PS 31-179 - Visualization of in-situ organic chemical flow through sand using neutron radiography

Abstract: Accurate tracking of organic chemical flow in soils and sediments is essential for understanding interactions in chemical ecology, for monitoring contaminant flow in ecotoxicology, and other diverse applications. However, in practice it is difficult to monitor in real time without tracers or labels and with a high spatial and temporal resolution. Computer modeling of flow dynamics is powerful, but produces highly variable results that are model dependent. We present a new method for imaging organic chemical flow in soils in real time without adding tracers. Projecting a beam of neutrons onto a sample allows for the imaging of internal structures within the sample, including hydrogen rich chemicals. Water flow can also be imaged, and differentiated from organic chemical flow, by using D₂O instead of H₂O because deuterium and hydrogen interact with neutrons differently. We grew prickly pear (Opuntia humifusa) and tomato (Solanum lycopersicum) in 1-cm thick rhizotrons made out of aluminum and filled with sand. Aluminum and sand are transparent to neutrons and allowed for clear imaging of plant roots, D₂O and organic chemicals. At the CG-1D beamline at the High Flux Isotope Reactor at Oak Ridge National Laboratory we placed rhizotrons in a neutron beam. After collecting background images, we injected rhizotrons with either D₂O, a D₂O and imazamox solution, or undiluted hydrophobic cineole and acquired multiple images per minute over 8-hour periods. In order to track chemical flow over time, we processed images to remove background and then calculated flow rates in multiple directions from the injection port by determining the location of the wavefront in each image. Neutron radiography successfully differentiated D₂O and organic chemicals, and allowed us to follow the heterogeneous spread of multiple wavefronts simultaneously. Imazamox flowed more quickly than cineole, and the two organic chemicals also showed different patterns of spread through the rhizotrons. Imazamox spread outward evenly from the injection port, while cineole rapidly dropped to the bottom of the rhizotron.