DukeSpace :: Browsing by Subject "Mercury" (2024)

After a pair of toxic releases in Bhopal, India and Institute, West Virginia in the 1980s, environmental groups and members of the public demanded more information on toxic chemical releases into the community. As a result, United States facilities that manufacture, use, or process above-threshold amounts of one of 650 listed toxic chemicals must publicly report their releases and transfers via the annual Toxics Release Inventory (TRI). In 2000 and 2001, the EPA lowered the reporting thresholds for mercury and lead, respectively. This project assesses how the increased TRI reporting for mercury and lead changes our understanding of the geographic distribution and industry composition of mercury and lead-releasing facilities, as well as the demographic characteristics of the areas surrounding the facilities. Through a geospatial analysis of mercury and lead TRI reporting before and after the threshold changes, this project evaluates the effectiveness of the TRI program at achieving its founding purpose: to provide information to empower communities. The findings suggest evidence of an overall geographic and industry diversification across the threshold changes, but point to specific states and industry types that tend to account for larger than average portions of releases. Reporting was found to be concentrated in working poor block groups, with no change in income distribution across the threshold changes. Further geospatial and statistical analysis of income and other demographic variables is recommended in order to confirm reporting and release trends. Given the limitations to interpretation of TRI data, more outreach and education would be prudent in order to maximize communities’ utility of the increased mercury and lead data available after the threshold changes.

There have been few studies investigating the association between mercury exposure and blood pressure, with inconsistent results. In this study, the association between hair mercury concentration with mean arterial pressure (MAP) and hypertension were evaluated using data collected in a 2015 cohort study, which sampled 23 communities in Madre de Dios, Peru. This area has recently experienced a rapid increase in artisanal and small-scale mining, which is the main anthropogenic source of mercury emissions. Generalized estimating equations were used to account for correlation within communities. Analyses for MAP and hypertension were performed using linear and logistic models, respectively, and confounding variables were included in both models. Due to the significant (p-value < 0.05) interaction between sex and mercury in both models, the analysis was stratified by sex. In women, there was an inverse association between hair mercury concentration with hypertension (OR: 0.84; 95% CI: 0.50–1.41) and MAP (gMR: 0.99; 95% CI: 0.98–1.008), but these associations were not significant at a 5% significance level. In men, the associations between hair mercury concentration with hypertension (OR: 3.07; 95% CI: 1.36–6.92) and MAP (gMR: 1.024; 95% CI: 1.01–1.04) were positive and significant at a 5% significance level. Differences observed between sex could be attributable to differences in exposure, men eating greater amounts of mercury-contaminated fish, or sex hormones, which regulate the distribution and excretion of mercury in the body.

Following the discovery of acute mercury toxicity from seafood consumption in the 1950s and subsequent research into mercury in the environment, scientists and managers now recognize the health threats of mercury poisoning from seafood consumption, especially in fetuses, infants, and children. Unfortunately, consumers remain confused or uneducated about species-specific mercury concentrations, thus perpetuating the risks associated with contaminated seafood. This study models the bio-economic dynamics of a system involving two species consumed by humans: a highly mercury-contaminated predator, bluefin tuna, and a tuna prey fish with low levels of contamination, Atlantic mackerel. Model scenarios evaluate varying levels of mercury pollution, consumer aversion to mercury, and fishes’ biological resistance to mercury poisoning to determine optimal harvest rates and population sizes for both species. The results demonstrate that while the mackerel fishery remains largely unaffected by the influence of mercury, optimal harvest and population of tuna depend greatly upon their biological resistance to mercury and consumers’ aversion to purchasing mercury-contaminated fish. When resistance to mercury is low, both tuna population and harvest decrease. When consumer aversion is high, harvest decreases and population increases. Increased mercury pollution exacerbates both effects. Due to lack of previous such studies and the paucity of empirical data, this research is both exploratory and qualitative in nature. Effective fisheries conservation and management requires understanding the strength of both fish resistance and consumer aversion to mercury. Future research should address the lack of empirical data, both biological and economic, as well as refine the above model in order to assist managers in appropriate consumer education and setting fisheries management goals that couple sustainability and public health.

Coal fired power plants generate approximately 45% of the electricity produced in the United States every year, and each year, over 100 million tons of coal ash are produced as a by-product of electricity generation. Coal ash is a solid waste made up principally of bottom ash, fly ash, and flue gas desulfurization materials. The chemical composition of coal ash varies depending on the feed coal source, combustion parameters, and the presence and type of air pollution control devices that remove contaminants from the flue gas into the solid waste stream. Although a significant portion of coal ash waste is recycled, the majority of coal ash is disposed in landfills and holding ponds. Coal ash impoundments have a long history of environmental degradation, which includes: contaminant leaching into groundwater, the discharge of contaminant-laden effluent into surface waters, and catastrophic impoundment failures and ash spills. Despite these known problems, coal ash is not considered a hazardous waste, and thus is not subject to stringent disposal requirements. The current coal ash management system is based on risk assessments of coal ash that do not include environmental parameters that have a profound impact on coal ash contaminant mobility, particularly for the toxic elements such as mercury, arsenic, and selenium. This dissertation research focused on the biogeochemical transformations of mercury, arsenic, and selenium associated with coal ash materials in an effort to: (1) define the key environmental parameters controlling mercury, arsenic, and selenium fate during disposal and ash spills; and (2) delineate the relationship between coal ash characteristics, environmental parameters, and leaching potential.

