“Changes in geochemistry from the natural breakdown of petroleum hydrocarbons in groundwater promote mobilization of naturally occurring arsenic from aquifer sediments into groundwater. This geochemical change can result in potentially significant and overlooked arsenic groundwater contamination. Arsenic is a toxin and carcinogen linked to numerous forms of skin, bladder, and lung cancer. Of particular concern to public health is elevated arsenic in groundwater used for drinking water.For the past 32 years a collaborative group of scientists have investigated the natural attenuation of a petroleum hydrocarbon spill in the shallow, glacial aquifer at the National Crude Oil Spill Fate and Natural Attenuation Research Site near Bemidji, Minnesota. Scientists have found that the natural attenuation of petroleum hydrocarbons promotes a reduction of iron hydroxides, and they wondered if naturally occurring arsenic in the glacial aquifers in this area might be mobilized under these conditions. To address this question, for several years arsenic concentrations were measured in groundwater and sediment upgradient, within, and downgradient from the hydrocarbon plume. Results from this field investigation reveal that arsenic concentrations in the hydrocarbon plume can reach 230 micrograms per liter (µg/L), or 23 times the current drinking water standard of 10 µg/L. Arsenic concentrations are below 10 µg/L upgradient and downgradient from the plume. A scientist completes anaerobic processing of sediment from selected core depths prior to geochemical analyses.  The scientists attributed the elevated arsenic in the hydrocarbon plume to a series of interrelated geochemical and biochemical processes. In hydrocarbon plumes, anoxic (low oxygen) conditions occur when microbes metabolize the carbon–rich petroleum and consume oxygen in the process. Once oxygen has been consumed, then iron hydroxides are used to metabolize the carbon, which can result in the release of iron and arsenic from the sediments to the groundwater. The migration of the iron–reducing zone and shifts in the biodegradability of the carbon sources within the hydrocarbon plume result in changes in the concentration and extent of the dissolved arsenic plume over time. Results from this work also suggest that the arsenic released in the plume may reattach to aquifer sediments downgradient from the plume. This reattachment could be good news for limiting the extent of the arsenic contamination. However, this attachment process may be reversible, highlighting the need for long–term monitoring of arsenic and other chemicals that pose a water quality concern in areas associated with petroleum hydrocarbon leaks and spills. This research was supported by the U.S. Geological Survey (USGS) Toxic Substances Hydrology Program and Hydrologic Research and Development Program, 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.”