From: Andy Soos, ENN
Published June 26, 2012 09:33 AM

Natural Contamination

Groundwater is water located beneath the earth's surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Ground water, of course, can be contaminated. However, it is not only contaminated due to man made activities. Potentially harmful levels of naturally occurring arsenic, uranium, radium, radon and manganese have been found in some bedrock groundwater that supplies drinking water wells in New England, according to a new U.S. Geological Survey study. While the presence of contaminants, such as arsenic, in some groundwater was already known, this new study identifies several that hadn’t been previously identified. This new report also provides information on the type of bedrock geologic formations where high concentrations are most likely to be found, which will help identify areas most at risk of contamination.

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The New England results highlight the importance of private well owners testing and potentially treating their water. While public water supplies are treated to ensure that water reaching the tap of households meets federal requirements, there are no such requirements for private supplies, which serve more than 2.3 million people in the region. Private well owners can find information on how to have their wells tested here. All of the contaminants identified can be reduced or eliminated through a variety of treatments.

"The same geologic forces which gave rise to the spectacular mountains and architecturally significant rock quarries of New England are also responsible, over time, for leaching trace contaminants into the groundwater that can be harmful to human health," said USGS Director Marcia McNutt. "This study helps focus attention on where and what the risk factors are such that citizens who depend on private wells can get their water tested to ensure peace of mind."

Another example of widespread groundwater pollution is in the Ganges Plain of northern India and Bangladesh where severe contamination of groundwater by naturally occurring arsenic affects 25% of water wells in the shallower of two regional aquifers. The pollution occurs because aquifer sediments contain organic matter that generates anaerobic conditions in the aquifer. These conditions result in the microbial dissolution of iron oxides in the sediment and thus the release of the arsenic, normally strongly bound to iron oxides, into the water. As a consequence, arsenic-rich groundwater is often iron-rich, although secondary processes often obscure the association of dissolved arsenic and dissolved iron.

Among the New England findings, arsenic in untreated samples exceeded federal safety standards for public drinking water at 13 percent of sites — nearly double the national rate. Manganese exceeded its human-health benchmark in more than 7 percent of wells tested. Radon exceeded the U.S. Environmental Protection Agency’s proposed standards in 33 percent of wells. Additionally, uranium, which is easily measurable, was found to be a significant predictor of the presence of other forms of radioactivity (radon, radium, gross alpha radioactivity) that are a cause of concern for human health.

"The concentrations above human health benchmarks and the wide variety of natural and man-made contaminants found show the vulnerability of crystalline rock aquifers that millions of people rely on to produce safe drinking water," said USGS scientist and lead author Sarah Flanagan. "The well-to-well variability of water quality from bedrock aquifers in the region underscores the importance of testing public and private wells individually."

For this study, scientists examined water-quality data from more than 4,700 public-supply wells that were sampled for the USEPA Safe Drinking Water Program from 1997 to 2007 and 117 private wells sampled by the USGS National Water-Quality Assessment Program from 1995 to 2007. The samples included only well water from crystalline rock aquifers found in most of New England and small portions of northern New Jersey and southern New York State.

"This study confirmed many areas already known to have groundwater with high levels of arsenic and radiochemicals and revealed for the first time, the potential fluoride hotspots in parts of the White Mountain region of northern New Hampshire," said Flanagan.

"We also found that high concentrations of many naturally occurring compounds in groundwater were related to specific bedrock formations," added Flanagan.

For further information see New England Water.

Map image via USGS,

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