Biogeochemical mapping of bacteriogenic iron oxides in a freshwater wetland at Chalk River, Ontario, Canada

Abstract

Large accumulations of bacteriogenic iron oxides (BIOS) form near Fe(II)-rich groundwater springs in a freshwater wetland located at Chalk River, Ontario, Canada, covering an area of approximately 12 000 m2. BIOS are efficient sorbent and may potentially serve as an in situ bioremediation method to sequester contaminants, such as heavy metals (e.g. Sr2+ and potentially radioisotopes, 9OSr 2+, 129I- and 14C), present in the groundwater. This study focused on the aqueous geochemistry of the wetland, as well as, the solid phase biogeochemistry of BIOS sediments. The mineralogy of BIOS was mainly composed of poorly ordered 2-line ferrihydrite, with minor amounts of lepidocrocite. Goethite was also detected in sediment samples collected during the warmer seasons at two of the sites. Crystallinity of the reactive solid phase Fe in BIOS increased downstream (approx. 10m) from groundwater discharge areas, whereas, Fe(III) became less bioavailable. Analyses of surface water sampling revealed that dissolved Fe(II) and dissolved inorganic carbon (DIC) were significantly correlated (r s =0.83). These two parameters are thought to be involved in a complex biogeochemical cycling in BIOS-rich environments. Both, dissolved Fe(II) and DIC are consumed by chemosynthesis used by iron-oxidizing bacteria during the formation of BIOS or produced during the microbial reduction dissolution of Fe(III)-oxides coupled with the oxidation of organic matter. These results provided valuable information concerning the long term redox stability of BIOS in the natural environment.