The Bonanza Mine and Mill Site is located near the small community of Nonpareil, 6 miles east of Sutherlin, Douglas County, Oregon. The Site is located in the SW ¼ of Section 16 of Township 25 South, Range 4 West, Willamette Meridian (latitude N43° 23’46”, longitude W123°10’54”).

Except for one former building used as a residence, other mine and mill buildings are no longer present, leaving only the mill concrete foundations, calcine, and waste rock. The mine had 12 adits and more than three miles of subterranean tunnels and shafts. The mine adits have been abandoned and it is likely that they are open.

Five residences are located close to the mine, including two residences within 200 feet of the former mill. Besides roads and driveways leading to the residences, the land is undeveloped. The nearest off-Site residences are located about a half mile away, to the northeast, along Banks Creek Road. 

The Bonanza Mine has an operation history extending from the mid-1860s through 1960. The main mercury-containing mineral is cinnabar, although metacinnabar and native mercury were also reported in the mine workings. Total recorded mercury production was 39,540 flasks (or 3,005,040 pounds).

1.1.2.2 Description of Threat

The data from numerous environmental investigations shows that environmental media are contaminated by elevated concentrations of mercury, arsenic, and other metals, and the source of metals is from historical mercury mining, processing, and disposal operations.   Elevated metals concentrations are present in calcine, waste rock, and soil at the former mill site, the surrounding hillside, and valley floor.  

1.1.3 Preliminary Removal Assessment/Removal Site Inspection Results

Data regarding the nature and extent of the contaminants of concern found at the Site are summarized below. 

            1999 – Preliminary Assessment

Ecology and Environment, Inc. (E&E) completed a Preliminary Assessment (PA) of Red Rock Road (Road) for EPA in May 1999.  The PA evaluated the potential for exposure to public health and the environment from potential metals contamination associated with the Road.  The Road is a former railroad grade approximately 17 miles long that was constructed of calcine from the Bonanza Mine.  The amount of material used in construction of the Road is estimated at 316,000 cubic yards (yd³).  As a result of the PA, further investigation was recommended.

2000 - Site Inspection

E&E completed a Site Inspection (SI) of Red Rock Road and surrounding watersheds for EPA in May 2000.  As part of this SI, nine surface soil samples were collected from potential source areas at the Bonanza Mine Site, including the former mill, calcine, waste rock, and an abandoned adit.  Mercury concentrations in these areas ranged from 74 to 12,000 milligram per kilogram (mg/kg), arsenic concentrations ranged from 71.3 to 246 mg/kg, and lead concentrations ranged from 6.5 to 1,240 mg/kg.  The total on-Site volume of calcine was estimated at 2,080 yd³ and waste rock was estimated at 400 yd³.  

2000 – Removal Assessment

In September 2000, Hart Crowser, Inc. (HC) performed a Removal Assessment (RA) at the former mill site for ODEQ to gather additional data to delineate the extent of metals contamination at the Site.  As part of this RA, 31 surface and near-surface soil samples were collected from the former mill site and surrounding hillside.  Mercury concentrations ranged from 67.7 to 12,000 mg/kg, arsenic concentrations ranged from 20.3 to 314 mg/kg, and lead concentrations were generally below 70 mg/kg.  Calcine, waste rock, and roadway soils also had elevated mercury and arsenic concentrations ranging up to 179 mg/kg and 246 mg/kg, respectively.

One sample each of the former mill soil and calcine were analyzed for mercury speciation.  Methyl mercury was detected at 0.03765 mg/kg in soil and 0.00246 mg/kg in calcine.  Sequential extraction on soil and calcine indicated that most of the mercury was sulfide-bound, primarily in the form of cinnabar or metacinnabar.  Volatile mercury was detected at 2,100 and 2,360 microgram per cubic meter (µg/m³).

Water samples were collected from the on-Site well and water storage tank.  Arsenic was detected at 0.0536 milligram per liter (mg/L) in a sample collected from the on-Site well and this concentration exceeds the Federal Maximum Contaminant Level (MCL) of 0.005 mg/L for drinking water.  Reportedly, well water is used only for agricultural purposes and not for drinking water.  Based on the findings of the removal assessment, the first of two removal actions described in Section 2.1.2 (Response Actions to Date) was performed by ODEQ in 2000 in certain areas to achieve prompt human health risk reduction.  Water samples have been collected from the spring water storage tank and have consistently had no detections of mercury and arsenic using standard drinking water analytical methods.

2003 – Site Visit

HC returned to the Site on behalf of ODEQ in 2003 to assess whether ecological receptors and/or exposure pathways were present or potentially present at or in the Bonanza Mine Site and along Foster Creek.  Impacts to the Site and surrounding properties attributable to contaminated environmental media were not observed during the Site visit.  Physical impacts from historical mining operations included the waste rock pile, mine access roads, and mine excavation.  Based on the results of the Oregon Natural Heritage Information Center data search and information from the Oregon Department of Fish and Wildlife, HC concluded that there is a possibility that rare, threatened, and endangered species may be present at or near the Site.

