The public should avoid eating too many striped bass out of southern Lake Powell, state agencies warned this week, because of possible exposure to toxic mercury that could affect the nervous system and brain.
They recommend no more than 4 ounces of striped bass per month for pregnant women and children younger than 6, no more than two 8-ounce meals of bass per month for women who could become pregnant and kids ages 6 to 16, and no more than eight 8-ounce meals for all other adults.
This makes 15 Arizona lakes -- including Upper and Lower Lake Mary -- where people are advised to limit fish consumption due to mercury.
Very small amounts of mercury -- "sub-part-per-trillion" levels -- can pose a problem, explained Dave Naftz a research hydrologist at the U.S. Geological Survey Salt Lake City.
He explains it this way: A slightly different form of mercury interacts with bacteria that use iron, and sulfates in these oxygen-poor zones deep in Lake Powell, where they create something called methylmercury -- extremely toxic.
The toxic kind of mercury works its way into algae, small organisms and ultimately fish.
It accumulates most heavily -- biomagnifies -- in the fish that eat other fish, which is why striped bass are the subject of this warning and other fish are not.
So, where is this mercury originating? That's the big question.
Mercury comes from the ocean and from natural events like volcanic eruptions.
Fossil fuel burning, gold mining and cement production are some of the bigger human-controlled sources, with power plants responsible for about half of the human-made mercury released in the United States.
Here's what the Arizona Department of Environmental Quality (ADEQ) said on the matter about how it got into Arizona's lakes, in its frequently asked questions:
"Mercury occurs naturally in the environment and is found in varying concentrations in soils throughout Arizona. Cinnabar, a natural solid form of mercury, occurs as reddish veins in or near recent volcanic rocks. Seven of Arizona's 15 counties contain significant deposits of cinnabar, with historic mining and exploration for the metal occurring in numerous areas. Mercury has also been used in many industrial and agricultural applications, placer mining and has been associated with some smokestack emissions," the state's environmental agency wrote.
Lake Powell has mostly sandstone and sedimentary rock, and very little volcanic rock, staff with the Glen Canyon National Recreation Area confirmed.
The Sierra Club has a different opinion: That coal-burning Navajo Generating Station power plant is a likely source.
"ADEQ is ignoring the fact that Navajo Generating Station is the single largest contributor to mercury pollution in the region. This warning is another reason why we need to clean up toxic pollution from Navajo Generating Station and transition to clean energy sources from the wind and sun," said Andy Bessler, of Flagstaff's Sierra Club office.
The Navajo Generating Station released 1,040 pounds of mercury in 2010, documents from the U.S. Environmental Protection Agency show.
The EPA has proposed tighter pollution measures in coming years to cut mercury emissions substantially, along with arsenic, chromium and nickel, and those kinds of proposals have been much debated this election season.
The Southwest has coal-burning power plants, but the majority of mercury released in the world -- and it can travel -- comes from Southeast Asia, said one of Utah's regulators.
"They are putting out quite a bit of mercury in the process of burning their coal," said John Whitehead, Utah Department of Environmental Quality's head of water quality.
Data from agencies like the United Nations and the National Research Council supports his thoughts.
"Plumes of harmful air pollutants can be transported across oceans and continents -- from Asia to the United States and from the United States to Europe -- and have a negative impact on air quality far from their original sources," these agencies and more warned in a 2009 paper about mercury and "persistent organic pollutants."
East Asia and Southeast Asia were by far the biggest source of human-made mercury emissions in the world in 2010, at 45.7 percent compared to North America's 4.4 percent, a report prepared for the United Nations in July stated.
State environmental quality organizations can't very well control creation of new power plants in China, Naftz said, but his agency is looking to do research about where Lake Powell's mercury is concentrated, and it can be caught before it transforms into the more toxic type.
He's proposing a big inventory and measuring whether the mercury is accumulating mostly in deep, oxygen-poor places such as at the bottom of Glen Canyon Dam.
If it is, Utah's Department of Environmental Quality might consider intervening as it has in another, smaller lake, Naftz said.
That could involve stirring up the water column in a given hot spot to keep oxygen in it, and keep the less harmful mercury from becoming the kind more toxic for fish and for people.