For my New York Times column, Poison Pen, I wrote this week about arsenic in drinking water, or more specifically on the increasing scientific awareness that this element – odorless, tasteless – is slowly being revealed as major public health threat.
The focus of that piece is the way that arsenic, in the part-per-billion level, is able to undermine basic health, from immune system damage to corrosive effects on cells. And that’s important when we think about how to protect ourselves.
But my focus here is to talk a little more about why we find it in water at all. That’s not only true here, of course. There’s high levels of contamination reported in Canada, in South America, in Southeast Asia and elsewhere.
But the United States makes a good case study for why the soil and the rocks under our feet so often contain one of our most reliable poisons. The map above, from the U.S. Geological Survey, is mostly focused on results drilled into bedrock aquifers. The legend is given in micrograms per liter which is equivalent to parts per billion. To put that in context, the EPA safety standard for arsenic in drinking water is 10 parts per billion.
Arsenic is a naturally occurring metallic element, sometimes referred to as a metalloid. It’s the 33rd most common element in the Earth’s crust.
I talked to USGS geochemist David Smith, from the agency’s Denver office, about why we might see it in such different locations as California’s Central Valley (you’ll see it glowing like warning lights on the map, right up the middle of the state) and coastal New England.
In the west, says Smith, arsenic is often deposited by the same hydrothermal fluids that make the area such a hotbed of mineable ores. “It’s not uncommon to see high arsenic in areas where you find gold mining or tungsten mining,” he says. In New England (where a belt of arsenic-tainted water stretches from coastal Maine down into Massachusetts) the source is more often ancient marine shales and schists.
It’s a reminder of long-ago arsenic rich oceans which seeped into the clay-dense materials at the bottom of those seas. The fine clays trapped the arsenic and kept it there, even as they gradually turned to rock. In both cases, as underground water erodes the rock, it releases the minerals and elements from within.
And, of course, as ground water is replenished by rain seeping through arsenic-laced soils, there’s a chance of additional contamination. The USGS is putting the finishing touches on a new survey and detailed of elements in minerals in soil in this country – among them arsenic.
The agency hopes to have that data online by late October. But Smith very kindly provided me with the upcoming map of arsenic in U.S. soils and I want to show it to you here. You’ll notice that while there’s still plenty of arsenic, it’s a different picture than the ground water map:
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