Floodplain soils and aquifers are composed of heterogeneous soil and sediment bodies with varying organic matter composition, texture, mineralogy, and dimensions. Being in hydrological contact with the unsaturated zone, these aquifers are particularly vulnerable to climatic or other changes in hydrological patterns. Moreover, redox conditions in floodplain aquifers typically vary dramatically, which has a major impact on the mobility and transport of redox active contaminants. It has been postulated that inflow of oxygenated groundwater would stimulate oxidation and release of contaminants (with a higher mobility in oxidized form, e.g. uranium) from reducing zones. Contrastingly, recent experiments and field investigations have indicated that oxygenation of groundwater can in fact lead to decreased uranium mobility, proposedly due to stimulation of microbial activity and a strong buffer capacity of reduced zones. However, the mechanisms and resilience of this effect has not been investigated in detail.  Here we present data from a dual domain column experiment, where varying numbers of small sulfidic, organic-rich, and fine-grained sediment spheres, containing elevated uranium concentrations and a natural abundance of arsenic, were inserted into a coarse-grained aquifer material. Oxygenated groundwater was pumped through the columns and depth resolved pore-water data was collected from inside sulfidic zones and between them. Contrary to the prevalent hypothesis (but supporting recent findings) that reduced zones release oxidation-sensitive contaminants when exposed to aerated groundwater, our results show no indications of oxidation inside the reduced spheres. Instead, we observed that the reducing conditions spread into the coarse-grained aquifer material. The diffusion of sulfide into the groundwater was accompanied with a mass transfer of uranium (presumably in its reduced form, transported as organo-complexes), organic carbon, and reactive iron to the aquifer solid phase. Oppositely, arsenic moved from the aquifer sand into the reduced spheres. Importantly, our results show that aqueous iron to sulfide ratios regulate arsenic mobility and speciation; slightly sulfidic groundwater can contain toxic levels of arsenic even if sediment concentrations are low. These results highlight the importance of redox interfaces for controlling contaminant behavior in groundwater and suggest reducing conditions may spread in response to groundwater aeration.

Influence of redox interfaces on metal(loid) contaminant mobility in shallow alluvial groundwater aquifers

2019 Fall National Meeting

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