            2005 – Post-Removal Assessment Report

HC compiled and assessed available information for the Bonanza Mine in 2005 to assist in preparation of a forthcoming Remedial Investigation (RI) Work Plan.  This report also developed a preliminary conceptual site model (CSM) for both human and ecological receptors at the Site and identified tasks to be performed during the RI to address data gaps.  Volatile mercury was measured in soil from the former mill and calcine.  No other environmental media samples were collected as part of this activity.  The RI Work Plan has not yet been prepared.

            2013 – Soil Assessment

In December 2013, ODEQ screened 118 soil samples using a field portable X-Ray fluorescence spectrometer (FPXRF) to gather additional data to identify those areas where soil concentrations are below a site-specific background concentration for arsenic and a residential risk-based concentration for mercury.  Nine discrete soil samples were also collected and sent off-Site for laboratory analysis.  The results of this assessment indicated that arsenic and mercury contamination is more widespread in the northern portion of the property than previously anticipated.  The results also showed that arsenic and mercury contamination extends into the southern portion of the Site near two existing residences.

2.1.2.a  The following removal actions have been undertaken by the Oregon Department of Environmental Quality (ODEQ) in the past:

            2000 – Removal Action

Based on the findings of the 2000 HC RA, HC performed a removal action at the former mill site for ODEQ from 14 through 29 September 2000.  The objective of this action was to provide prompt risk reduction by excavating soil exceeding 230 mg/kg mercury in the mill area, and for arsenic and lead the cleanup goals were 50 mg/kg and 400 mg/kg, respectively.  Eight yd³ of soil were excavated from the mill furnace area, and this material was transported off-Site for disposal as hazardous waste.  Approximately 240 yd³ of mercury-contaminated soil was excavated from the mill area and placed in a lined and covered temporary storage cell near the base of the waste rock pile.  This material was removed from the Site in April 2004 and transported off-Site for disposal.  Larger debris such as concrete, firebrick, and a metal furnace were placed in a subsurface vault located at the former mill site.  Disturbed areas were restored, as closely as possible, to the original site conditions.

Confirmation soil samples were collected after the removal action.  A few samples exceeded the mercury cleanup goal (up to 6,400 mg/kg); however, these sample areas are beneath two to six feet of clean material.  Characterization samples collected from the surrounding hillside, calcine, waste rock pile, roads, driveways, and cell base had mercury concentrations ranging from 1.53 to 220 mg/kg.  Four samples with greater than 230 mg/kg mercury were from the mine adit (306 mg/kg), the temporary repository (500 mg/kg), an area south of the former mill (930 mg/kg), and a small area where free mercury was observed (5,100 mg/kg).

            2014 – Removal Action

NRC Environmental, with technical support and documentation from APEX, performed a second removal action at the Site for ODEQ in February 2014.  The objective of this action was to achieve prompt human health risk reduction by removing and capping soil in certain inhabited areas of the Site that were impacted by elevated concentrations of mercury and arsenic.  At the time this removal action was performed, the contaminants of concern were mercury and arsenic, and the cleanup goals were 23 mg/kg and 17 mg/kg, respectively.

Prior to implementation of the removal action, FPXRF screening was performed at 118 points scattered across the Site.  Arsenic ranged from non-detect to 471 parts per million (ppm), and mercury concentrations ranged from non-detect to 1,200 ppm.  Using this information, six areas were identified that had arsenic or mercury concentrations above the cleanup goals.  During conduct of the removal action (12 through 21 February 2014) and follow-up site visit (12 March 2014), 39 additional data points were collected from across the Site with the purpose of better understanding the metals distribution across the Site.  Arsenic concentrations in these points ranged from non-detect to 81 ppm, and mercury concentrations ranged from non-detect to 459 ppm. The results indicated that the mine-waste contamination from the mill site area is more widespread than previously anticipated, including contamination encountered near two existing home sites.

The overall scope of the removal action, as described below, is intended to mitigate the potential human health and ecological threats posed by exposure to mercury and arsenic, including direct contact, ingestion, and inhalation pathways.

Adits

Two adits are located on the Site.  The adits will be closed to prevent human entry.  In consultation with federal and state agencies, if it is determined that the adits merits habitat preservation, the closure will be designed to both prevent human entry into the adits and to suit the specific needs of resident wildlife species.

            Excavations

Mine-waste contaminated materials exceeding cleanup levels for mercury (i.e., 23 mg/kg) and arsenic (i.e., 17 mg/kg) will be excavated from certain areas and to a depth of 24 inches below ground surface.  The limits and depths of the excavation areas will be determined in the field using field screening methods such as a field portable mercury screening instrument, FPXRF, and visual recognition.  The excavated materials will be hauled to the vicinity of the waste rock pile using existing on-Site roadways.  Disturbed areas will be restored, as closely as possible, to original conditions.

