Ethanol-Containing Fuel Spills Enhanced Natural Trace Element Release from Sediments in an Experimental Setting

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Researchers conducted an intensive 2-month field experimental study to look at the mechanisms controlling arsenic and other trace element (nickel, chromium, and cobalt) releases from experimental fuel spills at the National Crude Oil Spill Fate and Natural Attenuation Research Site, near Bemidji, Minnesota. The researchers determined that the co-presence of ethanol and nitrate mixed with hydrocarbons had significant effects on the rates and timing of arsenic and other trace element releases to groundwater.

Because arsenic and other trace element releases are microbially controlled, the rate and extent of arsenic release depends, in part, on what the microbial community has available to metabolize as a food source. Researchers determined that when petroleum hydrocarbons were released in the subsurface (simulating a petroleum fuel spill), the microbial community metabolized these hydrocarbons and released iron at slow rates. As a result, arsenic associated with that iron was released to groundwater at slow rates and never exceeded 10 micrograms per liter (μg/L) (the U.S. Environmental Protection Agency drinking water standard) during the 2-month experiment; however, when ethanol (organic carbon source) was also added to the sediments (simulating a mixed ethanol-petroleum fuel spill), metabolism in the microbial community increased, and iron and arsenic were released to groundwater at higher concentrations due to elevated rates of biodegradation and iron reduction. The rapid release rates resulted in a maximum observed arsenic concentration of 99 μg/L, almost ten times the drinking water standard, in a short time period (5 days). The presence of ethanol also significantly decreased the timing of the onset of arsenic release, with the reaction beginning 23 hours after the start of the experiment with ethanol, compared to 189 hours without ethanol. The release to groundwater of other trace elements including nickel, chromium, and cobalt was observed at rates similar to arsenic.

To test the mechanics of how microorganisms release metals into groundwater, nitrate was added in some experiments. The release of arsenic and other trace elements was not observed in those experiments where nitrate was added because microorganisms preferentially use nitrate compared to iron, so any biodegradation in the presence of nitrate is coupled with the reduction of nitrate, not iron. As a result, iron remains in the aquifer sediments and arsenic remains adsorbed to iron, inhibiting its release to groundwater.

Results from this study highlight the importance of monitoring trace elements at fuel spill sites where organic contaminants might be biodegraded under iron-reducing conditions. These complex reactions can introduce new, unexpected types of contaminants that are not commonly monitored at organic-contaminant sites under current (2015) regulatory framework. Depending on the geochemical conditions and the nature of the organic matter, trace element concentrations in groundwater can exceed drinking water standards on short time frames (days), reinforcing the importance of screening the rapid changes in geochemistry following releases of organic carbon sources such as petroleum or ethanol.

Environmental Health Implications

Arsenic is a naturally occurring element and commonly exists in aquifer sediments adsorbed to iron hydroxides. When it remains attached to sediment, arsenic does not pose an immediate human health threat; however, changing geochemical conditions, such as those initiated by the introduction of organic carbon sources such as petroleum byproducts and ethanol, can release arsenic and other trace elements into groundwater to potentially toxic concentrations if the groundwater were to be used as a drinking water source or transported to streams where aquatic biota could be exposed. Arsenic is of particular concern because of its known toxicity and links to liver, bladder, and lung cancers.

This research was funded by the USGS Ecosystems Mission Area’s Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology) and Hydrologic Research and Development Program, University of St. Thomas, the Virginia Polytechnic Institute and State University, and the National Crude Oil Spill Fate and Natural Attenuation Research Site, a collaborative venture of the USGS, the Enbridge Energy Limited Partnership, the Minnesota Pollution Control Agency, and Beltrami County, Minnesota.