Arsenic contaminated water is a growing global concern. World wide, it threatens ecological and human health.
Source of organic carbon fueling arsenic mobilization in aquifers of South Asia
For over a decade, researchers have debated the source of organic carbon fueling microbial reactions responsible for mobilizing arsenic off aquifer sediments into groundwater within the deltaic environments of South and Southeast Asia. The debate centers on the involvement of sedimentary organic carbon co-deposited when the aquifers were formed versus that of surface derived carbon transported into the subsurface with recharging water. Identifying the source of organic carbon involved with arsenic mobilization is important because it clarifies if human activities can affect the arsenic-contamination problem. Previously, Dr. Neumann and her colleagues used conservative tracers and groundwater flow modeling to connect high arsenic groundwater with recharge from man-made ponds and low arsenic groundwater with recharge from irrigated rice fields. They found that pond recharge contained the biologically available organic carbon needed to fuel arsenic mobilization, while rice field recharge lacked biologically available organic carbon. Recently, the hydro-biogeochemistry group at UW discovered an unexpected reservoir of biologically available organic carbon within the sandy aquifer sediment. The organic carbon was mobilized off the sediment during an incubation experiment and rapidly consumed by the native microbial community. Results indicate that in the aquifer, the bioavailable sedimentary organic carbon must be either physically or energetically unavailable to the microbes, suggesting that perturbations to the aquifer matrix could promote mobilization and utilization of this carbon. An example of such a perturbation is the influx of surface-derived organic carbon that stimulates microbial reactions that target the solid phase. Conceptually, this pool of stabilized sedimentary organic carbon signifies a possible role for both surface derived organic carbon and sedimentary organic carbon in the arsenic contamination problem, bridging the apparently conflicting lines of evidence driving the carbon debate.
Arsenic mobilization, bioaccumulation and eco-toxicity in urban lakes
Heavy metal exposure is a continuing health risk in the south-central Puget Sound basin due to air pollution from almost 100 years of smelter operation in Tacoma, WA. Heavy metals, including arsenic, settled on surface soils throughout the region, requiring ongoing management and remediation over the last 20 years. More recently, high concentrations of arsenic have also been found in surface sediments of recreational lakes in this region. In this project, we are working to identify the mechanisms by which arsenic may be mobilized from contaminated lake sediments into the overlying water column. Initial investigation suggests that arsenic is maintained at high concentrations in oxic surface waters in shallow lakes with high inputs of anthropogenic nutrient pollution. We are also assessing arsenic bioaccumulation in these lake ecosystems up the food chain from small plankton to fish that may be consumed via recreational fishing. Our findings will provide information to evaluate arsenic water quality criteria and lake management strategies. This project is part of the Superfund Research Program at UW.
Remediation of arsenic-contaminated groundwater
Washington State also suffers from arsenic contaminated groundwater, largely due to past to past industrial, mining, and disposal activities. We are working at a site outside of Tacoma, WA where a groundwater arsenic plume developed as a result of the past disposal of arsenic-contaminated slag into an unlined landfill. At this site, the Washington Department of Ecology has overseen the application of an arsenic-remediation strategy called induce microbial sulfate reduction. The strategy works by injecting the appropriate microbial substrates into the subsurface, creating biogeochemical conditions that favor the formation of minerals that incorporate arsenic during precipitation or create surfaces upon which arsenic adsorbs. We are interested in assessing the long-term sustainability of this arsenic removal method, which incudes identifying the minerals and surfaces involved with scavenging arsenic out of the groundwater, determining the capacity of the treated sediment to continue to scavenge arsenic out of groundwater, and testing under what conditions the sequestered arsenic will be mobilized back into the groundwater.
