planting, and bio-filters for erosion and sediment control.

Willow cut stakes were placed on 18-inch centers through an anchored coir fabric mat to provide protection for the three key bank areas along the new stream channel. Meandered willow wattling were installed on a 50-foot spacing along the over-banks, to trap surface sediment, direct flows back to the channel, and provide upland niches for wildlife. Vegetation between the wattles were established using staggered block plantings. This involved random placing of 16 to 20 containerized plants into a 15-foot by 25-foot area. Block plantings allow shade between plants, and colonization of other areas by wind, water, and animal vectors. Native plant species selected for these block plantings include shrubs and mid-story trees appropriate for the elevation and climate found at this site. Hardiness (western cedar), litter production (cottonwood, aspen), nitrogen fixation (alders) and wildlife preferences (mountain ash, elderberry, snowberry) were additional considerations used in the plant selection process.

Because of the length of the disturbance to the vegetation on-site, it is anticipated that there may be an extended period for the plant communities to recover. Soil amendments and composted vegetation from on-site clearing were used to augment the existing soils. It is anticipated that over time, the conifers from the adjacent slopes will colonize to the site. A grass-forb mixture was used to seed the remaining bare areas between the blocks and the existing vegetation. The slopes remaining after the waste rock was removed were stabilized using vegetation and bio-filter bags. These are recycled plastic mesh tubes each about five feet in length and filled with recycled wood chips. These bags are installed by staking into the slope on a 20-foot spacing, similar to wattling. In addition to sediment trapping ability, the wood chips form a weak ion exchange agent that may attenuate metals remaining in the surface sediment.

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