- Low-temperature Geochemistry
My research focuses on quantifying and understanding low-temperature geochemical processes using sensitive major and trace element analytical techniques in conjunction with radiogenic and stable isotope measurements.
Specific interests include:
- Mercury photochemistry in freshwater environments
- Mercury emission from surfaces of waterbodies, plants, and sediments
- Fate and transport of toxic trace metals in groundwater
- Metal speciation and adsorption/desorption on natural and synthetic materials
- Watershed biogeoscience
- Mineral weathering and its impact on local and global cycles
- Biotic controls on metal transformations
- Use of GIS techniques to address geochemical problems
- Advancement of analytical methods in geochemistry.
Mercury emission from coastal wetlands
Mercury (Hg) is a globally occurring pollutant that bioaccumulates and persists in the environment. The global Hg cycle is highly dependant on air/water exchange, as it is one of the primary pathways to deliver Hg to the atmosphere. Although open water systems appear to be net sinks for Hg sequestration (e.g. Chesapeake Bay), nearshore wetland systems may be significant sources of Hg emission due to 1) biogenic release from plant leaves and 2) the increased quantity and quality of dissolved organic carbon. Because estuarine environments are naturally and anthropogenically enriched in Hg, we hypothesize that the evasion of Hg from contaminated wetlands (e.g. estuaries) may be a critically important and currently underestimated flux of Hg to the atmosphere. Two pathways exert primary control over the release of Hg° from estuarine environments: the abiotic reduction of Hg(II) to Hg°(aq) as moderated by complex interactions of UV radiation, DOC, salinity, and pH in the water column, and by the diffusive release of bacterially reduced Hg° from plant leaf surfaces during transpiration. Our objectives are to 1) determine the relative importance of these two different evasion pathways, 2) investigate the fundamental processes governing chemical interactions within each pathway, and 3) evaluate the net contribution of evasion from Hg contaminated wetlands to the global Hg budget. These objectives will be studied by experimentally and empirically testing hypothesized relationships between measured environmental parameters and Hg behavior in a wetland. We hope to improve our understanding of the ultimate fate and transport of Hg in estuarine environments, including possible predictions of responses to increasing UV-B radiation and environmental change. Especially useful to environmental managers will be what we learn about the relative pathways of Hg transport in contaminated wetland ecosystems, and what steps would be most/least appropriate for long-term management.
Arsenic behavior in a fractured bedrock aquifer
In the arsenic project, I am tackling a very practical subject of concern to our society: that of metals contamination in groundwater. Previously, arsenic contamination was thought to occur only in obviously impacted areas, such as economic mineralizations and pesticide manufacturing sites. My research has demonstrated that disseminated arsenic sulfide minerals associated with both pegmatites and hydrothermal systems heavily impact the New England states. In the laboratory, we've developed highly sensitive methods to measure arsenic at the sub-ppb level, including determination of As(III), As(V) and organic species.Using these techniques, we have closely studied the mechanisms and processes that control the concentration and speciation of arsenic in groundwater. Our results demonstrate that both saturation with the arsenic oxide mineral scorodite, and pH-dependant adsorption of arsenic on iron oxyhydroxides are the primary controlling mechanisms. We have carefully studied the source rocks,and arsenic appears to be concentrated along with other incompatible elements such as boron, during anatectic melting of calc-silicate metapelites and subsequent crystallization as sulfide minerals in late stage pegmatites. Our study breaks new ground, suggesting mechanisms that now help explain several arsenic problem areas across New England, and can contribute to a better understanding of arsenic mobility elsewhere. Overall, this project has contributed to the awakening in the United States of the recognition of national arsenic problem, and we are working now to assist in developing remediation technology for these issues.
This project is part of a larger overall program, supported by the NIEHS/EPA Superfund Basic Research Program, entitled: Toxic Metals in the Northeast: From Biological to Environmental Implications
Calcium supply in a forested ecosystem
This project addresses the long debated discrepancy between field calculated and lab measured weathering rates of silicate minerals. The watershed scale silicate weathering experiment is a facet of a larger program studying the depletion and replenishment of base cations in an acid-deposition impacted forested ecosystem. We have recently added base cations back to Hubbard Brook watershed-1 in the form of wollastonite, CaSiO3. Using strontium isotopes, we can carefully track the progress of strontium, and by inference, calcium through the ecosystem. The first aspect of this study is the study of the dissolution of the wollastonite in the stream channel itself. Using strontium isotope mixing relations within the stream, we can carefully and precisely determine the wollastonite dissolution rate. This dissolution rate is approximately one order of magnitude below the rates described in the literature from laboratory experiments. In parallel, we are currently running dissolution experiments with the same materials in the laboratory to directly compare with the data from the watershed. These studies will have an impact on the estimation of silicate weathering rates, and by extension, to hypotheses about silicate weathering controls on climate and replenishment of cations in soil exchange sites following acid deposition.
Some other projects I have worked on include:
- Analysis of acidic precipitation, including aerosols, fog/clouds, and rain on Mount Washington, NH.
- Development of laboratory automation technology related to liquid sample handling and nebulization for magnetic sector ICP-MS analysis.
- LabVIEW programming for laboratory applications including ion chromatography and online HPLC speciation techniques
- GIS techniques for presenting and evaluating regional groundwater geochemical data
- Tracing the origins of maple syrup using 13C and 206Pb/207Pb isotopes
I serve as the director of the Lehigh University Field Camp.