MOBILIZATION AND ATTENUATION MECHANISMS OF GEOGENIC TRACE ELEMENTS IN A CRUDE OIL CONTAMINATED AQUIFER

Trace elements including manganese (Mn), cobalt (Co), nickel (Ni), arsenic (As), strontium (Sr), and barium (Ba) pose numerous health complications when ingested by way of drinking water. Geogenic trace elements occur in aquifer sediment through inclusion in mineral structures and by sorption to mineral surfaces. When geochemical disequilibrium occurs via microbially mediated changes in pH and/or redox state, minerals can become soluble, prompting trace elements to mobilize from sediment into groundwater.

In a crude oil contaminated aquifer near Bemidji, MN, biodegradation of petroleum has lowered groundwater pH by about 1.5 units and formed methanogenic, iron (Fe)-, and Mn-reducing zones. In the former Fe-reducing (now methanogenic) zone near the oil source, As, Sr, Ba, Co, and Ni have been stripped from the sediment, forming co-occurring trace element plumes in groundwater. Sr, Ba, Co, and Ni are attenuated downgradient by sediment in the current Fe-reducing zone. We use scanning electron microscopy with energy dispersive X-ray spectroscopy to elucidate the mineralogic origins of trace elements in sediment and identify the mechanism(s) attenuating trace metals in the Fe-reducing zone.

Our investigations of uncontaminated background sediment show As and Mn (oxides) co-occur with Fe-hydroxides, while Sr associates with feldspars. Co, Ni, and Ba tend to co-occur with Fe-hydroxides, carbonates, and silicates. In preliminary analysis of the Fe-reducing sediment, where Co, Ni, Ba, and Sr are attenuated, Sr remains strongly correlated with silicon, as does As with Fe. Co, Ni, and Ba associate most strongly with Fe. From these observations, we interpret that in the methanogenic zone (previously Fe-reducing zone), Fe/Mn-(hydr)oxides were reductively dissolved, mobilizing Fe²⁺, Mn²⁺, and As. Acidity produced during biodegradation weathered silicates and carbonates to mobilize Sr²⁺ and Co, Ni, and Ba, respectively. In the current Fe-reducing zone, concentrations of Fe²⁺ and bicarbonate increased due to biodegradation and caused the precipitation of ferrous carbonates, which we hypothesize can co-precipitate trace metals. Results from this study provide new knowledge about secondary impacts to water quality following oil spills and the aquifer’s capacity to naturally attenuate secondary contaminants.