The impact of coal ash on mercury transformations in anaerobic systems was assessed using anaerobic sediment-ash microcosms to mimic an ash spill into a benthic aquatic system. Anaerobic sediments are the primary zones for the microbial conversion of inorganic mercury to methyl mercury (MeHg), a process that is mediated by anaerobic bacteria, particularly sulfate reducing bacteria (SRB). MeHg is a potent neurotoxin that biomagnifies up the aquatic food chain, presenting a human health risk-- especially to children and pregnant women. The results of the sediment-ash microcosm experiments indicated negligible net production of MeHg in microcosms with no ash and in microcosms amended with the low-sulfate/low-Hg ash. In contrast, microcosms amended with sulfate and mercury-rich ash showed increases in MeHg concentrations that were two to three times greater than control microcosms without ash. The enhancement MeHg production in the microcosms was likely due to large quantities of leachable sulfate that stimulated the activity of methylating bacteria. Overall, these results highlight the importance of considering both the geochemical conditions of the receiving environment and the chemical composition of the coal ash in assessing the MeHg potential of coal ash.

The hypothesis that sulfate-rich coal ash can change sediment microbial communities, enhancing MeHg production, was tested by analyzing coal ash impacts on the SRB community in the sediment-ash microcosms using Terminal Restriction Fragment Length Polymorphism (T-RFLP), Quantitative Polymerase Chain Reaction (q-PCR), and Reverse Transcription-qPCR (RT-qPCR). Coal ash did not appear to cause significant changes to the structure of the overall bacterial community, though results showed that it may have caused a decrease in the evenness for species distribution for both SRB and the overall microbial community. During the five-day incubation experiment, the coal ash had a temporary significant effect on SRB abundance during the first one to two days of the experiment and a more sustained effect on SRB activity. This stimulation of SRB population growth and activity also corresponded with increasing net MeHg production. Overall, results indicate that coal ash amendments do not cause large shifts in the overall microbial community or the SRB community, but results indicate that there are connections between SRB abundance/activity and MeHg production. More research is needed to determine how coal ash directly impacts Hg methylating microorganisms, which include diverse array of microorganisms outside of SRB.

The effect of aerobic and anaerobic conditions on arsenic and selenium leaching from coal ash in an ash spill scenario was also assessed using sediment-ash microcosms. The fate of arsenic and selenium associated with coal ash is of particular concern due to the leachability of these elements at neutral pH and their tendency to bioaccumulate in aquatic organisms. Both the redox speciation of arsenic and selenium, and the pH of the aquatic system, are known to influence leaching into the environment, yet current environmental risk assessments of coal ash focus on pH alone as the primary driving force for arsenic and selenium leaching from coal ash and do not take into account the effects of anaerobic conditions and microbial activity. In this research, total dissolved concentrations of arsenic and selenium, dissolved speciation of arsenic, and solid phase speciation of selenium were monitored to determine the biogeochemical transformations and leaching of arsenic and selenium under differing redox conditions. The results from the sediment-ash microcosm studies showed that redox potential was the major determinant of arsenic and selenium mobility in the microcosm systems with greater arsenic leaching occurring in anaerobic microcosms and greater selenium leaching occurring in aerobic microcosms. Furthermore, the experiments provided clues to how coal ash influences the geochemistry of the benthic environment and how these influences affect the speciation and longer term solubility of arsenic and selenium.

Finally, experiments were conducted to determine how differing CaO, SO3, and Fe2O3 concentrations in coal ash affect the release of arsenic and selenium from sediment-ash mixtures in a simulated ash spill environment. Aerobic and anaerobic sediment-ash microcosms were constructed to mimic an ash spill into a benthic aquatic system, and a variety of coal ash materials were tested as amendments, including seven fly ashes, one lime-treated fly ash sample, and two FGD samples. Results showed that, in most cases, the sediment in the microcosm buffered the system at neutral, which counteracted leaching impacts of differing CaO and SO3 concentrations in the microcosms. Regardless of ash material, leaching of selenium was greater under aerobic conditions and was correlated with the total selenium content of the microcosm. Maximum leaching of arsenic occurred in anaerobic microcosms for some ash materials and in aerobic microcosms for other materials, suggesting that ash material chemistry played a significant role in controlling arsenic mobility. In both aerobic and anaerobic microcosms, dissolved arsenic concentration was correlated with total arsenic content of the ash material and in anaerobic microcosms, dissolved arsenic concentrations also correlated with the total iron content of the ash material. Overall, the results of these experiments showed that arsenic and selenium release under environmentally relevant conditions cannot be predicted by the CaO and SO3 content of the ash material. Rather, the total arsenic, total selenium content, and total iron content of the ash material are good predictors of the worst case environmental leaching scenario.

These investigations illuminated two major conclusions: (1) microbial activity and differing redox conditions are key in determining the impact of coal ash on the environment and in determining the mobility of coal ash contaminants, and (2) coal ash characteristics, such as sulfate and iron content, can change the redox chemistry and microbial activity of the surrounding environment, further influencing the fate of ash contaminants. This work will be useful in designing a framework that accurately predicts the leaching potential of ash contaminants under environmentally relevant conditions. The results will also be helpful in developing treatment technologies for ash impoundment effluent, guiding decisions on ash pond closure and remediation, and in designing long-term monitoring plans and remediation strategies for ash-impacted sites.

