THE ROLE OF SEDIMENTS IN AS CYCLING IN HYDROCARBON PLUMES

Arsenic (As) is present in groundwater at elevated concentrations in a petroleum-hydrocarbon plume near Bemidji, MN. Previous work documents that As is naturally occurring in aquifer sediments in the region, and is associated with iron (Fe) minerals. Biodegradation of petroleum hydrocarbons is coupled with reduction of Fe(III) minerals, releasing aqueous Fe(II) and associated As to groundwater.

The goal of this study is to assess how Fe and As are redistributed in aquifer sediments resulting from changing redox conditions induced by petroleum biodegradation. Results from acid extractions show that As is associated with Fe(III) in background sediments. Arsenic is below detection in the methanogenic zone. In the Fe-reducing zone, Fe and As are spatially heterogeneous. Close to the methanogenic zone, long-term Fe reduction has resulted in depleted As in sediment. Further downgradient in the Fe-reducing zone, where Fe-reduction is more recent, Fe(II) comprises >50% of total Fe in some areas; these sediments contain As concentrations greater than background sediment. At the leading edge of the plume where groundwater is sub-oxic, sediments are slightly elevated in As, and Fe(III) is the dominant Fe oxidation state. Combined, these data suggest that areas of the Fe-reducing zone that have been exposed to hydrocarbons for a greater period of time have undergone significant Fe-reduction and As mobilization into groundwater. In contrast, more recent Fe-reducing areas of the plume accumulate As and limit its transport in groundwater. Fe(II), likely Fe-carbonates, may play an important role in the sequestration of As in the Fe-reducing zone, whereas in the more oxic groundwater at the leading edge of the plume, Fe(III)-hydroxides sequester As.

We are developing a mass balance of As cycling in the plume to quantify As distribution between groundwater and sediment. Preliminary results suggest that most of the As mass exists in sediments, not in groundwater. Data also suggest that petroleum biodegradation occurring over several decades at the site has resulted in dynamic redox conditions that allow different sections of the aquifer to act as sources, sinks, or both sources and sinks for As. Results from this study will be used to assess aquifer vulnerability for As mobilization under current and future geochemical conditions.