Areas to be excavated include:  Area 2 where an estimated ± 2,400 yd³ of mine-waste contaminated material will be excavated, including the roadway and sediment from the intermittent unnamed tributary; Area 4 where an estimated ± 3,200 yd³ of mine-waste contaminated material will be excavated, including the roadway and sediment from the intermittent unnamed tributary; Home Sites1 and 2 where an estimated ± 2,750 yd³ of mine-waste contaminated material will be excavated, including the road way and driveways; and the mill site where an estimated ± 2,000 yd³ of mine-waste contaminated material will be excavated, including the mill concrete foundations.

            Replacement of Property

The trailers used as residences at Home Sites 1 and 2 will be replaced because they cannot be saved and restored.  The aged trailers have fallen in disrepair since the families were relocated in late 2013.  The trailers are located on pads constructed of calcine.

The trailers must be relocated to enable excavation of the calcine pads.  Because of the structural condition of the trailers, they cannot be moved and saved without incurring further damage and then restored to their original condition upon completion of the action.  Additionally, contaminated items made of soft, permeable materials such as mattresses and sofas that cannot be decontaminated in an efficient and cost-effective manner may also be targeted for disposal and replaced with property of similar value.

            Temporary Relocation

Five families live on-Site.  ODEQ has provided for the temporary relocation of two of these families and is expected to continue to do so pending completion of the removal action described herein.  The proposed removal action may also require the relocation of one or more of the remaining three families temporarily to ensure their health and safety or to allow EPA to conduct cleanup activities.

Repository

Excavated mine-waste materials, along with other ancillary debris such as the concrete mill foundations, will be consolidated at Area 1, against the calcine and waste rock piles and hillside in the north end of the valley floor.  Other materials such the trailers and abandoned household furnishings, may be disposed of at the repository or transported off-Site for disposal at an approved disposal facility.  Woody debris will be placed elsewhere on-Site.  The calcine and waste rock piles will be consolidated to achieve a smaller footprint for the protective barrier.  A protective barrier consisting of an impermeable geomembrane liner and a minimum of 24 inches of clean rock and/or soil imported from off-Site will be placed over the contaminated material.

Best Management Practices

Best Management Practices (BMPs) will be implemented during construction activities to protect workers, the community, and the environment from short-term construction impacts such as erosion, sedimentation, fugitive dust, noise, and other similar potential impacts.

Non-hazardous materials and wastes such as inert construction debris, will be disposed of or recycled in accordance with appropriate solid waste disposal or recycling requirements.

Greener Cleanup Best Management Practices (BMPs)

Appropriate and practicable greener cleanup BMPs will be implemented during cleanup activities, including, but not limited to, minimizing energy consumption, minimizing generation and transport of fugitive dust, minimizing waste generation through reuse and recycling, minimizing impacts to water resources, minimizing areas requiring activity or use limitations, minimizing unnecessary soil and habitat disturbance, and minimizing lighting and noise disturbance.

Long-term Monitoring and Maintenance and Repair (MM&R)

A long-term MM&R program to be performed by the property owner and with assistance and oversight provided by ODEQ will be implemented to ensure the continuing effectiveness of the removal action and to monitor Site conditions.  EPA will prepare the MM&R in concert with ODEQ and the landowner.  As part of the monitoring component, annual, semi-annual and/or episodic inspections of the protective barrier integrity and performance and surface water drainage systems will be required, and as part of the maintenance and repair component, the landowner will be required to perform the needed maintenance and repairs.

Engineering and Institutional Controls

Engineering and institutional controls will regulate access to and use of the repository and other certain areas such as hillsides.  An enforceable environmental covenant will be developed and put into effect to limit certain activities, thus promoting the long-term durability and protectiveness of the cleanup action.  Additionally, ODEQ will ensure that environmental covenants restricting the disturbance of contaminated soil and land or resource use, as appropriate, are provided by the property owners.

 
2.2.1.2 Next Steps

The following removal activities are expected to occur during the next two week period (08/18/14 - 08/29/14):  complete the excavation of mine-waste contaminated material from Areas 2 and 4; demolish the trailers and excavate mine-waste contamnated material from the trailer pads and driveways; continue placing mine-waste contaminated material into the repository and grading and shaping; and continue to monitor and measure Site conditions and to maintain Site BMPs.

2.2.2 Issues

The extraordinary hot and dry conditions have required that additional personnel and equipment be assigned to ensure that BMPs for minimizing generation and transport of fugitive dust are effective and efficient and to ensure that adequate precautions regarding wild fire prevention and suppression are in-place.  The effort has included mobilizing an additional 4,000-gallon water truck with driver, and the purchase of other field equipment such as shovels, pulaskis, and fire extinguishers.