The properties of nanoparticle sensors intended for real- time monitoring of low concentration of elemental mercury (Hg) vapor and volatile organic compounds (VOCs) are presented and discussed. This sensor for mercury vapors is composed of gold (Au) nanoparticles on single-walled carbon nanotubes (SWNTs) networks. Surface topography was determined by scanning electron microscopy (SEM). The electrical resistance of Au-SWNTs networks drastically increased upon exposure to mercury vapor. The experiment result shows that higher deposition amounts of Au nanoparticles on SWNTs lead to higher sensing responses. A detection limit of this senor to vapor mercury concentrations in the parts-per billion (ppb) was seen. Response features of current mercury sensors are discussed concerning sensitivity, reproducibility and regeneration at room temperature (25°C).

Nanosensors made of conducting polypyrrole (PPY) and tin dioxide (SnO2) on SWNTs were tested for the detection of volatile organics such as benzene, methyl ethyl ketone (MEK), hexane and xylene. The greater sensitivity of these two sensors to lower analytes concentrations compared to previous research studies was demonstrated. Experiments were conducted at room temperature, and the response was shown to be fast and highly sensitive to low concentration of VOCs. Using PPY and SnO2 sensors in a sensor array can identify polar and nonpolar analytes. Sensing mechanisms of these two sensors to analytes are discussed in this thesis.

Further work to improve the sensors that were tested was identified. The main challenge of this sensor is that the response and regeneration time is relatively slow at room temperature, especially for Au nanoparticle sensors. Also, with respect to PPY and SnO2 nanosensors, a high reproducibility in the making of sensors is desired. This improvement can help PPY and SnO2 sensors to have consistency. Finally, since nanosensors that can detect VOCs are not very specific, array sensing and numerical methods that can be used to quantify individual compounds in mixture from nanosensors array data are needed.

This study examined developmental toxicity of different mercury compounds, including some used in traditional medicines. Medaka (Oryzias latipes) embryos were exposed to 0.001-10 µM concentrations of MeHg, HgCl2, α-HgS (Zhu Sha), and β-HgS (Zuotai) from stage 10 (6-7 hpf) to 10 days post fertilization (dpf). Of the forms of mercury in this study, the organic form (MeHg) proved the most toxic followed by inorganic mercury (HgCl2), both producing embryo developmental toxicity. Altered phenotypes included pericardial edema with elongated or tube heart, reduction of eye pigmentation, and failure of swim bladder inflation. Both α-HgS and β-HgS were less toxic than MeHg and HgCl2. Total RNA was extracted from survivors three days after exposure to MeHg (0.1 µM), HgCl2 (1 µM), α-HgS (10 µM), or β-HgS (10 µM) to examine toxicity-related gene expression. MeHg and HgCl2 markedly induced metallothionein (MT) and heme oxygenase-1 (Ho-1), while α-HgS and β-HgS failed to induce either gene. Chemical forms of mercury compounds proved to be a major determinant in their developmental toxicity.

Mercury (Hg) is a globally circulating heavy metal released through both natural and anthropogenic sources. The largest anthropogenic source of mercury to the global atmosphere is artisanal and small-scale gold mining (ASGM). During the ASGM process, miners add elemental mercury to large quantities of sediment or soil in order to create gold-mercury amalgams that separate alluvial gold from the remaining geological host material. Miners then heat the amalgam using a blowtorch or similar device to separate the mercury and gold, exposing themselves to mercury vapor and releasing mercury to the environment. Following amalgam heating, mercury can deposit into aquatic ecosystems. There, anaerobic microorganisms can convert mercury to methylmercury (MeHg), a potent neurotoxin that rapidly accumulates in aquatic food webs. A high concentration of MeHg in fish poses serious human health risks, especially to pregnant women and children.

In Peru’s Region of Madre de Dios (MDD), mercury use for ASGM is widespread due to increasing global demand for gold. This region in the tropical Amazon is one of the world’s most biodiverse ecosystems and home to more than 150,000 Indigenous and non-Indigenous people, 40% of whom live below the poverty level. Recently, people living in the region have become more aware of negative impacts of Hg pollution through popular press. However, there is lack of controlled scientific studies to examine the environmental impacts of Hg from ASGM and subsequent exposures to surrounding communities.

This dissertation addresses four questions in order to better understand how mercury from ASGM impacts environmental health in Madre de Dios: (1) How is mercury distributed along the Madre de Dios River in areas of active ASGM activity, and what is the risk for mercury exposure to downstream communities? (2) How does land use change associated with ASGM activity affect soil-mediated mercury transport in the Colorado River, Madre de Dios, Peru? (3) Can sulfurized carbon be manufactured in a feasible way for developing countries and used to capture mercury during ASGM amalgam burning? (4) What is the mercury methylation potential of easy-to-manufacture spent, sulfurized carbon sorbents?

