Date of Award

5-14-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Charles Driscoll

Abstract

The natural cycling of mercury (Hg) has been augmented by anthropogenic activities, including mining, waste incineration, and the combustion of fossil fuels. The subsequent enrichment of Hg in air, soils, and surface waters has caused concern, due to the adverse health effects of mercury on humans and wildlife. In the United States, policies like the Clean Air Act Amendments of 1990 and the Mercury and Air Toxics Rule have led to decreases in domestic emissions, but global emissions have increased in recent years and now dominate U.S. emissions. As a result, uncertainties remain regarding how quickly and to what extent changes in environmental Hg concentrations will occur. Such evaluations often rely on the availability of long-term monitoring data or a robust reference point for comparisons. In this dissertation, I utilize a series of publicly available long-term monitoring datasets to evaluate changes in Hg concentrations in air, precipitation, and soil. I present a formal analysis of soil Hg stocks in the top 1 m of soil of the conterminous United States (CONUS), which may be used as a future reference point for the evaluation of changes in national soil Hg reservoirs. Finally, I measured Hg in a time-series of archived organic soils from the Hubbard Brook Experimental Forest, NH. In Chapter 2 of this dissertation, I perform trend analyses on monitoring data of Hg in air and precipitation across CONUS. Monitoring data were obtained from the Mercury Deposition Network (MDN; precipitation Hg concentrations, wet deposition, precipitation volume), Atmospheric Mercury Network (AMNet; air Hg concentrations), and the National Emission Inventory (NEI; mercury emissions). Trends in precipitation mercury concentration and wet deposition were primarily found to be decreasing over time, though trends exhibited a distinct spatial pattern; most sites located in the eastern U.S. were decreasing, while sites in the western United States mostly included non-significant or increasing trends. Precipitation quantity increased at several sites, offsetting decreases in Hg concentration. Atmospheric fractions of Hg largely decreased across CONUS, particularly for gaseous oxidized Hg. Broadly, the changes in precipitation and atmospheric Hg matched the spatial patterns of decreased emissions. In Chapter 3, I utilize a large-scale US Geological Survey (USGS) soil survey to examine the distribution of Hg in the top meter of soil across CONUS. Soil Hg concentrations were found to be highest in surface soils, suggesting atmospheric inputs are the dominant Hg source to soils. Landcover exhibited a significant impact on soil Hg concentrations, with the highest concentrations being associated with forested, developed, and cropland landcover types. Conversely, barren and shrubland landcover types were found to be the lowest in Hg concentration. Ecoregion also had a significant effect on Hg concentration, with colder, wetter climates containing higher concentrations of Hg. Soil Hg pools were calculated for 11 layers, cumulatively representing the top 1 m of soil, and totaling 158 ± 2 Gg Hg for the conterminous U.S. Assessment of the provenance of soil Hg using bedrock titanium normalization suggests that 62%–95% of soil Hg is unexplained by parental sources. In Chapter 4 of this dissertation, I utilize archival soil samples from the Hubbard Brook Experimental Forest to examine trends in forest floor Hg spanning nearly 50 years. Hg concentrations were found to be significantly decreasing over time in both the Oie and Oa horizon at a rate of 1.6% and 0.8% per year, respectively. Only the Oe horizon was found to consistently differ in Hg content between paired sampling events, suggesting that this stage of soil organic matter decomposition was particularly important for the fate of Hg in soil. Overall, this work demonstrates that reductions in Hg emissions confer meaningful benefits to the environment in the form of decreasing Hg concentrations in air, precipitation, and soils.

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Open Access

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