The natural breakdown of petroleum underground can lead to elevated levels of arsenic in groundwater, a long-term study has shown.

U.S. Geological Survey (USGS) and Virginia Tech scientists found that changes in geochemistry from the natural breakdown of petroleum hydrocarbons can promote the chemical release (mobilization) of naturally occurring arsenic into groundwater.

Arsenic is a toxin and carcinogen and long-term exposure has been linked to cancer of the bladder, lungs, skin, kidneys, nasal passages, liver and prostate. It naturally present in most soils and sediments, and it only becomes a public health concern when concentrations are high.

The current drinking water standard is 10 micrograms per liter.

The USGS-led study was carried out at a crude-oil-contaminated aquifer near Bemidji, Minnesota. Researchers wanted to find out whether naturally occurring arsenic found in the glacial aquifers in this area might be mobilized in the presence of hydrocarbons because of chemical interactions involving iron hydroxides which also occur naturally.

Arsenic concentrations were measured for several years in groundwater and in sediment up-gradient, within and down-gradient from the hydrocarbon plume at Bemidji.

Analysis of the samples revealed that arsenic concentrations in the hydrocarbon plume were as high as 230 micrograms per liter — 23 times the current drinking water standard. But up-gradient and down-gradient from the plume, concentrations were below 10 micrograms per liter.

The scientists attributed the elevated arsenic in the hydrocarbon plume to a series of interrelated geochemical and biochemical processes that involve arsenic and iron oxides, commonly found in sediments, and the metabolization of carbon-rich petroleum by microbes in anoxic (low oxygen) conditions, USGS explained.

Results also suggested that the arsenic released in the plume may reattach to aquifer sediments down-gradient from the plume, a process that might limit the extent of arsenic contamination in the groundwater but may also be reversible. This highlights 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.