TRACE ELEMENT MOBILIZATION IN A CRUDE OIL CONTAMINATED AQUIFER

Reduction of ferric (Fe(III)) hydroxides is an important electron accepting process within a crude oil-contaminated aquifer near Bemidji, MN, USA. Fe(III) hydroxides commonly sorb trace elements in aquifer sediments; when they are reductively dissolved, trace elements can be mobilized into groundwater. Previous studies have shown that arsenic (As) is elevated in groundwater of the Bemidji plume due to this mechanism. New analyses of spatial distributions of other trace elements in groundwater (nickel, cobalt, rubidium, molybdenum, barium, strontium, and zinc) show similar spatial distributions to dissolved Fe and As, suggesting that they may also be mobilized due to Fe(III) reduction.

In this study, we use new (2019) and previously collected (2013-2016) groundwater and sediment samples from the aquifer to assess the extent of trace element mobilization due to oxidation of hydrocarbons coupled with Fe(III) reduction. Sediment samples were digested using concentrated nitric acid at 180° C and analyzed for trace elements using inductively coupled plasma-optical emission spectrometry (ICP-OES); groundwater samples were analyzed using ICP-mass spectrometry.

Mobilization of trace metals associated with iron reduction was determined via mass balance models. Results show depleted concentrations in sediments adjacent to the oil body due to mobilization during Fe(III) reduction and enriched concentrations at the anoxic/suboxic transition zones due to sorption onto freshly precipitated Fe(III) hydroxides. These results will help evaluate the risk of long-term secondary groundwater contamination from naturally occurring trace elements at oil-contaminated sites.