State’s iron legacy still toxic to soil

PENN STATE (US) — Some areas of soil in the state of Pennsylvania contain 14,000 parts per million of manganese—more than 17 times as much as in the surrounding bedrock.

Pennsylvania’s history in the iron industry has left the soil saturated with manganese—toxic to trees, especially sugar maples, and other vegetation.

Research which quantifed the amounts was part of work done at the Shale Hills Critical zone Observatory.

“Our group’s focus was to study the soil chemistry,” says Elizabeth M. Herndon, graduate student in geosciences at Penn State.

“We saw excess manganese in the soil and decided that we needed to quantify the manganese and determine where it came from.”

Typically, manganese in soils comes from the disintegration of the bedrock as soil forms.  Bedrock in the studied area is shale and the average amount of manganese in the shale is commonly about 800 parts per million.

Researchers sampled 21 sites along a ridge at Shale Hills.  They took core samples from the surface down to bedrock.  At 20 of the sites they found elevated manganese.  The core samples, which are about 12 inches long, encompass about 7,000 years of soil formation.

“We needed to quantify how much extra manganese there actually was in the samples,” Herndon says. “While soil formation puts manganese into the soil, chemical weathering and physical erosion remove manganese from the soil, so we used a mass balance model to account for these inputs and outputs.”

The researchers found that 53 percent of manganese in ridge soils can be attributed to atmospheric deposition from anthropogenic sources.

Results are reported online in Environmental Science and Technology.

“Because the amount of manganese in the soil was highest near the surface, the added manganese was very likely industrial pollution,” says Herndon.

The area of central Pennsylvania tested was the site of numerous iron furnaces beginning in the late 1700s.  While some furnaces stayed in operation into the 20th century, most were abandoned by the 1860s.  The legacy of the ores and fuels they burned remained behind in the soil.

The researchers knew there was added manganese, they needed to show that the element came from industry.  They looked at a location near a steel mill in Burnham, in Mifflin County and found a similar pattern of manganese concentrations in the soil suggesting that the steel mill was the source of the manganese.

They also examined datasets for soils across the United States and Europe and found that a majority of these soils have excess manganese.  This may indicate that manganese pollution is not just a local phenomenon but could be widespread throughout industrialized areas.

Because manganese is naturally found in soils and is readily taken up and cycled by trees, the researchers looked to see if the pattern of manganese deposition matched that of areas where trees were manipulating the manganese.

In those cases, trees move manganese from deep in the soil creating deficits near the bedrock, but concentrate the manganese nearer the surface.  The manganese pattern did not show a depletion near bedrock and the case for industrial pollution was strengthened.

Manganese is an exceptionally reactive element and is considered toxic if inhaled, but its presence in the soil, where it occurs naturally and is less likely to be inhaled, is not typically a danger to humans.

Trees, however, may be adversely affected.  While sugar maples can be detrimentally affected if they have a manganese deficiency, too much manganese can be toxic especially for saplings.  High levels of manganese can also damage other vegetation and crops.

“Manganese oxides could also change the chemical properties of the soil,” says Herndon.  “Even if the sources of manganese pollution are no longer active, the remnants remain in the soil. I find it interesting that we have to consider the kinds of contamination left over from the past that might impact us today.”

The National Science Foundation supported this work.

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