Influence of rhizosphere processes on biogeochemical cycles governing metal contaminant mobility in floodplains
EMSL Project ID
49874
Abstract
Floodplains are important repositories for metals, regulating their transport to surface and groundwater. The heterogeneous lithology and seasonal wet-dry cycling associated with alluvial deposits, result in highly complex biogeochemical redox cycles. As a consequence the mechanisms that control metal contaminant mobility in floodplains are not well understood. Our work aims to decipher the controls on contaminant partitioning between organic and mineral hosts in the solid phase and dissolved, colloidal and complex-bound mobile phases in the soil solution. Particularly, we are examining the influence of plant roots on metal behavior in these systems. Plants orchestrate the biogeochemistry around their roots through water and nutrient uptake, ion and organic exudate release, and respiration. Further, wetland plants aerate the rhizosphere. Thus, the floodplain rhizosphere is highly heterogeneous at small spatial scales and with a strong temporal transiency driven by changing root activity, diurnally, seasonally, and with root age. In response to spatial and temporal variability, the fate and transport of contaminants becomes complicated, depending on redox transformations of the metal itself (as for U) or of associated phases, such as Fe (oxyhydr)oxides or sulfides. Moreover, metals form complexes with organic matter (OM) and, thus, are mobilized/retained in accordance with their organic hosts' behavior. Focusing on a hydrologically dynamic floodplain in semi-arid, high plateau Riverton (WY), we will examine the influence of plants on mobilizing/stabilizing mechanisms for existing groundwater contaminants at this site: U and Mo. Because these contaminants are associated with different mineral and organic host phases depending on redox conditions and OM composition, we aim to elucidate i) the molecular to pore scale changes in Fe/S mineralogy, C chemistry, and microbial community composition in relation to plant root activity; and ii) the resulting speciation and partitioning of contaminants between solution, colloidal, and solid host phases. Accomplishing this task will require integrating a number of unique EMSL capabilities (see Table 1) allowing us to combine high resolution imaging (e.g., SEM, TEM, NanoSIMS, fluorescence microscopy) with high resolution chemical fingerprinting and quantification methods (e.g., FT-ICR-MS, NMR, EPR, Mossbauer). We will combine our EMSL analyses with synchrotron-based x-ray imaging (STXM, nano-XRF) and spectroscopy (XAS) capabilities available to us through active user proposals at Stanford Synchrotron Radiation Lightsource (SSRL) and Canadian Light Source (CLS). Further, we will use isotopic labeling to trace plant derived C into the rhizosphere using IRMS methodology through collaboration with PNNL staff.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2017-10-01
End Date
2018-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members