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Davis - Abandoned Mine Lands Project


Location

The abandoned Davis Tailings site is located approximately two miles west-northwest of Nevada City, California off North Bloomfield Road. The site is under the administration of the BLM Mother Lode Field Office.

Site History

The Davis Mill was constructed in 1915 and operated intermittently until the 1940's. The ore was fed through a rock crusher and then to a battery of five stamps where the ore was pulverized and mixed with water to form a pulp. The pulp was then transported from the stamp battery by cart to amalgamation pans. Here the pulp was mixed with mercury, salt and copper sulfate and fed into settling tanks. The resulting amalgam was then carted to a retort where the mercury was vaporized and recovered for re-use and the remaining silver and gold residue was melted in a furnace and poured into molds to form bars of silver-gold bullion.
The mill tailings were transported away from the mill by gravity in a slurry across an unnamed tributary of Rock Creek and into the floodplain area, roughly 600-800 feet northwest of the Davis mill.

The Davis Mill building is still intact and contains much of the mill equipment including the stamp battery. The ore was fed into the mill at the top building into a rock crusher and thence into a stamp battery where it was crushed to fine sand and mixed with water. The force of gravity was used to help move the ore into each successive stage. Before or after the stamps, mercury was added for amalgamation with gold and the amalgam was separated on concentration tables. The waste materials (tailings) were flumed or piped out of the mill for disposal where they came to rest across the unnamed tributary of Rock Creek. Because of the presence of large conifers and associated litter, tailings can be identified in the floodplain only by excavating.

The nature of the tailings chemistry was first identified in February 2003, when  mill tailings samples were analyzed by the USGS and BLM technical specialists, . Elevated levels of arsenic, lead and mercury were identified in the samples, using ICP-MS and XRF methodologies during May-October 2003.  Arsenic values ranged from non-detect to 554 ppm, lead ranged from non-detect to 168 ppm, mercury ranged from non-detect to 13 ppm, and zinc ranged from non-detect to 138 ppm.

Mill Tailings Sampling/Characterization

A sample grid was created to cover the suspected tailings area and staked on 25-foot centers. A portion of the tailings near the northwest perimeter may be on adjoining private land. The grid nodes were hand augered by BLM personnel and samples will be collected at 0-1' and every 1' interval until native soil or refusal is encountered. Samples were placed into sample bags, identified with unique sample numbers. Samples were then sieved to the -125 micron size for analysis. Samples were measured using a Niton 723S X-ray fluorescence (XRF) spectrometer  using EPA Method 6200. Samples were also analyzed for leaching characteristics using EPA Method 1312 Synthetic Precipitation Leaching Procedure, acid-base accounting (ABA), and a composite tailings sample was analyzed for bioaccessibility. Additional samples were checked for gold content, in order to consider the feasibility of remining as a potential remedial cleanup option. 

Davis water samplesSurface water and sediment samples were collected upstream, onsite and downstream of the site by the USGS. Surface water samples were collected for total and dissolved metals and mercury by ultra-trace analyses. Sediment samples were collected, dried and sieved to -62 microns and were analyzed for total metals. Additional sampling in the watershed has been performed by USGS. Two nearby and one downstream well water samples were also collected and analyzed for total metals via EPA method 200.
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Sample Objectives

* Determine if toxic metals were released from the site to the surface waters of the Rock Creek drainage.
* Determine the 3-dimensional volume and chemistry of the mill tailings
* Determine the depth to groundwater from local well records and determine its chemistry.

Proposed CERCLA Actions 

The objective of this CERCLA investigation is to evaluate the abandoned Davis Tailings site and characterize the sources, amounts and releases of hazardous substances and to determine if a CERCLA removal action is warranted. Environmental site characterization data has been compiled into a Removal Site Inspection (RSI) document. This document summarizes the contaminants of concern, potential targets that may be impacted and the pathways by which contaminants may impact potential human and biota.  The Engineering Evaluation Cost Analysis (EECA) report has also been compiled and identifies contaminants of concern, potential targets that may be impacted and the pathways by which contaminants may reach the targets.

The historic Davis Lode Mine was actively mined for lode gold during the 1893-1940?s period. The historic Davis Stamp Mill was constructed in 1915 and the 5-stamp milling circuit was in operation from 1915-1945. Mill tailings from the Davis Mill probably were discharged into tailing dams and ponds within the Little Rock Creek floodplain area. This area is now an AML hazmat site. Sample results from water, sediment, and biota indicate elevated arsenic and mercury level exist in the floodplain sediments. A CERCLA non-time critical remediation process has been implemented. An RSI (Removal Site Investigation) document has been compliled. The EE/CA (Engineering Evaluation/Cost Analysis) document will be completed by December 2005. Remediation options will be listed in the EE/CAThe PRP compilation data has been furnished to BLM by CA-DTSC (Dept. Toxic Substance Control) and several key PRP?s (responsible parties) have been identified. Coordination with upstream RP?s on the Hoge Mine property (upstream of the Davis Mine) will be required to effectively remediate arsenic tailings located downstream from both the Davis and Hoge Mine Mine sites. 

CERCLA Action Benefits: Proposed improvement of water quality to the Little Rock Creek-Rock Creek watershed. Proposed remediation of arsenic and mercury-bearing lode tailings also represents a tangible benefit. The Removal of arsenic-mercury contaminated sediments within active stream channel/100-year floodplain will change its present status as a pollution "point source" under the Clean Water act and prevent downstream discharge into the Yuba River watershed. The California State Water Control Board and other regulatory agencies require that BLM mandate significant reductions in arsenic and mercury loads from its managed lands within the Sacramento River watershed - Bay Delta region.
 
Feasibility: This site of BLM's high priority AML 1010 sites for pilot cleanup. During FY03-05, technical specialists collected water, sediment, and biota samples. During FY05, a hazardous materials cleanup of the interior of the Davis Mill building was undertaken by a qualified hazmat contractor team. During FY06 BLM technical specialists finished compilation of the required CERCLA environmental documents. During late FY06-07  BLM specialists and BLM contractors will undertakeremoval action of the arsenic-bearing mill tailings, located on a 2-acre size northwest of the Davis Mill building.
 
Support: There is widespread support from interagency Federal partners (USFS, BOR, EPA), State Partners (DOC AMLU, CWQCB, RWQCB, CA Fish&Game, Public Health), County Partners (Dept. Envtl Health) and various watershed stakeholders (American River Conservancy).

Abandoned Mine Lands
   Project Sites

Davis Mill Site  Davis Mine site location
Davis Mill Building and Site Location

Davis mill building    5-Stamp mill battery
Davis Mill Building and 5-Stamp Mill Battery