The breakdown of petroleum hydrocarbons underground can mobilize naturally occurring arsenic in groundwater, according to a study from the U.S. Geological Survey and Virginia Tech. The authors collected groundwater samples from a crude-oil-contaminated aquifer near Bemidji, Minn., annually from 2009 to 2013. Sampling revealed arsenic concentrations up to 230 µg/L, a level that is 23 times as high as the current drinking water standard. Outside the hydrocarbon contamination plume, arsenic concentrations fall within the accepted range. Chronic exposure to arsenic in drinking water is a public health concern because arsenic is linked to skin, bladder, and lung cancers. The results suggest a series of related geochemical and biochemical processes that involve arsenic and iron oxides and the metabolization of petroleum by microbes in low-oxygen conditions, USGS says. Because the study’s geologic setting is not unusual in the U.S., and arsenic may go through cycles of reabsorption and dissolution near a plume, USGS proposes monitoring for arsenic at oil-spill sites.