Rice field irrigation management to reduce arsenic exposure
In the Ganges Delta, dry-season groundwater irrigation has successfully increased rice production and food security. However, groundwater in this region is severely contaminated with arsenic, and use of this contaminated water is loading hazardous levels of arsenic to rice field soils. The arsenic is entering rice plants, decreasing yields, and jeopardizing food safety. Low-cost and environmentally sustainable management solutions are required to address these problems and maintain the long-term viability of dry-season rice production in the region. Using knowledge of hydrologic processes in irrigated rice fields, the hydro-biogochemistry group developed and tested an irrigation management approach that involved sealing (i.e., waterproofing) rice field boundaries to minimize water use and reduce arsenic loading to soils. We found that waterproofing rice field boundaries at a site in Bangladesh reduced groundwater irrigation demand by ~50%. By virtue of minimizing groundwater use, the paddy soil received ~32% less arsenic from irrigation water; a reduction that, if sustained over decades, could translate into notably lower rice grain arsenic concentrations. We estimated that waterproofing bunds could, on a decadal time scale, mean the difference between rice representing 80% versus 50% of a Bangladeshi adult’s tolerable daily intake of arsenic.
Oxygen dynamics in the rice rhizosphere
A lab-based project in the hydro-biogeochemistry group is focused on visualizing the oxygenation zone surrounding root of growing rice plants and understanding the sensitivity of this zone to external variables. Diffusion of oxygen from the root tissues of plants in otherwise anoxic soil plays an important role in the chemistry of the soil around rice roots, particularly the oxidation of iron(II), which in turn can impact availability and root uptake of arsenic. We are probing rhizosphere oxygen dynamics using planar optical oxygen sensors ("optodes"), which allow for real-time two-dimensional visualization of concentrations. We use optode oxygen profiles to direct sampling of porewater and soil from mm-scale oxic and anoxic soil zones for key solutes and parameters that help us assess the impact that oxygenation dynamics have on the availability of arsenic in the soil zone around rice roots and on arsenic uptake by the plants.
Methane Oxidation in the Rhizosphere of Wetland Plants
Methane is a potent greenhouse gas, with a global warming potential 20-times larger than that of carbon dioxide. The objective of this research project is to improve predictions of future methane emissions by examining the conversion of methane to carbon dioxide (i.e., methane oxidation) within the soil zone surrounding roots (the rhizosphere) of wetland plants. Wetlands are the largest natural source of methane to the atmosphere, and a majority of methane emitted by wetlands travels from soil through plants to the atmosphere. Plants also support the movement of atmospheric oxygen into the soil where it can oxidize methane; up to 90% of the methane produced in wetlands can be converted to carbon dioxide in this way. As the climate changes, plant species composition, plant behavior, and subsurface chemistry will change, altering the fraction of methane oxidized within wetlands. Thus, understanding the dynamic response of methane oxidation to these expected climate-induced changes is key to accurately predicting future methane emissions. The Hydro-biogeochemistry group is assessing the potential for future changes in rhizosphere methane oxidation using a combination of field measurements taken in two Alaskan wetlands, laboratory experiments conducted on field-collected material, and modeling investigations informed by field and laboratory results.
Measuring and modeling plant-mediated water flow process to improve ecosystem models
Vegetation changes due to climate shifts or anthropogenic conversions will alter water, carbon and nutrient fluxes in soils, which in turn will feedback to impact the climate. Although plant roots bind the atmosphere to the subsurface and affect terrestrial water, carbon and nutrient cycles, the influence of plant-controlled belowground processes on water and carbon balances remains poorly quantified. At present, the treatment of roots is often extremely simplistic in dynamic vegetation and ecosystem models. This simplicity is largely due to the fact that the current understanding of root hydraulics and water-uptake behavior is not advanced enough to improve modeled representations of roots. The hydro-biogeochemistry group has worked to advance mechanistic understanding of two plant-mediate water flow processes: hydraulic redistribution (the movement of water from moist to dry soil layers through the root systems of plants) and nighttime transpiration. Both processes are known to occur globally in many different ecosystems. We have synthesized data from published empirical and modeling studies to quantify how much water is moved through the environment by HR, we have conducted controlled greenhouse experiments that demonstrated modeled rates of hydraulic redistribution could only match observed data after including nighttime transpiration, and with a reactive transport model we have clarified the impact that both processes can have on nutrient availability to plants and microbes living in the soil surrounding roots.