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:

For instance, Wisconsin, where I live, is a spatter of contaminated wells on the water map but looks fairly benign on the soil map. Arizona, which has just few hot spots on the water map becomes "an arsenic province", as Smith describes it, on the soil map. Why such a difference? Part of the issue, Smith says, is that the water map reflects limited sampling. Some states have done extensive testing, some almost none. Unlike the soils map, "the arsenic map doesn't even approach national coverage" so this is not a very good comparison. Part of the issue is that soil and bedrock, of course, are not mirror images of each other.

There are some similarities - both maps show elevated arsenic in the Mississippi River in Arkansas and Mississippi, both show concentrated arsenic levels in New England, parts of New Jersey, in southwestern Nevada, western Montana, among others. And, as you may have noticed, there is one other notable difference. The water map cites parts per billion. The soil registers in the parts per million. And it's the latter issue that has led public health experts to look at arsenic levels in crops grown in the "elevated" regions. For instance, both Mississippi and Arkansas grow rice in their river flood plain areas. (I just wrote a piece for Discover magazine about arsenic and rice but it will be behind a paywall for the next 30 days).

However imperfect, these two maps still contain essential information - and an essential message.

They remind us that we need to pay attention to what's under our feet if we want to be smart about dealing with problems like arsenic. "It's critical to understand the natural variation of potentially toxic elements in water and soil," says Smith.

Couldn't agree more.

Images: 1) Arsenic in ground water map/USGS 2) Arsenic in soil map/courtesy of USGS