Despite significant information on the direct health impacts of mercury to ASGM miners, the impact of mercury contamination on downstream communities has not been well characterized, particularly in Madre de Dios. In this area, ASGM has increased significantly since 2000 and has led to substantial political and social controversy. The second chapter of this dissertation examines the spatial distribution and transport of mercury through the Madre de Dios River with distance from ASGM activity. It also characterizes risks for dietary mercury exposure to local residents who depend on fish from the river. River sediment, suspended solids from the water column, and fish samples were collected in 2013 at 62 sites near 17 communities over a 560 km stretch of the Madre de Dios River and its major tributaries. In areas downstream of know ASGM activity, mercury concentrations in sediment, suspended solids and fish within the Madre de Dios River were elevated relative to locations upstream of mining. Fish tissue mercury concentrations were observed at levels representing a public health threat, with greater than one-third of carnivorous fish exceeding the international health standard of 0.5 mg/kg. This research demonstrates that communities located hundreds of kilometers downstream of ASGM activity, including children and indigenous populations who may not be involved in mining, are at risk of dietary mercury exposure that exceed acceptable body burdens.

This research involved extensive field sampling in an active mining region and indicated suspended particulate transport may be an important source of mercury from mining areas to downstream communities. Chapter three of this research focused on understanding how land use changes can influence soil and sediment transport from mining regions. Within the MDD, a large portion of mining in concentrated within the Colorado River watershed. In the Colorado River watershed, mining and deforestation have increased dramatically since the 1980s, largely concentrated in the Puquiri subwatershed. Field sampling in Feb 2015 identified a strong correlation between Hg and suspended solids concentrations, with especially high suspended solids concentrations downstream of ASGM activity. This supported the hypothesis that Mercury transport in this region is facilitated by soil mobilization and runoff. In order to understand how ASGM activity in the Puquiri affects sediment mobilization from the watershed over time, we employed a watershed-scale soil mobilization model using satellite imagery from 1986 to 2014. The model estimated that soil mobilization in the Colorado River watershed increased by 2.5 times during the time period, and increased by six times in the Puquiri subwatershed, leading to between 10 and 60 kg of mercury mobilized in 2014. If deforestation continues at its current exponential rate through 2030, soil and heavy metal mobilization may increase by five times. This research shows that deforestation associated with ASGM in the Colorado River watershed can exacerbate soil mobilization and mercury contamination. While the impacts of mercury and deforestation are often considered separately, here we studied how deforestation associated with ASGM in the Madre de Dios region can significantly increase soil mobilization and mercury transport to downstream communities.

With a substantial portion of mercury releases coming from a non-industrialized process in developing countries, low-cost and low-tech mercury capture is becoming increasingly necessary. While impregnated activated carbon sorbents are well studied for mercury-capture in developed countries and large industrialized settings, there exist few suitable low-cost alternatives for mercury capture from artisanal and small-scale gold mining (ASGM) in developing countries. Chapter four sought to develop an easy-to-manufacture carbon sorbent using elemental sulfur and activated carbon or hardwood-based biochar for potential use during ASGM gold-amalgam heating. Consumer-grade sulfur powder was melted on granular activated carbon or hardwood biochar in a process feasible for a cook stove setting. Activated carbon and biochar were successfully sulfurized to more than 5% sulfur by weight using powdered, elemental sulfur. The sorbent products were tested for elemental mercury sorption from an air gas stream at room temperature. The sulfurized activated carbon achieved higher elemental mercury adsorption capacity in air stream (500 μg Hg m-3, 2 L min-2) relative to unsulfurized activated carbon and sulfurized biochar. Sorption isotherms were used to examine the sorption mechanism, and indicated that likely a pseudo first order reaction was occurring. This research provides a possible option for mercury control by modifying established mercury capture technologies to be easy to manufacture, locally available, and less hazardous to produce.

In Chapter 5 of this research, the sulfurized sorbents were examined further to understand methylation potential in sediment slurries. Anaerobic sediment slurries were constructed to examine methylmercury (MeHg) production of spent sorbents. Five sorbent types with approximately 10 mg/kg Hg each were added to slurries at 5 % by mass. Dissolved mercury was used as a control to simulate atmospheric deposition or highly reactive mercury. After a 5 d incubation at room temperature, MeHg production was ten times greater with low-technology sulfurized sorbents as compared to activated carbon or biochar alone. Sulfurized sorbents leached significantly more mercury than their non-sulfurized counterparts during desorption experiments and led to greater dissolved mercury concentrations. This research shows that low-cost mercury-contaminated sorbents can have unintended consequences with increased MeHg production and potential for more harm to local communities than atmospheric release.

Mercury releases from ASGM are expected to grow, leading to higher concentrations of mercury in the atmosphere that may affect ecosystems throughout the globe. Understanding the importance of mercury from ASGM to toxicity and accumulation requires in depth research on mercury transformations and MeHg production associated with ASGM. This research examines mercury distribution and transport from ASGM active regions. It identifies that deforestation, erosion, and particulate transport play important roles in overall mercury transport, leading to hazardous mercury concentrations downstream of ASGM activity. Effective point-of-use mercury capture technologies would dramatically decrease the mass of mercury released to the environment. The final chapters of this research serve as a proof of concept for using sulfurized activated carbon for mercury capture in developing countries.

Our research team has built strong relationships with several governmental and non-governmental organizations in Peru who will aid in distributing information. This research will provide invaluable environmental health information to residents, inform political intervention, and reveal a new potential avenue for low-cost mercury control.

