2g-2: Legacy site assessment and management

The Black Butte Mine is a former mercury (Hg) mine in central western Oregon that is on the US National Priorities List due to off-site migration of mercury in surface water, and due to significant fish contamination in a nearby flood control reservoir. Two decades of environmental assessment and various remediation activities have been conducted at the site by state and federal agencies. This presentation provides an overview of the key findings from the environmental assessments to provide a holistic understanding of Hg transport pathways from the mine site, as well as methylmercury (MeHg) production in the downstream reservoir and accumulation in fish. At the mine site, water samples were collected from groundwater and surface water during baseflow and stormflow conditions for dissolved and particulate total mercury (THg) and MeHg. In the reservoir seasonal measurements of lake water column, sediment and porewater were collected for THg and MeHg as well as several ancillary parameters. The results for the water measurements at the mine site showed that surface water transport was the dominant pathway for offsite Hg mobilization, mostly through erosion during stormflow events. Peak stream THg concentrations reached 94,000 ng/L and mostly consisted of particulate-bound Hg. Downstream in the reservoir, there was large spatial variability in sediment THg concentrations (0.05 to 2.3 mg/kg) representing a range of influence from the upstream mine. Methylmercury production in the reservoir was also highly variable and was affected by variations in sulfate, organic carbon, and reservoir water level fluctuations. Fish Hg concentrations in the reservoir were up to 1.8 mg/kg. Overall, these results highlight the spatial and temporal variability in Hg transport and transformation from the mine site as well as impacts on a downstream fishery. These findings are being used to help guide upcoming remediation activities at the site.

An increase in intensity and frequency of extreme events resulting from climate change is expected to result in extreme precipitation events on both regional and local scales. These events have the potential to mobilize large volumes of mercury (Hg) in watersheds where mine wastes reside in flood plain deposits downstream from historic mines that operated with little or no environmental regulations. Most mine remediation has focused on mine sites and rarely have Hg contaminated flood plain deposits been evaluated or remediated. Several types of ore deposits have released Hg contaminated mine wastes that are now stored in flood plain deposits that extend from 10s to 100s of km from the mine site. These deposits have varying degrees of risk to release bioavailable Hg based on the volume, concentration, and speciation of Hg in the flood plain deposits. These mines include: 1) Hg and placer Au mines that have the highest potential to release bioavailable Hg from flood plain deposits during extreme events; 2) massive sulfide, sediment and hot spring type gold deposits that have moderate potential; and 3) porphyry copper and MVT deposits that have low potential. The various types of downstream aqueous environments receiving Hg contaminated mine wastes resulting from extreme events are an important factor to consider in evaluating the risk posed by extreme events in mine impacted watersheds. Reservoirs are at highest risk for a significant increase of MeHg in the food web after an extreme event because the Hg contaminated sediment also supplies sulfate, iron and organic carbon. Several reservoirs in the California Hg mineral belt have had a significant and rapid increase in MMeHg production that impacted the food web after Hg mine contaminated sediment was released into the reservoir.

An analysis of the Hg mine-impacted watersheds, where Hg is stored in contaminated flood plain deposits, in more than 50 Hg mines from the California Hg mineral belt, indicates that this Hg source poses a major environmental risk to aquatic systems during a period of climate change. An extreme Atmospheric River 1000 Storm (ARkStorm) similar to the 1861-1862 storms has the potential to remobilize Hg stored in flood plains throughout the entire Hg mineral belt and impact reservoirs, estuaries, and wetlands throughout much of California.

The East Fork South Fork Salmon River (EFSFSR)is a critical spawning habitat for bull trout and Chinook salmon in central Idaho. Monthly water-quality monitoring on Sugar Creek, a tributary to the EFSFSR downstream from the Cinnabar mercury mine site, has demonstrated elevated mercury (Hg) concentrations, with the greatest concentrations occurring during peak snow melt in late May and during high-intensity summer thunderstorms. Mercury mineralization at the Cinnabar mine site is hosted by carbonate rocks, which generate waters dominated by Ca2+ and HCO3- at pH 7 to 9. Sampling along Sugar Creek and its tributary Cinnabar Creek was conducted near baseflow conditions to determine groundwater contributions to methylmercury (MeHg). Total unfiltered Hg concentrations in Cinnabar Creek increased from 8 to 14 ng THg L-1 above the mine site to 170 to 213 ng THg L-1 in the main reach flowing through the mine site. Unfiltered MeHg concentrations from same reaches of Cinnabar Creek increased from <0.04 ng MeHg L-1 to 0.07 to 0.15 ng MeHg L-1, or 0.03 to 0.09 percent of the total mercury, in unfiltered water from Cinnabar Creek flowing through the mine site. Groundwater seeps and mine adits were sampled to evaluate contributions from groundwater in direct contact with cinnabar-bearing carbonate rocks. There are several fault-controlled seeps with associated wetlands in the mine site with up to 960 ng THg L-1 and 6 ng MeHg L-1, reflecting the enhanced dissolution and methylation of mercury in these discrete areas of high organic matter in what is otherwise a low-nutrient watershed. Incorporating existing fault-controlled groundwater seeps into a remediation design is critical for reducing downstream transport of mercury and other metals in legacy mercury mine sites.

