Phytoremediation is the use of green plants to clean up environmental contamination from a variety of sources (Cunningham and Berti 1993). It has been suggested as a method for remediating metal contamination (Baker 1981, Chaney 1983), and numerous plants have since been identified as either tolerant to, or accumulators of various heavy metals, including lead (Huang and Cunningham 1996, Vassil et al. 1998, Chin 2007), cadmium (Dhankher et al. 2003, Podar et al. 2004), and arsenic (Zhaol et al. 2002, Meharg 2003, Rathinasabapathi et al. 2007).
Most phytoremediation studies to date have focused on the physiological mechanisms involved in the extraction, transport, and biomagnification or accumulation of metals in plants, primarily within a laboratory environment (Lasat 2002, Hansen et al. 2005, Maestri et al. 2010) and often focus on known accumulators of metals. While the mechanisms by which plants uptake and translocate metals are still being examined, identifying native species that thrive in contaminated soils as well as remove threats of metal exposure should be a primary objective. Approaching phytoremediation from both a theoretical perspective (i.e., which plants accumulate and how?) as well as an applied one (i.e., how effectively can phytoremediation be deployed at restoration sites?) leverages the potential two-fold benefit of both contamination removal and ecological restoration.
We took an exploratory approach to assess potential for three California native plant species to grow and accumulate heavy metals on gold-mine waste-rock-tailings piles within the Sierra Nevada Gold Country at an abandoned gold extraction site. We also looked at the potential heavy metal extraction capacity of these species as a phytoremediation-feasibility assessment for potential implementation as part of a broader clean-up effort.
We established open-air growth containers with plastic basins to hold all plants at the Providence Mine in Nevada City, California. The site was one of the richest mines during the California gold rush, and mining activities have left a legacy of heavy metal contamination. Preliminary soil sampling found elevated concentrations of arsenic (As), cadmium (Cd), and lead (Pb) with mean concentrations of 68.41 mg/kg As, 5.8 mg/kg Cd, and 240.77 mg/kg Pb. Soil screening levels (SSLs)—regulatory limits at time of [End Page 210] site assessment—for each of these contaminants vary by location and sampling time, with recent California SSLs for each of the above metals being 0.07 mg/kg As, 1.7 mg/kg Cd, and 80.0 mg/kg Pb (CalEPA 2010). This restoration note focuses primarily on Cd due to significant uptake of Cd across sampled species as well as discovery of a previously unreported Cd accumulator.
Candidate species for this study were chosen following a previously conducted field-based-pilot feasibility analysis using a large selection of species (Lauder 2013). Species chosen for that pilot were selected based on their presence in the exhaustive PhytoRem phytoremediation literature database (McIntyre et al. 2003), as well as being described as locally native, and augmented with species yet untested in terms of metal tolerance. Pilot study results led to the selection of Festuca rubra (red fescue) and Helianthus annuus (sunflower) as candidates for controlled pot studies due to their nativity and demonstrated uptake capacity as well as biomass production (Lauder 2013). Stipa pulchra (= Nassella pulchra [purple needlegrass]) was added to the potted plant study due to its growth form, nativity, seed availability, and as an exploratory species that has not yet been tested for heavy metal uptake or tolerance. All three species were also selected due to their presence in commercially available hydro-seed mixtures which may be applied as part of larger, engineering-based clean-up efforts.
We planted candidates in equal amounts of contaminated homogenized soil taken directly from the mine tailings. All plants were direct-sown to simulate broadcast seeding similar to hydro-seed application. Each species was planted in triplicate with each replicate using the same amount of seed by weight (1.5 ounces [42 g] for both grasses and eight seeds per pot for H. annuus). Three one-gallon (5.7 l) pots of each plant contained no amendments, and three pots containing each species were...