Non-essential trace metals (lead, mercury, arsenic, and cadmium) are ubiquitous in our environment and have overlapping routes of exposure, yet mixed trace metal exposures are rarely considered in epidemiological studies. Instead, research often follows a single research question that focuses on a single trace metal of concern and does not incorporate potential co-exposures. The published literature of artisanal small-scale mining in the Amazon is a prime example as it has predominantly focused on mercury exposure, due to its use in the mining process. Once exposures of concern are identified, further studies evaluate health outcomes; however, the health effects cannot be accurately determined without accounting for co-exposures. This verification is becoming more important as there is a growing recognition that mixed trace metal exposures are more common than previously believed.To address the prevalence of mixed trace metal exposures and their health effects in the Peruvian Amazon region of Madre de Dios, I use epidemiological data from the COhorte de NAcimiento de MAdre de Dios (CONAMAD) birth cohort study (2018-Present), and two cross-sectional epidemiological studies (Amarakaeri Communal Reserve study (ACR, 2015), and Etiology and Toxic Metals study (EATM, 2018)). CONAMAD collected survey data along with maternal and cord blood samples at birth, which were processed for minerals and trace metals. The cross-sectional studies collected venous blood for trace metal analysis and hair samples for total hair mercury. Blood samples from the ACR were also processed for amino acids. In-depth demographic and health survey data were collected in all three studies. Structural equation models and random mixed effect models were used to evaluate research questions.The cross-sectional studies demonstrate a high correlation of lead and mercury exposure in communities that rely on wild fish and wild game as protein sources, which is prevalent throughout the Amazon. Consuming a meal of wild game resulted in an estimated lead dose of 500 µg, with those who eat wild game (Yes/No) associated with 1.41 µg/dL (95% CI: 1.20 – 1.70) higher blood lead levels compared to those who do not. This furthers the notion that mixed exposures are likely more common than previously believed. Mixed exposures target the same toxicological pathway, which may lead to synergistic or antagonistic effects. My research found that lead disrupts the arginine pathway and is associated with increased blood pressure. Mercury exposure was a modifier of the arginine pathway, with high blood mercury levels changing the effect of global arginine bioavailability from 17.16 (95% CI: 9.09 – 25.84) to -14.17 (95% CI: -31.88 - -0.33) on systolic blood pressure. Interestingly, mercury was not directly associated with the arginine pathway. Results from the birth cohort demonstrate the importance of nutrition and prenatal care for fetal development, which had a large positive effect on birthweight and gestational age. However, even low maternal lead exposure had detrimental effects on fetal health. A 1% increase in maternal blood lead was associated with a shorter gestational age of 0.05 days (β: -0.75, 95% CI: -1.51 - -0.13), even with the CONAMAD birth cohort having lower blood lead levels than other birth cohorts. There is a need for an integrated approach of nutritional and exposure assessments to better understand neonatal health outcomes.

An increasingly large fraction of Earth’s surface has been reshaped and contaminated by humans, leading experts to suggest we’ve entered into a new geologic epoch – the Anthropocene. Nowhere are these changes more obvious than in mining-impacted landscapes. Mining reshapes landscapes, liberates trace elements, and alters the fate, transport, and transformation of trace elements. In this dissertation, I examine the extent to which mining both alters landscape features and mobilizes trace elements, and how these paired changes together determine the bioavailability of toxic trace elements. Specifically, in this dissertation, I focus on the mobilization and transformation of selenium (Se) and mercury (Hg) from mountaintop mining (MTM) in West Virginia; the biogeochemical interactions between Se and Hg in ecosystems and organisms; and the fate of Hg derived from artisanal and small-scale gold mining (ASGM) in Senegal and Peru.

In chapter 2, I examine the fate of Hg and Se from MTM of coal. Coal is naturally enriched in trace elements, including Hg and Se. Alkaline mine drainage from MTM – the dominant form of surface coal mining in Appalachia, USA – releases large quantities of Se into streams draining mined catchments, resulting in elevated bioaccumulation of Se in aquatic and riparian organisms. Yet, the release of Hg into these streams from MTM has not yet been studied. I measured total Hg, methyl Hg (MeHg), and Se in stream water, sediment, biofilm, cranefly larvae, and riparian spiders in alkaline streams (pH range: 6.9-8.4) across a mining gradient (0-98% watershed mined) in central Appalachia. Hg concentrations ranged from below detection limit (BDL)-6.9 ng/L in unfiltered water, BDL-0.05 μg/g in bulk sediment, 0.016-0.098 μg/g in biofilm, 0.038-0.11 μg/g in cranefly larvae, and 0.046-0.25 μg/g in riparian spiders. In contrast to Se, I found that Hg concentrations in all environmental compartments were not related to the proportion of the watershed mined, suggesting that Hg is not being released from, nor bioaccumulating within, MTM-VF watersheds. I also did not find clear evidence for a reduction in Hg methylation or bioaccumulation under elevated Se concentrations: water, sediment, biofilm, and riparian spiders exhibited no relationship between Hg and Se; only cranefly larvae exhibited a negative relationship (p=0.0002, r2=0.42). I suggest that the type of surface mining matrix rock, with resultant alkaline or acid mine drainage, is important for the speciation, mobility, and bioaccumulation of trace elements within watersheds affected by mining activities.