Persistent mercury (Hg) impact from legacy mines continue to present risk to the environment and human health globally. The Stibnite Mining District is a historical mining area located near the headwaters of the East Fork South Fork Salmon River in central Idaho. Past mining of antimony, gold, silver, mercury and tungsten from the early 1900s to the 1990s has resulted in elevated Hg as well as other trace elements in the surrounding ecosystem. The Cinnabar and Fern legacy Hg mine sites, within the Stibnite Mining district are primary source areas for elevated Hg entering the ecosystem. The extraction of Hg from cinnabar was carried out via two methods over the 45 years of Hg production in this mining district: 1) Hg was initially retorted in rotary kilns (1921 to 1956), a process during which the ore is heated to 500-600°C volatizing the Hg to gas with recovery as elemental Hg(L) via condensation and 2) Hg was later produced by wet flotation and electro-separation (1956 to 1966). These distinct methods of extracting Hg from cinnabar resulted in variations of Hg concentration and isotopic composition being introduced into the environment during the active processing of the Hg ore, as well as long term legacy input of Hg from the weathering of calcine tailing piles and other residual Hg contamination sources related to the original mining and processing of Hg ore. Mass dependent fractionation of Hg isotope ratios and variation in Hg concentrations resulting from the processing of cinnabar ore has been reported previously for produced Hg metal and residual waste fractions (Hg(L), Hg(g), and calcines). These Hg fractions possess the potential of releasing Hg into the local environment over sustained periods of time. In order to better understand Hg behavior in legacy mining areas, establish a priority of effort for remediation, and determine differences in mobility and bio-accessibility of Hg sources in this ecosystem, we investigated Hg concentrations and isotopic compositions in a wide range of samples: two distinct calcine produced tailing piles, lichen, periphyton, stream sediment, soil, tadpoles, tadpole eggs, tailed frog, sculpin, and bull trout. We will present Hg concentrations and isotopic compositions of samples collected over multiple years within the mining district and provide a post-mining baseline of Hg distribution. This baseline will help inform environmental assessments for future landuse activities, including recommencing of mining operations and onsite remediation in this ecosystem.

The Cache Creek watershed, located in the coast range of California, has had a legacy of mercury (Hg) mining. Hg levels in Cache Creek are elevated above regional background; Cache Creek exportstotal Hg (THg) and methylmercury (MeHg) to the San Francisco Bay Delta.The regulatory approach for controlling Hg in the Cache Creek watershed finalized in 2007 focused on reductions in THg loading from legacyHg mines in tributaries (e.g., Harley Gulch). The Hg mines in Harley Gulch were remediated in 2007, reducing loads from the mines by 95%. However, an analysis of fish tissue THg data conducted in 2015 has indicated thatHg concentrations in fish tissue THg collected from Cache Creek in 2011 were not declining. The regulatory agencies responsible for implementing the Hg control approach haveconcluded that additional remediation of Hg mines in the CacheCreek watershed will be required for water quality to improve.

