Geologic sources of radium to municipal wells in Wisconsin
Radium commonly occurs in groundwater obtained from the Midwestern, Cambrian-Ordovician aquifer system at activities (a measure of concentration) close to or exceeding the U.S. Environmental Protection Agency’s Maximum Contaminant Level (MCL) of 5 pCi/L for combined 226Ra and 228Ra. Many communities in Wisconsin and the north-central United States rely on these regionally extensive, dolomitic and sandstone aquifers for their principal source of drinking water. A recent U.S. Geological Survey study of radium occurrence and geochemistry found this aquifer in the Midcontinent and Ozark Plateau region has the highest frequency of Ra occurrence among the 15 major aquifer systems in the country. The 95 public water systems in Wisconsin with Ra levels exceeding the MCL have adopted various strategies to bring their systems into compliance. These strategies include well reconstruction, water treatment by ion exchange or blending, and/or abandonment of groundwater systems in favor of surface water sources. However, the source of Ra contamination to the groundwater has not been identified: therefore it is difficult to plan municipal well construction to avoid Ra contaminated water. This research project is designed to address this gap in knowledge by developing a quantitative relationship between sediment and aqueous geochemistry and the concentration of Ra in groundwater.
Organic Contaminant Transformation by Mn(IV) Oxides
Naturally occurring and synthetic manganese oxides (e.g., MnO2) are among the strongest, naturally occurring oxidants and can oxidize hazardous organic compounds containing amine or phenolic groups to potentially less hazardous species. Organic compounds containing amine and phenolic groups are widespread aqueous pollutants and include many endocrine disrupting compounds, biologically active hormones and antibiotics.
These classes of organic contaminants are quite common in municipal wastewater, storm water, runoff from confined animal feeding operations (CAFOs), and/or landfill leachate. These chemicals are generally not removed nor significantly degraded by traditional primary and secondary wastewater treatment technologies, resulting in their discharge to and presence in drinking water sources, including surface and groundwater.
In addition to occurring naturally, manganese oxide minerals are created during treatment of drinking water that relies on chemical oxidation and filtration to remove reduced transition metals (e.g., Fe(II) and/or Mn(II)) prior to introduction into the water distribution system. Currently, these water treatment solids are either landfilled or disposed of in the sanitary sewer. Due to their high reactivity, Mn oxides could instead be utilized to remove recalcitrant organic contaminants in a variety of water treatment applications. We are investigating which Mn oxides are ideally suited for use as inexpensive, in situ, passive oxidants to remove a variety of organic contaminants from water.
Refining our understanding of methylmercury production and bioavailability in the St. Louis River Estuary
The St. Louis River Estuary (SLRE) lies at the mouth of the St. Louis River, just prior to its discharge into the western tip of Lake Superior. The diverse and abundant habitats in the estuary make it a valuable fish spawning ground for the western arm of Lake Superior and a valuable recreational resource for northeastern Minnesota and northwestern Wisconsin. Methylmercury (MeHg), the bioaccumulative form of mercury, is primarily produced by microbial activity in anaerobic wetlands, soils, and sediments that are abundant in estuarine environments, such as the SLRE. Preliminary Hg stable isotope data collected from fish in the SLRE suggests that higher Hg levels are found in walleye that feed within the SLRE as opposed to those that feed in Lake Superior. Although the underlying cause of elevated mercury levels in fish tissue in the SLRE is presently unknown, it is likely due to a combination of biogeochemical factors including solid-phase Hg speciation, coupled with variations in water chemistry (e.g., dissolved sulfate and organic carbon). These factors control both Hg bioavailability and methylation and hence its entry to the base of the SLRE food web. Developing an understanding of these fundamental biogeochemical processes is critical to the ability of state and federal resource management agencies to make effective management decisions concerning the beneficial use of future dredging materials and habitat restoration in the SLRE. Out proposed work aims to address these uncertainties by relating mercury methylation potential to the speciation of mercury in solid and dissolved phases of SLRE sediments, and its subsequent bioavailability to the base of the food web. To assess spatial variability, we will evaluate samples collected from several distinct settings and geochemical conditions (e.g., estuary flats, sheltered bays, near-shore wetlands, clay-influenced river mouths).
Coupled Fe and C Biogeochemistry
Soil organic matter (SOM) is one of the largest carbon pools on the Earth’s surface, in fact it’s larger than the amount in the biosphere and atmosphere combined. Despite the obvious importance of soil organic matter in relatively little is understood about the biogeochemical processes controlling its mobility and speciation, particularly in cyclically anaerobic environments. Recent studies have shown that the cycling between Fe(II) and Fe(III), which occurs in many near-surface environments promotes the transformation, dissolution and mobilization of soil organic matter (SOM). Currently, global climate models assign SOM residence times without taking these underlying biogeochemical processes into account, resulting in large uncertainties in the residence time of SOM and in projections of its response to a changing climate.
The chemical nature of OM can dictate the amount and chemical components of OM released during Fe(III) reduction, and consequently the microbial oxidation of OM under aerobic conditions. However, the exact nature of interactions between organic carbon and iron under redox fluctuating conditions remains unclear. Additionally, the use of solid-state OM as an electron source for Fe(III) reduction has not been extensively studied. We are investigating the molecular-level changes in chemical nature of soil OM and OM-iron oxide interactions upon the reduction of Fe(III).
Carbonate Mineral Formation
The formation of inorganic carbonates (e.g., CaCO3 polymorphs) through biomineralization is thought to proceed through a concentrated liquid phase, induced through biological molecules, just prior to mineral formation. The presence of charged, organic molecules such as in seawater and biological systems, has a profound effect on the abundance of the condensed liquid phase and its coalescence into droplets, and are associated with biomineralization processes affecting nucleation, and modulating crystal growth through manipulation of the droplet size and abundance. These condensates are an important pathway through which solid phase nucleation occurs.