In chapter 3, I examine evidence for an interaction between Hg and Se. Hg is a pervasive environmental pollutant and contaminant of concern for both people and wildlife that has been a focus of environmental remediation efforts for decades. A growing body of literature has motivated calls for revising Hg consumption advisories to co-consider Se levels in seafood and implies that remediating aquatic ecosystems with ecosystem-scale Se additions could be a robust solution to Hg contamination. Provided that elevated Se concentrations are also known toxicological threats to aquatic animals, I performed a literature search to evaluate the strength of evidence supporting three assertions underpinning the ameliorating benefits of Se: (1) dietary Se reduces MeHg toxicity in consumers; (2) environmental Se reduces Hg bioaccumulation and biomagnification in aquatic food webs; and (3) Se inhibits Hg bioavailability to, and/or MeHg production by, microbial communities. Limited or ambiguous support for each criterion indicates that many scientific uncertainties and gaps remain regarding Se mediation of Hg behavior and toxicity in abiotic and biotic compartments. Significantly more information is needed to provide a strong scientific basis for modifying current fish consumption advisories on the basis of Se:Hg ratios or for applying Se amendments to remediate Hg-contaminated ecosystems.

In chapter 4, I examine evidence for Hg and Se interaction at the base of the food web. Hg, a potent neurotoxin, can biomagnify through food webs once converted into MeHg. Some studies have found that Se exposure may reduce MeHg bioaccumulation and toxicity, though this pattern is not universal. Se itself can also be toxic at elevated levels. We experimentally manipulated the relative concentrations of dietary MeHg and Se (as selenomethionine [SeMet]) for an aquatic grazer (the mayfly, Neocloeon triangulifer) and its food source (diatoms). Under low MeHg treatment (0.2 ng/L), diatoms exhibited a quadratic pattern, with decreasing diatom MeHg concentration up to 2.0 g Se/L and increasing MeHg accumulation at higher SeMet concentrations. Under high MeHg treatment (2 ng/L), SeMet concentrations had no effect on diatom MeHg concentrations. Mayfly MeHg concentrations and biomagnification factors (concentration of MeHg in mayflies: concentration of MeHg in diatoms) declined with SeMet addition only in the high MeHg treatment. Mayfly biomagnification factors decreased from 5.3 to 3.3 in the high MeHg treatment, while the biomagnification factor was constant with an average of 4.9 in the low MeHg treatment. The benefit of reduced MeHg biomagnification was offset by non-lethal effects and high mortality associated with ‘protective’ levels of SeMet exposure. Mayfly larvae escape behavior (i.e., startle response) was greatly reduced at early exposure days. Larvae took nearly twice as long for all to metamorphose to adults at high Se concentrations. The minimum number of days to emergence did not differ by SeMet exposure, with an average of 13 days. We measured an LC50SeMet for mayflies of 3.9 μg Se/L, with complete mortality at concentrations ≥6.0 μg Se/L. High reproductive mortality occurred at elevated SeMet exposures, with only 0-18% emergence at ≥4.12 g Se/L. Collectively our results suggest that while there is some evidence that Se can reduce MeHg accumulation at the base of the food web at specific exposure levels of SeMet and MeHg, Se is also toxic to mayflies and could lead to negative effects that extend across ecosystem boundaries.

In chapter 5, I examine the fate of total and MeHg from ASGM in Senegal. The largest source of global Hg anthropogenic inputs to the environment is derived from ASGM activities in developing countries. While our understanding of global Hg emissions from ASGM is growing, there is limited empirical documentation about the levels of total Hg (THg) and MeHg contamination near ASGM sites. I measured THg and MeHg concentrations in soil (n=119) sediment (n=22), and water (n=25) from four ASGM villages and one non-ASGM reference village in Senegal, West Africa with active ASGM. Nearly all samples had THg and MeHg concentrations that exceeded the reference village concentrations and USEPA regulatory standards. The highest median THg concentrations were found in huts where Hg-gold amalgams were burned (7.5 μg/g), while the highest median MeHg concentrations and percent Hg as MeHg were found in river sediments (4.2 ng/g, 0.41%). Median river water concentrations of THg and MeHg were also elevated compared to values at the reference site (22 ng THg/L, 0.037 ng MeHg/L in ASGM sites). This study provides direct evidence that Hg from ASGM is entering both the terrestrial and aquatic ecosystems where it is converted to the neurotoxic and bioavailable form of MeHg in soils, sediment, and water.

In chapter 6, I examine pathways of Hg deposition and storage from ASGM in the Peruvian Amazon. Hg emissions from ASGM now exceed coal combustion as the largest global source of Hg to the atmosphere and are being released into some of the most biodiverse ecosystems on Earth. Hg, following microbial conversion to MeHg, is a potent neurotoxin with deleterious impacts on people and wildlife. However, while we know ASGM is an important source of Hg to the atmosphere, we know very little about the fate of this source of Hg. Here, I examine Hg deposition and storage in the Peruvian Amazon by analyzing THg and MeHg in atmospheric, precipitation, leaf, and soil samples from remote and mining-impacted areas. I found that intact forests in the Peruvian Amazon near ASGM receive extremely high inputs of Hg in throughfall (71 µg m-2 yr-1) and litterfall (66 μg m-2 yr-1) and have accumulated significant quantities of soil Hg (9100 μg Hg m-2 within the top five cm). My findings show for the first time that intact forests near ASGM are intercepting high levels of Hg deposition, and that songbirds inhabiting these forests have elevated levels of mercury. Our findings raise important questions about how mercury pollution may constrain modern and future conservation efforts in these ecosystems.

