I am a watershed biogeochemist who works at a variety of scales, from microsite to landscape to global. My research focuses on the transport, fate, and consequences of environmental contaminants. My approach recognizes humans as part of the ecosystem and investigates the impact of ecosystem perturbations on humans and wildlife.
Current Projects
Implications of Sulfur Loading from Vineyards and Agricultural Activity
With the implementation of the Clean Air Act, deposition of sulfur (S) as acid deposition has drastically reduced. Today, the largest source of S is from agricultural activity, where S is added as a pesticide/fungicide, fertilizer, soil conditioner, and nutrient carrier. For example, vineyards in Napa Valley, California use S to prevent powdery mildew infection. But the high levels of S are leaching into nearby aquatic environments, which also have elevated mercury (Hg) concentrations from historical gold mining. As a postdoc on a USDA-funded project and in partnership with local vineyards, I am examining whether S inputs from vineyards are impacting the rate at which inorganic Hg is converted by microbial activity into the bioavailable and toxic form of Hg. I am also examining how the global S cycle has changed over the past few decades, the implications this has for plant nutritional needs as well as environmental consequences, and how we can better manage S for a sustainable future.
Past Projects
Looking Below the Surface: The Role of Redox Dynamics in Deep Carbon and Greenhouse Gas Dynamics
The largest flux of C from terrestrial ecosystems originates from soil respiration, which in turn is a function of soil moisture and microbial processes. As a postdoc on the NSF Critical Zone Collaborative Network, I explored how soil depth and subsurface hydrology influence greenhouse gas dynamics. I led the team in the use of continuous in situ oxygen (O2), carbon dioxide (CO2), and moisture sensors across five different ecosystem types across the United States to understand local and global patterns in C processing. I also measured the flux of iron (Fe), nitrogen (N), phosphorus (P), and other key drivers of microbial function and diversity across soil depth and time. My goal was to understand how fluctuating metabolic and redox conditions impact the release of greenhouse gases, as well as how the biogeochemical processes interact and relate to soil microbial communities.
Image from CZO Archive
Transport and Fate of Mercury from Illegal Artisanal and Small-Scale Gold Mining in Senegal and Peru
In the process of illegal artisanal and small-scale gold mining (ASGM), mercury is utilized to isolate gold from sediments. After the formation of a mercury-gold amalgam, the mixture is heated to create pellets of gold, while the mercury is released into the atmosphere or deposited onto the landscape as mining waste. ASGM represents the largest source of global anthropogenic mercury emissions. I examined the fate and transport of mercury from ASGM in Senegal and Peru. In Senegal, we found that the bioavailable and toxic form of methylmercury can be elevated in terrestrial and riparian ecosystems near mining areas. In Peru, I analyzed mercury concentrations in atmospheric deposition and found that the canopy of intact forests located near artisanal gold mining has the highest inputs of mercury ever reported globally. While this Hg is currently sequestered, erosion and deforestation pose an enormous risk of Hg contamination to nearby aquatic ecosystems. Aquatic ecosystems in Peru are also directly disturbed by gold mining, which simultaneously increases Hg loading and alters its processing, leading to a synergistic enhancement of Hg methylation in these mining-impacted landscapes.
Can Selenium Provide Protection from Mercury Bioaccumulation and Toxicity?
Mercury is a potent neurotoxin that can impact both humans and wildlife. Selenium, while an essential nutrient, can cause deformities in organisms when present in high concentrations. However, some studies suggest that when both mercury and selenium are present at elevated levels, selenium can reduce mercury methylation, bioaccumulation, and/or toxicity. To better understand the interaction between these two trace elements, I led a literature synthesis to evaluate studies examining this relationship. Given the toxicity of the individual elements to aquatic organisms and their potential synergistic effects, we suggest a cautionary approach when considering Se-Hg interactions in the context of environmental management. I also conducted a laboratory experiment using diatoms and mayfly larvae to investigate whether the presence of selenium can reduce mercury uptake at the base of the food web. We found that Se moderation of Hg bioaccumulation is dependent upon the magnitude of methyl Hg and Se exposure, though Se is highly toxic to mayflies.
