The Providence Mine Brownfield site is a 45-acre property owned by the City of Nevada City located at the end of Providence Mine Road along the south side of Deer Creek, approximately one mile downstream of downtown Nevada City. The Providence Mine was a large hard rock gold mine which operated from 1851 to 1919 and produced an estimated $20 million in gold and associated minerals. The operation included a 40-stamp mill and a chlorination works where releases of mercury and concentrated heavy metals likely occurred. The City-owned property includes the eastern portion of the former mine site. A popular-use trail traverses the site, passing directly through the mine features in the northern portion of the site.

The Providence Mine Brownfields Cleanup Project will be performed on two areas in the northwestern portion of the property. The "Mine Features Area" is located on a benched area above Deer Creek and adjacent to the east of Providence Mine Road where several mine structures and the Providence Mine Shaft were located. The "Waste Rock Area" is located on a steep slope extending from the Mine Features area to Deer Creek.

Cleanup work proposed for the Mine Features Area includes excavation and off-site disposal of a limited amount of highly contaminated soil, trail construction, capping with clean soil and phytoremediation (the use of plants to extract contaminants from near-surface soil).

Cleanup work proposed for the Waste Rock Area includes slope benching and re-grading to lower mine waste slope angles below the current angle of repose, soil stabilization using physical methods, stream bank restoration using rock walls or rock armoring, and phytoremediation to extract contaminants, stabilize soil, and create access deterrents.

UPDATE. Work completed in 2015. Read the poster-format report by Kyle Leach and Jeff Lauder.

The Stiles Mill site is a 2.14-acre city-owned property located across Deer Creek from downtown Nevada City just upstream of the Pine Street Bridge at the confluence of Gold Run and Deer Creek. The site was operated in the mid to late 1800s as a wood planning mill and a custom quartz stamp mill which crushed ore from many small local mines. The mill has been completely removed and no visible tailings remain; however, a partially collapsed shaft and waste pile and a stockpile of mine waste with relatively high levels of lead and arsenic remain on the site along the banks of Deer Creek. Brownfields Cleanup work proposed for the Stiles Mill site includes re-grading of mine waste piles and capping with clean soil, stream band restoration to stop erosion of mine waste into Deer Creek and cleanup of a trail leading from the corner of Sacramento and Clark Streets onto the site. A phytoremediation pilot study may also be performed to test the use of plants to extract contaminants from near-surface soil.

Sierra Streams Institute set out in 2012 to assess the ability of local native plants to extract lead, cadmium, and arsenic from mine waste at Providence Mine and Stiles Mill, two abandoned mine sites in Nevada City. The project was made possible with generous funding from the Oakland-based Rose Foundation for Communities and the Environment. Both of the mine sites have high levels of arsenic, cadmium, and lead in their waste rock piles. All three of these metals are highly toxic to both local wildlife and people, and their proximity to Deer Creek could result in their downstream transport, resulting in degradation of downstream environments. SSI's experiment tested uptake of all three heavy metals into plant tissue and associated decreases in soil concentration in both field plots and controlled potted scenarios. The design also assessed the relative effects on plants' uptake capacity of amending the soil using rock phosphate or compost.

The project is an example of a type of remediation strategy known as phytoremediation, the process of using plants to clean up contaminated or polluted soils, waterways, and materials. There are three main methods: phytoextraction--uptake of contaminants into the plant; rhizofiltration--the filtering of contaminants from water by the roots of plants (typically in aquatic systems); and phytostabilization--the "locking up" of contaminants by roots to render them mostly harmless.

Our phytoextraction research project demonstrated excellent metal uptake capabilities in Red Fescue (F. rubra), Sunflower (H. annuus), and Purple Needlegrass (S. pulchra). Red Fescue, previously known to extract lead from soil and hydroponic solutions, may now be a newly discovered hyperaccumulator of cadmium. A hyperaccumulator is defined as any plant that extracts the desired contaminant to a concentration in plant tissue that exceeds that of the host soil. In our case, Red Fescue extracted cadmium to a concentration that was almost 120% that of the soil. While cadmium concentrations in the soil were relatively small, cadmium is a known carcinogen and may be linked to incidence of breast cancer, and so any remediation of this metal in the Gold Country is significant. Fescue and Purple Needlegrass also both extracted up to 5 times as much lead as other, non-accumulating plants.

This study is an important step in establishing the feasibility of phytoremediation and outlining the steps necessary to apply this new remediation technology to contaminated sites. Unlike our field experiment, other phytoremediation studies carried out to date implement strict laboratory controls and use hydroponic solutions or spiked soils. By demonstrating uptake of metals by these native plants, as well as showing that these plants can grow from seed and establish healthy roots that stabilize steep mine waste piles, our research lays the groundwork for future use of native plants to assist in the cleanup of metals in the soil, and potentially help limit human exposure to these dangerous materials in the Gold Country and beyond. We hope to publish our results soon and contribute to this exciting new field. For more information see the Phytoremediation Feasibility Assessment report, or contact jeff@sierrastreamsinstitute.org.