Processes Controlling the Fate and Transport of Trace Metals in the Subsurface During Changing Redox Potential

Citation

Farnsworth, Claire Elizabeth (2011) Processes Controlling the Fate and Transport of Trace Metals in the Subsurface During Changing Redox Potential. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/TB3A-Z569. https://resolver.caltech.edu/CaltechTHESIS:05132011-121157483

Abstract

This study investigated the cycling of Mn and Fe as oxic surface water entered shallow sediments and was reduced, and as reduced groundwater became exposed to oxygen and was oxidized. A Mn-oxide doped gel probe sampler was developed to study in situ rates of reductive dissolution and was validated with laboratory studies with ascorbic acid and Shewanella oneidensis MR-1 as model reductants. The sampler was deployed in the bank sediments of Lake Tegel, Berlin, Germany. Modeling of the diffusion-controlled reaction converted the mass loss from the gels in the sampler to a profile of pseudo-first-order rate constants as a function of depth. The rate constants were highest at depths with high dissolved Fe and low operationally defined fractions of reducible oxides of Fe and Mn in the sediments.

A laboratory column experiment showed that 1.3-m water table fluctuations, as observed in bank filtration sites around Berlin, were able to provide sufficient dissolved oxygen delivery for Pseudomonas putida GB-1, an obligate aerobe, to oxidize Mn(II) in situ. Accumulation of Mn on the quartz sand in the column at the end of the experiment was limited to the top 60 cm, as measured with X-ray fluorescence (XRF), and X-ray absorption spectroscopy (XAS) analysis confirmed that the solid formed was a Mn(IV) oxide characteristic of biogenic origin. After a period of “filter ripening” in the column, rates of in situ oxidation were still lower than rates in engineered aerated sand filters.

Adjacent to a production well with a water table that fluctuates up to 7 m annually at Lake Tegel, however, sediments collected from a borehole did not show any significant accumulation of Mn or Fe with depth, as measured by XRF; analysis of the speciation of Mn and Fe in the solid phase by XAS suggested a slight increase in the proportions of total Mn as Mn(II) and of total Fe as Fe(II) with depth. At this location, vertical zonation of groundwater may preclude the co-occurrence of reduced Mn and Fe with dissolved oxygen entrapped by water table fluctuations. Whether groundwater changes from oxidizing to reducing conditions or vice versa, the behavior of Mn and Fe reflects a complex interaction between sediments, solutes, microbial activity, and hydrology.

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