Browsing by Subject "East Fork Poplar Creek"

Mercury (Hg) is a toxic pollutant whose speciation will highly impact its fate and transformation in the environment. One understudied form is that of liquid elemental mercury (Hg0L), a dense non-aqueous phase liquid which, upon release to the environment, can persist in soil for decades. Over time, this residual Hg0L is gradually released to groundwater and eventually surface water, serving as a long-term source of contamination. Hg that reaches natural waters can eventually be transformed into methylmercury (MeHg), a powerful neurotoxin that can biomagnify in aquatic food chains and endanger human health. The presence of Hg0L in the environment brings up a number of interrelated topics relevant in Hg science, namely mobilization, reactivity, and exposure. Hg0L contamination exists worldwide due to the historic and current use of Hg0L in various industrial and mining practices. Many former Hg processing sites, such as chlor alkali plants, contain Hg-contaminated soils. Today, however, artisanal and small-scale gold mining (ASGM) is the primary utilizer of Hg0L. This largely unregulated, rudimentary mining process uses large quantities of Hg0L to separate and amalgamate gold from excavated soils. This gold-Hg amalgam is later heated to evaporate the Hg0L, resulting in large losses of Hg to the atmosphere. As a result of the ongoing and past use of Hg0L, threats of Hg exposure due to contaminated soil environments will remain prevalent. This dissertation addresses three questions in order to better understand Hg0L in the environment and reliable ways to assess human exposure to the metal: (1) How does Hg0L interact with sulfide, and what impact do these interactions have on Hg0L dissolution and mobility? (2) What is the Hg speciation in streambank soils downstream of a site with historical Hg0L soil contamination, and what is the potential for future mobility and transformation of Hg in these soils? (3) How does exposure to Hg0L impact the efficacy of hair total Hg (THg) as a proxy for MeHg exposure in ASGM communities? These questions are explored through the lens of two different sites: Madre de Dios, Peru, a region contaminated from rapidly growing ASGM activity, and East Fork Poplar Creek (EFPC), a superfund site with historical Hg0L contamination in Oak Ridge, TN. The spillage of Hg0L, with a density that is 13 times the density of liquid water, results in deep penetration of the metal into soils and entrapment of Hg0L droplets in soil pore spaces. Over time, this residual Hg0L can undergo various chemical and biological transformations that will ultimately impact its fate in the subsurface. Of particular interest is the corrosion of Hg0L via reaction with reduced inorganic sulfur species to form mercury sulfide (HgS), a process that enables long term sequestration of mercury in soils and generally reduces its mobility and bioavailability. The second chapter of this dissertation examines the natural corrosion of Hg0L in the presence of sulfide by quantifying rates of Hg release under different sulfide doses and aging environments. For droplets aged in ambient air, no differences in Hg release were observed among all sulfide doses. However, for droplets aged in the presence of a strong oxidant (H2O2) and reacted under low sulfide doses, we observed an increase in Hg release relative to droplets aged in air. However, the release of Hg from droplets aged in H2O2 was suppressed upon addition of adequate sulfide. These results suggest two critical factors dictate the corrosion of Hg0L in the presence of sulfide: surface oxidation of the droplet and sufficient sulfide dose. Surface oxidation controls the release of Hg into solution; without adequate oxidation, no Hg will be available to be sequestered through precipitation of HgS. However, in the case of an oxidized Hg0L surface, sufficient sulfide is needed to prevent large amounts of Hg from being released into solution. These results suggest that the mobilization of Hg0L will largely depend on aging conditions in the subsurface. Hg mobilized from the dissolution of Hg0L will likely be transported to other environmental compartments downstream. For sites located near alluvial systems, bank soils are one likely receptor of this mobilized Hg. The solid-phase speciation of Hg in these receiving streambanks are critical because erosion and flooding events may mobilize Hg into the adjacent waterbody and eventually be methylated. Therefore in Chapter 3 of this dissertation, we characterized mercury speciation in Hg-enriched soils (100-1100 mg/kg Hg) collected from the incised bank of the East Fork Poplar Creek (EFPC) in Oak Ridge, TN (USA). A combination of characterization techniques were used including scanning electron microscopy, X-ray absorption fine structure spectroscopy, and transmission electron spectroscopy. Altogether the data demonstrated the predominance of nanoparticulate HgS with crystal lattice defects in the streambank soils of this industrially impacted stream. The results of this work support further investigation of the impact of these nanocrystalline lattice defects on particle surface reactivity, including Hg dissolution rates and bioavailability at key microbial interfaces. Increased availability of Hg to methylators may give rise to hotspots of MeHg bioaccumulation. In order to identify and monitor human populations with elevated MeHg exposure, total mercury content (THg) in hair is used as a proxy for chronic MeHg exposure. However, while this biomarker may be useful for populations who are primarily exposed to Hg as dietary MeHg, recent studies have indicated that for those in mixed exposure communities such as ASGM areas, THg in hair may comprise a substantial proportion of inorganic mercury (iHg). With the enactment of the Minamata Convention on Mercury prompting renewed efforts for biomonitoring of vulnerable populations, the efficacy of hair THg as a proxy for MeHg exposures to populations needs to be evaluated for ASGM settings. Chapter four investigates the efficacy of hair THg as an indicator of MeHg exposure by quantifying both THg and MeHg contents in hair from a representative subset of participants in a large, population-level mercury exposure assessment near ASGM areas in Madre de Dios (MDD), Peru. We observed that 100% of hair samples contained MeHg, and 86% had levels reflective of dietary exposure, or non-occupational exposure. For a small subset of individuals living in mining towns (N = 15; ~20% of within mining sample), we observed the opposite result: hair THg mostly comprised of inorganic mercury. However, this subset did not influence population level trends; hair MeHg-THg correlations were high (r >0.7) for all communities, regardless of location or nativity. Our results support the use of hair THg for monitoring of MeHg exposure of populations in ASGM settings in addition to urine, the current recommended biomarker for ASGM communities. Altogether, this research seeks to improve our risk characterization of environments and populations impacted by soil Hg0L contamination by addressing gaps in our knowledge on Hg0L mobilization, downstream reactivity, and exposure evaluation. This work will contribute to our understanding of the formation and identification of biological Hg hotspots resulting from Hg0L contaminated sites. Additionally, our enhanced understanding of hair biomarkers will help countries with active ASGM sites develop monitoring programs to identify vulnerable populations and assess the effectiveness of programs designed to decrease environmental Hg release.