In chapter 7, I examine the combined effects of landscape change and Hg loading from ASGM in the Peruvian Amazon. ASGM is the largest global source of anthropogenic Hg emissions. However, little is known about how effectively Hg released from ASGM is converted into the bioavailable form of MeHg in ASGM-altered landscapes. Through examination of ASGM-impacted river basins in Peru, I show that lake area in heavily mined watersheds has increased by 670% between 1985 and 2018, and that lakes in this area convert Hg into MeHg at net rates 5-7 times greater than rivers. These results suggest that synergistic increases in lake area and Hg loading associated with ASGM are significantly increasing exposure risk for people and wildlife. Similarly dramatic increases in lake area in other ASGM hotspots suggest that ‘hydroscape’ (hydrological landscape) alteration is an important and previously unrecognized component of Hg risk from ASGM.

In chapter 8, I develop several of the emergent themes that connect the distinct elements of this dissertation research. Here I develop a conceptual framework for merging perspectives from geochemistry, landscape ecology, and toxicology to understand the movement, fate and impact of toxic trace elements in the natural world. In the Anthropocene, we typically study the increasing mobilization of toxic trace elements and the changing land cover of our planet as separate issues. Yet the way we alter our landscapes plays a critical role in the likelihood that any particular place will retain, sequester, and alter the transport and bioavailability of trace elements to people and wildlife. The goal of this chapter is to provide examples that demonstrate that the risk of contaminant exposure is not merely a function of loading, but arises through interactions among loading, landscape capture, and biological transformation, all of which are simultaneously altered by human activities. I posit that successful prevention and mitigation of trace element toxicity requires a merging of these diverse perspectives and traditions.

Many factors such as poverty, ineffective institutions and environmental regulations may prevent developing countries from managing how natural resources are extracted to meet a strong market demand. Extraction for some resources has reached such proportions that evidence is measurable from space. We present recent evidence of the global demand for a single commodity and the ecosystem destruction resulting from commodity extraction, recorded by satellites for one of the most biodiverse areas of the world. We find that since 2003, recent mining deforestation in Madre de Dios, Peru is increasing nonlinearly alongside a constant annual rate of increase in international gold price (∼18%/yr). We detect that the new pattern of mining deforestation (1915 ha/year, 2006-2009) is outpacing that of nearby settlement deforestation. We show that gold price is linked with exponential increases in Peruvian national mercury imports over time (R(2) = 0.93, p = 0.04, 2003-2009). Given the past rates of increase we predict that mercury imports may more than double for 2011 (∼500 t/year). Virtually all of Peru's mercury imports are used in artisanal gold mining. Much of the mining increase is unregulated/artisanal in nature, lacking environmental impact analysis or miner education. As a result, large quantities of mercury are being released into the atmosphere, sediments and waterways. Other developing countries endowed with gold deposits are likely experiencing similar environmental destruction in response to recent record high gold prices. The increasing availability of satellite imagery ought to evoke further studies linking economic variables with land use and cover changes on the ground.

In the United States, aquatic mercury contamination originates from point and non-point sources to watersheds. Here, we studied the contribution of mercury in urban runoff derived from historically contaminated soils and the subsequent production of methylmercury in a stream-wetland complex (Durham, North Carolina), the receiving water of this runoff. Our results demonstrated that the mercury originated from the leachate of grass-covered athletic fields. A fraction of mercury in this soil existed as phenylmercury, suggesting that mercurial anti-fungal compounds were historically applied to this soil. Further downstream in the anaerobic sediments of the stream-wetland complex, a fraction (up to 9%) of mercury was converted to methylmercury, the bioaccumulative form of the metal. Importantly, the concentrations of total mercury and methylmercury were reduced to background levels within the stream-wetland complex. Overall, this work provides an example of a legacy source of mercury that should be considered in urban watershed models and watershed management.

Mercury emissions from coal-fired power plants pose environmental and public healthconcerns in North Carolina. Once converted to methylmercury in aquatic environments,mercury compounds can bioaccumulate in fish and other species, including humans. Inhumans, mercury compounds can function as a neurotoxin to a fetus and impair neurological development of young children. Recent multi-pollutant strategies areexpected to reduce the amount of mercury emitted from coal-fired power plants in North Carolina. To test this assumption, this study examines the mercury capture of a similar piece of existing legislation, the Acid Rain Program. Holding other variables constant, this study finds that during the Phase II years of the Acid Rain Program, coal-fired power plants emitted on average 208 fewer pounds of mercury per year compared to the Phase Iyears. These results suggest multi-pollutant strategies can be an effective strategy to reduce mercury emissions.

The objectives of this study were to investigate trace elements in adobe houses and to characterize potential health risks from children’s exposure in Potosí, Bolivia. The city of Potosí sits at the base of the Cerro Rico Mountain, which has been mined heavily for its rich polymetallic deposits since the Spanish Colonial era in the 16th century, leaving a legacy of pollution that is not well understood. In this study, total trace elements were quantified in dirt floor, adobe brick, and surface dust samples from 49 houses. Mean concentrations of total mercury, lead, and arsenic in adobe bricks were significantly greater than concentrations in Sucre, Bolivia, a non-mining town used as a reference site, and exceeded US-based soil screening levels that are protective of human health. Adobe brick samples were further analyzed by simulated gastric fluid (GF) extraction, which approximates bioaccessibility. Mean GF extractable concentrations of mercury, arsenic, and lead were 0.841, 14.9, and 30.0 percent of the total concentration, respectively. Total and GF extractable concentrations of these elements were used to estimate exposure and potential health risks to one and six year old children following incidental ingestion of element enriched adobe brick particles. Although the majority of households have concentrations of total mercury and arsenic that represent a potential health risk, the percentage significantly decreases when GF extractable concentrations are considered. However, even when GF extractable lead is considered, the majority of the households have lead concentrations in adobe bricks that represent a potential health risk to children. This is the first study to quantify trace elements in adobe houses and the results show that the building materials in these houses are a source of exposure to potentially toxic trace elements in South American mining communities. Additional environmental sampling, biomonitoring, and exposure questionnaires are needed to fully characterize sources of exposure and to understand potential adverse health outcomes within the community.

