Arsenic contaminated groundwater is a growing global concern. World wide, it threatens ecological and human health.
In Bangladesh, arsenic contaminated groundwater has been referred to as the largest poisoning of a population in history1. Past work done by Dr. Neumann and her colleagues suggests that current patterns of arsenic concentration in the aquifer are related to groundwater flow and recharge chemistry. Constructed ponds and groundwater irrigated rice fields serve as the primary aquifer recharge sources, with pond recharge evolving into high-arsenic groundwater and rice field recharge evolving into low-arsenic groundwater. Recharge is largely controlled by the practice of groundwater irrigation, which removes water from the aquifer, creating a downward gradient that drives surface water into the subsurface. These dynamics suggest that rice field water management schemes that reduce irrigation water demand will necessarily change arsenic concentration patterns in the aquifer by reducing the total amount of surface recharge pulled into the aquifer and by altering the proportions of rice field and pond recharge. Our project in Bangladesh is field testing the water savings potential of specific field management schemes, modeling the subsequent effects of this scheme on groundwater arsenic concentrations, and experimentally probing the chemical changes that occur when different proportions or recharge enter the aquifer.
Groundwater contamination is a known threat to drinking and irrigation water throughout the world, but few investigations have considered future contamination of currently clean sources. The goal of our project in Cambodia is to determine the vulnerability of arsenic-free groundwater to future arsenic contamination. Research is focused at a field site situated in a transition zone with high “upstream” arsenic concentrations and zero or low concentrations “downstream.” Arsenic contamination may be geogenic (created at the point of measurement), allogenic (transported to the point), or a combination of the two. Investigation of the site is important given Cambodia’s continued economic development, which may change domestic and agricultural groundwater demands and alter recharge rates and chemistry. In our work, we will identify the processes that may stimulate future arsenic contamination and the time scales on which these processes may occur. Our findings will advance basic understanding of groundwater arsenic contamination, and will provide concrete, useful information about the sustainability of currently ‘safe’ wells – information that policy makers, development organizations, and individuals can use both locally to aid in decisions about specific water use options, and broadly to inform larger policy initiatives.
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. (Funding: State of Washington Water Research Center)
1Smith, A., Lingas, E., & Rahman, M. (2000). Contamination of drinking-water by arsenic in Bangladesh: a public health emergency. Bulletin Of The World Health Organization, 78(9), 1093–1103.
A major lab-based project in the Neumann lab focuses on visualizing the oxygenation zone of growing rice roots 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 toxins (e.g., arsenic) and micronutrients (e.g., zinc). We are probing rhizosphere oxygen dynamics using planar optical oxygen sensors ("optodes," Larsen et al, 2011), 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 toxins and micronutrients in the soil zone around rice roots. (Funding: Royalty Research Fund)
Plant roots can act as conduits for water movement between soil layers. The movement of water from one soil layer into another through the root system of plants is call hydraulic redistribution (HR). Field measurements and regional scale modeling indicate that HR enhances moisture availability to plants, leading to increased transpiration rates and carbon exchange with the atmosphere. Further, it has been hypothesized that the upward redistribution of deep soil water to dry, nutrient-rich surface soil may enhance soil microbial activity and nutrient availability to plants. Thus, through its direct impact on plants and indirect impact on soil biogeochemistry, hydraulic redistribution has the potential to alter terrestrial carbon, nitrogen and water cycles. As part of a larger collaborative project, the Neumann lab is using root-scale mechanistic modeling to explore how and to what extent HR influences land-atmosphere carbon exchange and nitrogen availability to plants. (Funding: DOE Terrestrial Ecosystem Science Program)
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 Neumann lab 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. (Funding: DOE Early Career Award)