Mercury Transport and Its Interaction with Selenium In Mountaintop Mined Watersheds
In the process of mountaintop mining (MTM), the tops of mountains are removed to expose underlying coal seams. This represents the largest form of land use change in the central Appalachian region and is a major source of contaminants to streams draining these watersheds. While numerous trace elements have previously been found in elevated concentrations as a result of MTM, no data currently exist for mercury. I examined mercury concentrations in water, sediment, biofilm, cranefly larvae, and riparian spiders across a gradient of mining. Although we found high concentrations of Se in mining-impacted streams and biota, Hg concentrations were very low in all sample types (water, sediment, biofilm, cranefly larvae, riparian spiders), suggesting that trace element fate and transport in mountaintop mined watersheds is dependent upon geochemical constraints (i.e., alkalinity) that differentially impact the solubility and mobility of individual elements. Additionally, I assessed the relationship between mercury and selenium ratios in these compartments to determine if the presence of selenium reduces mercury methylation and/or bioaccumulation. We found that the relationship between Hg and Se is inconsistent across taxa.
Mercury Deposition in the Montane Landscape of the Adirondacks, NY
For my Master's thesis, I analyzed patterns in total mercury and methylmercury across an elevational gradient at Whiteface Mountain in the Adirondacks. I analyzed inputs of total and methylmercury from throughfall, wet precipitation, cloudwater, and litterfall, as well as stocks of total and methylmercury in the soil. We found that soil methylmercury concentrations were greatest in the mid-elevation coniferous forests, consistent with trends observed in songbird mercury concentrations. We attribute the pattern in soil methylmercury concentration to internal soil processing of ionic mercury, which is likely derived from atmospheric inputs. This suggests that reducing mercury emissions could impact mercury concentrations in terrestrial ecosystems and terrestrial avian species.
Long-Term Mercury Dynamics in an Adirondack Watershed, NY
Mercury is a potent neurotoxin that can bioaccumulate across the food chain to reach high levels in some fish species. Mercury emissions have been declining in the United States, but less is known about concurrent long-term changes in mercury concentrations in remote aquatic systems. In particular, the Adirondack Mountains of NY have been known as a biological hotspot for mercury deposition and bioaccumulation. I examined how the Arbutus Lake watershed in the Adirondacks is responding to decreases in mercury deposition. Using litterfall and wet deposition data, as well as water samples collected from the inlet and outlet of Arbutus Lake from 2004-2015, we found that litter mercury inputs are decreasing, as are lake total mercury and methylmercury fluxes. These trends are apparent despite increases in dissolved organic carbon and shows the potential for ecosystem recovery.
Nutrient Dynamics in Adirondack Lakes Recovering from Acidification
As acid deposition decreases, aquatic ecosystems in the Adirondacks, NY are recovering. There is a concurrent expectation that nitrogen:phosphorus ratios in lakes could also decline, thereby decreasing phosphorus limitation of phytoplankton. I investigated changes in these nutrient ratios in 52 Adirondack lakes from 2008-2012. We found that changes in nutrient dynamics was dependent upon the different hydrologic flowpaths of seepage (lacking a surface outlet) and drainage (containing a surface outlet) lakes. As nitrate concentrations decline in Adirondack Lakes, seepage lakes may become nitrogen-limited, while drainage lakes may become less phosphorus-limited; these changes could result in increased lake productivity.
The Uptake of Pyrene Molecules onto Sea Spray Aerosol Particles
Aerosol particles are important to the regulation of climate, heterogeneous chemical reactions, and human health. Polycyclic aromatic hydrocarbons (PAHs), produced predominantly from anthropogenic sources, are often found adsorbed to these particles. To better understand the adsorption kinetics, I conducted studies on a simple PAH molecule - pyrene. I modeled the uptake of pyrene molecules onto sea spray constituents (NaCl, NaNO3, and MgCl2) using a Nd-YAG laser ionization scheme to gain a molecular and kinetic understanding of aerosol surface morphology and the factors controlling gas-particle partitioning. We found that a free energy barrier controls the uptake rate.
