The Argonaut Mine Tailings Site (the site) is located in the City of Jackson, Amador County, CA. The site is comprised of approximately 64.8 acres of privately owned undeveloped land and is currently the number two site on the EPA Region 9 Abandoned Mine Site Prioritization List. The site is bounded by Argonaut Lane and Argonaut High School to the west, Hoffman Street and Amador County Superior Court to the south, residential properties to the north and Argonaut Drive and Jackson Junior High School to the east. The site was used to store, process and dispose of unprocessed, semi-processed ore and mine tailings from the Argonaut Mine and possibly other mining operations in the area. Tailings materials on the site were generated primarily from hard rock gold mining and cyanide leaching processes from the 1850’s to 1942. The cyanide plant generated an estimated 1 million cubic yards of grey sands tailings which were deposted behing a 100’ high tailings dam. The mine shaft and head frame, stamp mill and associated mercury amalgamation processing areas, which are located approximately ½ mile northeast of the site, are not currently part of the removal assessment. A concurrent site investigation (PASI) is being conducted to score the site for potential listing on the NPL.
The site consists of six distinct areas: The Unprocessed and Semi-Processed Ore Storage Area (AOC-1); the Cyanide Plant Area (AOC-2); the Upper Earthen Tailings Dam, Grey Sands Tailings Pile Area, Lower Earthen Tailings Dam, Historic Impoundment and Concrete Dam Basin (AOC-3); Eastwood Multiple Arch Dam and Surface Water Drainage Areas (AOC-4); Vacant Lot used for tailings disposal (AOC-5) and; Mental Health Clinic soil contamination (AOC-6). The Ore Storage Area is approximately 5 acres and is one of the main focuses of our investigation. This area is barren and devoid of vegetation. EPA's preliminary assessment revealed high concentrations of arsenic (48,000 mg/kg), lead (4,000 mg/kg) and mercury (360 mg/kg) in residential soils. These levels are 50-60 times the metal concentrations found in other areas of the site. For comparison, the background arsenic concentrations ranged from 8 mg/kg to 20 mg/kg and the EPA Regional Screening Level (RSL) for arsenic in residential soils is 61 mg/kg based on a carcinogenic target risk of 10-4 and bioavailability factor of 60%. The RSL for lead is 400 mg/kg and mercury is 10 mg/kg. Additionally, the soils are very acidic and corrosive. The 0.39 acre vacant lot located at the corner of Argonaut Lane and Pioneer Street is another primary area of concern. This area also has high concentrations of heavy metals including arsenic, lead and mercury. Ponded liquid on the lot had a pH of 1-2. The Cyanide Plant area is approximately 6.5 acres and consists of various sized concrete cyanide processing tanks and vats and a thickening basin. There are eight partially buried round concrete tanks measuring 25’ in diameter and 10’ deep. Two of the tanks were filled with a cyanide processing waste. Several other vertical concrete tanks contained some type of by-product. The sands were cyanided in leaching tanks and vats while the slimes were cyanided in an agitator, and then filtered. The gold bearing solution was then precipitated and roasted. Both sands and slimes were discarded in a large ravine adjacent to the plant.
At the most eastern part of the site is a concrete tailings dam known as the Eastwood Multiple Arch Dam (EMAD). The EMAD was constructed in 1916 and is approximately 46’ high and 420' long. The dam impoundment is filled with sediment and water cascades over the top of the dam during storms. Runoff water from dam flows to Jackson Creek and Lake Amador. The U.S. Army Corps of Engineers (Corps) was commissioned to perform a dam safety evaluation. Based on the evaluation, the dam was determined to be structually unsafe. There is a concern about the static and seismic stability of the dam which is only 400 yards from downtown Jackson. A geotechnical investigation was conducted to determine the potential for liquification and mud/debris flow if the dam failied. The report is due in January. Additionally, ORD's contractor, CDM Smith, has commenced modeling the water/mud/debris flow to estimate the extent and magnitude of the debris flow and impacts to life and property. EPA is assisting the City and County officials in obtaining a FEMA Pre-Disaster Mitigation Grant or Flood Mitigation Assistance Grant to retrofit or reinforce the dam.
Click here for information on the Argonaut Mine Site School/Residential Yards Removal Action in July/August 2015.