Mercury contamination in aquatic ecosystems is a concern as anaerobic aquatic sediments are the primary regions of methylmercury production in freshwater and coastal regions. Methlymercury is a bioaccumulative neurotoxin, and human exposure to methylmercury can result in impaired functioning of the central nervous system and developmental disabilities in children. To minimize the risk of human exposure to methylmercury, it is important to be knowledgeable of the various sources which can supply mercury to aquatic ecosystems as well as have a complete understanding of the biogeochemical processes which are involved in methylmercury production in aquatic systems. In this dissertation work, both mercury biogeochemical speciation in anaerobic aquatic sediments and sources of mercury to aquatic systems were addressed.

The biogeochemical speciation of mercury is a critical factor which influences the fate and transformation of mercury in aquatic environments. In anaerobic sediments, mercury chemical speciation is controlled by reduced sulfur groups, such as inorganic sulfide and reduced sulfur moieties in dissolved organic matter (DOM). The formation of mercury sulfide nanoparticles through stabilization by dissolved organic matter (DOM) was investigated in precipitation studies using dynamic light scattering. Mercury sulfide nanoparticles (particle diameter < 100 nm) were stabilized through precipitation reactions that were kinetically hindered by DOM. To further investigate the interaction between DOM and metal sulfides, similar precipitation studies were performed using zinc sulfide and a number of DOM isolates (humic and fulvic acids) representing a range of DOM properties. The results of these experiments suggest that the mechanism of metal sulfide particle stabilization may be electrostatic or electrosteric, depending on the nature of the DOM molecule.

The mercury that is methylated in aquatic systems enters these environments via a number of sources, including atmospheric deposition, landscape runoff and other industrial and municipal activities. In two separate field studies, two potential sources of mercury to aquatic systems were investigated: landscape runoff and coal combustion products. The mercury loading to aquatic environments from these sources and their potential for transformation to methylmercury were investigated.

Landscape runoff from a Duke University campus catchment (Durham, NC) was identified as a source of mercury to a stream-wetland. The source of mercury to the runoff was likely from a `legacy' source of mercury; the historic application of mercury fungicide compounds to turf grass during the 20th century. Downstream of the point where the runoff was discharged to the stream-wetland, methylmercury concentrations were detected in stream sediments (up to 11% of total mercury), suggesting that this legacy mercury could be transformed to methylmercury.

The environmental impact of coal combustion products (CCPs) with respect to mercury and methylmercury was also investigated in a river system (Roane County, TN) that was inundated with fly ash and bottom ash from the Tennessee Valley Authority Kingston coal ash spill in 2008. Elevated total mercury and methylmercury sediment concentrations (relative to upstream sediments) were detected in regions impacted by the ash spill, and our biogeochemical data suggested that the ash may have stimulated methylmercury production in river sediments.

The results of this dissertation work address the formation of mercury sulfide (along with zinc sulfide) nanoparticles in anaerobic aquatic sediments. In the current mercury methylation paradigm, dissolved mercury species such as Hg(SH)02(aq) and HgS0(aq) are assumed to be the only mercury species that are available for methylation. The results of this dissertation work suggests that in previous studies, HgS0(aq) may have been mistaken as mercury sulfide nanoparticles which may be formed in under supersaturated conditions (with respect to HgS(s)) where DOM is present. Mercury sulfide nanoparticles are a mercury biogeochemical species that has been largely ignored in the research literature and whose role in the mercury biogeochemical cycle and in mercury methylation remains to be investigated.

This dissertation work also identifies potential sources of mercury to aquatic systems, namely, landscape runoff and CCPs. Atmospheric deposition is currently considered to be the major source of mercury to inland aquatic water bodies compared to sources such as landscape runoff and CCPs. However, in the watershed studied in this dissertation, landscape runoff was identified as a larger source of mercury than atmospheric deposition, suggesting that these so-called `minor' sources may actually be major sources of mercury to watersheds depending on land usage, and should be considered in watershed models. Furthermore, the environmental hazards of mercury-associated with CCPs has typically been determined through leaching experiments, such as the Toxicity Characteristic Leaching Procedure (TCLP), which are not representative of environmental conditions and do not predict that CCPs may influence mercury methylation in aquatic sediments. Thus, in this dissertation work, we suggest that leaching protocols such as the TCLP should be re-evaluated.

Overall, this dissertation work will be useful in future studies examining mercury speciation and bioavailability to methylating bacteria in aquatic sediments, and the formation of metal sulfide nanoparticles in aquatic systems. Additionally, data on sources of mercury will be useful in developing policies for the regulation of these sources and in assessing the risk to human health from mercury methylation.