Mercury stable isotope geochemistry as a tool for tracing sources and chemical transformations in the environment

Mercury (Hg) is a redox active global contaminant. Hg has two stable oxidation states in the environment, Hg(0) and Hg(II). Hg(0) is significantly less soluble than Hg(II) and is less reactive. Hg(II) is very soluble and highly reactive; being able to form methylmercury, a potent neurotoxin. Hg is a health concern due to its neurological effects, ability to cross the blood-brain barrier and umbilical cord in pregnant women, and get biomagnified. Anthropogenic inputs of Hg into the environment are dominantly due to coal burning and artisanal gold and silver mining. Hg stable isotope ratios have been developed since the early 2000s in order to understand Hg cycling in the environment. The three studies reported in this dissertation aim at developing Hg isotope ratios as tools for source apportionment, identifying Hg chemical transformations in a contaminated environment, and understanding the interaction between dissolved Hg(0) and Hg(II). In the first study, Hg isotopes were used to apportion different Hg sources into Emory and Clinch Rivers in Tennessee, USA. Following a coal ash spill into the Emory River, elevated Hg concentrations were detected; however, there is a known Hg contamination entering the Clinch from upstream of the spill extent. Hg isotope ratios were used to apportion all the Hg inputs into the Emory and Clinch Rivers. The second study attempted to use Hg isotopes in order to detect and quantify natural chemical transformations of Hg in a contaminated creek. The East Fork Poplar Creek is a contaminated creek that runs through the DOE Y-12 plant in Oak Ridge, Tennessee, USA. The Y-12 plant historically used large amounts of Hg which released into the creek and the surrounding floodplain. Hg isotopes were used to identify the probable chemical transformations occurring naturally in the creek’s waters and quantified them. Finally, laboratory experiments were conducted in order to better understand the interaction between dissolved Hg(0) and Hg(II). Hg(0) and Hg(II) species are expected to be in contact in any environment in which Hg is getting actively chemically transformed; however, no studies have looked into the interaction between these two dissolved species. The results of these exchange experiments show that Hg(0) and Hg(II) have the ability to exchange isotopes very quickly, which will have implications for interpreting field isotope data

Environmental Impacts of the Tennessee Valley Authority Kingston Coal Ash Spill. 1. Source Apportionment Using Mercury Stable Isotopes

Mercury stable isotope abundances were used to trace transport of Hg-impacted river sediment near a coal ash spill at Harriman, Tennessee, USA. δ²⁰²Hg values for Kingston coal ash released into the Emory River in 2008 are significantly negative (−1.78 ± 0.35‰), whereas sediments of the Clinch River, into which the Emory River flows, are contaminated by an additional Hg source (potentially from the Y-12 complex near Oak Ridge, Tennessee) with near-zero values (−0.23 ± 0.16‰). Nominally uncontaminated Emory River sediments (12 miles upstream from the Emory-Clinch confluence) have intermediate values (−1.17 ± 0.13‰) and contain lower Hg concentrations. Emory River mile 10 sediments, possibly impacted by an old paper mill has δ²⁰²Hg values of −0.47 ± 0.04‰. A mixing model, using δ²⁰²Hg values and Hg concentrations, yielded estimates of the relative contributions of coal ash, Clinch River, and Emory River sediments for a suite of 71 sediment samples taken over a 30 month time period from 13 locations. Emory River samples, with two exceptions, are unaffected by Clinch River sediment, despite occasional upstream flow from the Clinch River. As expected, Clinch River sediment below its confluence with the Emory River are affected by Kingston coal ash; however, the relative contribution of the coal ash varies among sampling sites

Environmental Impacts of the Tennessee Valley Authority Kingston Coal Ash Spill. 2. Effect of Coal Ash on Methylmercury in Historically Contaminated River Sediments

The Tennessee Valley Authority Kingston coal ash spill in December 2008 deposited approximately 4.1 million m³ of fly ash and bottom ash into the Emory and Clinch River system (Harriman, Tennessee, U.S.A.). The objective of this study was to investigate the impact of the ash on surface water and sediment quality over an eighteen month period after the spill, with a specific focus on mercury and methylmercury in sediments. Our results indicated that surface water quality was not impaired with respect to total mercury concentrations. However, in the sediments of the Emory River near the coal ash spill, total mercury concentrations were 3- to 4-times greater than sediments several miles upstream of the ash spill. Similarly, methylmercury content in the Emory and Clinch River sediments near the ash spill were slightly elevated (up to a factor of 3) at certain locations compared to upstream sediments. Up to 2% of the total mercury in sediments containing coal ash was present as methylmercury. Mercury isotope composition and sediment geochemical data suggested that elevated methylmercury concentrations occurred in regions where native sediments were mixed with coal ash (e.g., less than 28% as coal ash in the Emory River). This coal ash may have provided substrates (such as sulfate) that stimulated biomethylation of mercury. The production of methylmercury in these areas is a concern because this neurotoxic organomercury compound can be highly bioaccumulative. Future risk assessments of coal ash spills should consider not only the leaching potential of mercury from the wastes but also the potential for methylmercury production in receiving waters