The Clinton administration lowered the level from 50 parts per billion to 10 ppb. Bush raised the standard back to 50 ppb. Mesa County, McLean said, has only 2 or 3 ppb in its water. Levels spiked to 12 ppb in Clifton several years ago, she said, but have remained low since. Arsenic is naturally present in the earth's crust, McLean said, but it can also be found in agricultural products.
Mining can also add more arsenic to water sources.
Other Colorado communities aren't so lucky. McLean said Alamosa has had levels as high as 86 ppb, and has had to blend its water with water with low arsenic before it can be consumed. Arsenic is known to cause cancer, McLean said, and a National Academy of Sciences study showed a level of 50 ppb in the drinking water raised the risk of a fatal cancer to one in 100 people. But arsenic can come from other sources, McLean said. Some pressure-treated lumber uses chromated copper aresnate to preserve wood and protect it from termites.