To help the EPA and others understand where concentrations of naturally occurring arsenic are found, the USGS assessed arsenic data collected over 25 years from 18,850 samples of groundwater that was used as a source for public water supply systems. The survey estimated that 13.6 percent of all water systems (which are not necessarily current sources of drinking water) exceeded the targeted arsenic concentration of 5 parts per billion. Homeowners with wells are less likely to know if their water is tainted with high arsenic levels, says Alan Welch, a co-author on the USGS report released in May. Residents living in rural areas of Michigan, Wisconsin, the upper Midwest and also Minnesota, the Dakotas and southeast Oregon may all be surprised to find their counties marked dark brown on the map. Understanding the geochemical conditions that mobilize arsenic may help explain its presence:

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•   High concentrations of arsenic occur commonly in groundwater with pH levels above eight, Welch says. The West Coast, with its abundance of siliceous volcanic rock, gives the arsenic a naturally alkaline bath. The more weathered Appalachians and Blue Ridge Mountains are less prone to the contaminant.
•  Another source of arsenic is iron oxide. Organic material reacts with iron oxide and releases arsenic in groundwater as it moves through young sedimentary rocks. “Groundwater is probably dominantly where arsenic is found because of the anoxic conditions which favor dissolution of iron oxide,” Welch says. “If a lot of oxygen is present, such as in rivers, iron oxide will precipitate and remove arsenic with it.”
•   In Yellowstone, arsenic is found in the geothermal waters from the deeper part of the system. Hot springs and geysers carry the arsenic to the surface where it drains into the Madison River, back into the younger rock and into the groundwater.
•   Mined areas are particularly sensitive to arsenic. “As much as 5 to 7 percent of pyrite is actually arsenic trapped in the lattice of the mineral,” Welch says. Dissolution of pyrite and other sulfide minerals releases arsenic onto the ground and into surface waters.
•  Arsenic is also in highly evaporative systems, where water is trapped in a hydrologically closed basin, such as Mono Lake in California.

Christina Reed

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