All fish sampled in 291 streams throughout the United State contained mercury with 27 percent indicating levels above human health protection standards, according to a new study by the U.S. Geological Survey.

The U.S. Forest Service recently cleaned Pena Blanca Lake, a Superfund site that had methylmercury levels in fish in 2005 that were six times the level of EPA health criteria.

The agency did so by removing tailings upstream, creating sediment catchment ponds, draining the lake and removing the sediment and burying it under a soil cap with the premise that sediment was the main cause of the pollution. It could take up to eight years for the lake to refill and be fully restocked.

Patagonia Lake is probably next on the list of state water bodies to be cleaned up for a variety of metals that include mercury, but no plan is in place, according to Arizona State Parks officials.

The largest source of the sampling between 1998 and 2005 was believed to be atmospheric deposition from U.S. coal-fired power plants with substantial contribution in the Southwest from gold and mercury mining. Another major source has been cement kilns and both sources are under EPA review to tighten standards.

On Thursday, Aug. 20, the EPA called for proposals to reduce mercury from power plants in the Four Corners region, particularly in New Mexico.

The latest USGS study, part of a series spanning more than two decades, demonstrated that mercury was present in all fish sampled in U.S. streams.

The most widespread levels of methylmercury (MEHG), a very powerful neurotoxin that is especially dangerous to babies, were in the southern and southeastern portions of the country where large amounts of organic matter and acidic pine forests contribute to blackwater streams with low dissolved oxygen levels in the water. In these waters, it was concluded that mercury, largely deposited from the atmosphere, was converted to MEHG and that areas with wetlands concentrated the MEHG still more.

The same conditions are less predominant in arid areas of the West and Southwest where less organic matter or coniferous forests are present.

“In many of the southwestern states,” study author and USGS scientist Barbara Scudder, “atmospheric deposition of (MEHG) is low compared to the east and large concentrations of mercury were often generated by mine wastes.”

The National Atmospheric Deposition that measures mercury in rainfall does show far less rainfall in the Southwest but in some areas, such as Mesa Verde National Park in the Four Corners region of Colorado, the mercury levels exceeded many stations in the eastern United States in 2008. Other arid areas such as Sycamore Canyon in north-central Arizona had high readings during 2008.

“The main cause of the fish contamination is algae and water itself. A number of studies have been done comparing water, fish mercury and sediment and the evidence is strong that mercury in sediment is not a cause for high levels of mercury in fish,” said Scudder.

The USGS studies may call into question the efficacy of removing sediment and putting in sediment catchment ponds although the removal of mine tailings and the removal of the water may be major steps to at least temporarily keep mercury levels down.

“We’ve been sampling fish in streams for our studies and not lakes,” said Scudder. “I would think you would need to do a lot of samples in different places in a lake to characterize how mercury is getting into fish. They travel everywhere and eat algae throughout bodies of water.”

Previous USGS studies of fish, sediment and water have indicated that the mercury levels sampled in water are on average only one millionth of the levels of mercury in fish but that there was no consistent correlation of mercury in sediment and in fish. Algae in those studies had 100,000 to one million times the mercury that occurred in the water.

Along the west coast, California streams in Yolo and other counties that were part of the 19th century gold rush remain highly contaminated with mercury and MEHG.

Still farther west, 2009 USGS study of MEHG in portions of the north Pacific Ocean thousands of miles from land found that total mercury levels have risen about 30 percent over the last 20 years from air pollution, and project a 50 percent increase in Pacific Ocean mercury levels by the year 2050, largely from Asia. An estimated 40 percent of U.S. exposure to mercury in food comes from that region.

The new study indicated that among the fish measured nationally for mercury, perch, followed by spotted and largemouth bass, had the highest concentrations of mercury. Channel catfish had the lowest levels of mercury followed by various trout species. Sunfish, a small fish easy to catch and popular with children, was relatively high in mercury in samples tested.

(Editor’s note: Kamp is an environmental liaison for Wick Communications, which owns the Nogales International/Weekly Bulletin.)