A recently-completed data synthesis and comprehensive evaluation of Hg sources and cycling in the Harley Gulch watershed indicates thatfurther efforts to remediate Hg mines may not result in the attainment of local water quality targets oralter Hg levels downstream. This isdue to the presence of natural Hg sources inHarley Gulchandthe weak hydrological linkage between Hgmines in Harley Gulch and Cache Creek. Surface water and sediment data collected pre- and post-remediation of the mines in the vicinity of Harley Gulch has revealed that the remediation has successfully prevented the erosion of high THg concentration particles but aqueous MeHg targets have not been met. Additional Hg mine remediation is likely to have spatially limited benefit due to the nature of Hg transport in the watershed. The presence of naturally occurring sources of Hg in Harley Gulch, including connate groundwater and high THg concentration soil unimpacted by mining activities may prevent attainment of water quality objectives locally in Harley Gulch. Harley Gulch is an ephemeral stream and Hg loading even from un-reclaimed mine sites is limited to erosion of THg adsorbed to particles or in calcine wastes during rain events, which generally occur between October and May.The THg loading from historical mines is likely to be further reduced by climate change, which is predicted to lead to lower precipitation amounts in the ecoregion.Harley Gulch presents a case for why focusing on legacy Hg sites in complex watersheds may not lead to water quality improvements.

Mercury contaminated sediment from legacy gold mines in the Sierra continues to be a source of inorganic mercury (Hg) to the environment. The discharge from Malakoff Diggins, once one of the largest hydraulic mines in California, is a source of Hg and sediment to Humbug Creek. The purpose of this study was to estimate the load of particulate bound Hg and suspended sediment in Humbug Creek for Water Years 2012 and 2013. Grab samples were taken from baseflow conditions and from multiple storm events and analyzed for nonfiltered Hg, filtered Hg and total suspended sediment (TSS) (EPA 1669, EPA 1631, EPA 160.2). A stage discharge relationship was developed for the Humbug Creek gage station over a range of flow conditions. Samples were collected from Humbug Creek upstream of the Malakoff Diggins discharge point, from the discharge point and downstream of the discharge and Humbug Creek confluence at a stream gage. The annual load in Humbug Creek for suspended sediment and particulate bound Hg was calculated at the gage using relationships established with continuously monitored turbidity (15 min data) and grab samples of total suspended sediment (n = 25, R2 = 0.82) and particulate bound Hg (n = 15, R2 = 0.80). The annual load was 100-120 grams of particulate bound Hg and 475,000-575,000 kg of suspended sediment. For both water years, as much as half of the annual sediment load was from a single storm event during which 3-4g of particulate bound mercury was released per day. The contribution of mercury loads from legacy hydraulic gold mines should be quantified as it is a critical source control strategy for California Total Maximum Daily Load programs.

Many historical gold mining sites across the world are contaminated with mercury (Hg) and other potentially toxic elements, such as arsenic (As). The high level of Hg is due to the gold extraction process (mercury amalgamation) used in the late 1800`s to early 1900`s. Mercury amalgamation is still used at artisanal mines in developing countries and at smaller mines, due to its simplicity. The waste from this process, tailings, presents significant human health and ecological liabilities. Revegetation of tailing deposits could provide increased protection against wind erosion, toxic dust blowing across populated areas and prevent leaching of Hg and As into waterways. A recent Saint Marys University study demonstrated that growth of a reclamation grass seed mix in highly contaminated gold mine tailing material can be facilitated by low-dose selenium additions. However, commercial reclamation seed mixes often include non-native highly-invasive species which should be avoided. There are several advantages to using native plant species for reforestation and reclamation, including proven hardiness in growing regimes, avoiding introduction of new invasive species in vulnerable ecosystems, and maintaining / improving the biodiversity of impacted ecosystems. The objective of this project was to (1) identify wide-spread established native species which can be used in phytostabilization of gold mine sites, (2) assess whether seeds of native hardy species can be used directly in exposed tailing deposits to promote natural revegetation, and (3) assess if a low-dose of sodium selenite can promote growth of native species while limiting the plant bioaccumulation of Hg and As. Three widespread North American native plant species were selected for this study: Panicum virgatum, Juncus tenuis and Anaphalis margaritacea. Plants were grown in 7 replicates of untreated tailing material, 7 replicates of Se treated tailing material (3 mg Se/kg) and in 7 replicates of artificial soil (control) for nine weeks. The results from this study are currently being compiled and are promising; indicating that seeds of native hardy species can be used for cost-effective phytostabilization of tailing deposits. In addition, preliminary results indicate that a low dose of sodium selenite increased the biomass, % emergence, shoot height and root length in Juncus tenuis, increased % emergence in Anaphalis margaritacea, and increased root lengths in Panicum virgatum. The sodium selenite additive also decreased plant shoot accumulated [Hg] in Panicum virgatum by 36%. Accumulated Hg concentrations in Anaphalis margaritacea shoots did not differ significantly between untreated and Se treated tailings.