Coal fly ash, a byproduct of coal-fired power generation, is gaining a reputation as a green building material when used to make concrete, but due to toxins it contains, its use in agricultural applications may be cause for concern.
A large amount of fly ash that has found a home near downtown Minneapolis has been receiving lots of attention lately: The superstructure of the new 35W bridge is composed of 25 percent fly ash, and the bridge’s piers, footings and shafts all contain 16 to 18 percent fly ash. The ash is used as a substitute for Portland cement, a key component of concrete.
Portland cement is notorious for the large amounts of energy consumed and CO2 released during its production. Fly ash in concrete has recently be gaining a reputation as an alternative.
“If you use a 25 percent fly ash content instead of the standard 9 percent, you get an 11 percent reduction in CO2 emissions,” explains Daniel Handeen, a research fellow at the University of Minnesota Center for Sustainable Building Research. “A fly ash content of 35 percent reduces CO2 potential 21 percent.”
But fly ash contains toxins like arsenic, lead and mercury, as well as dioxins. Fly ash has also been shown to have elevated levels of Radium-226. According to the Center for Sustainable Building Research’s building material database, the debate among building health experts is still open:
Some experts maintain the metals are effectively locked into the [cement] matrix, preventing their release. Furthermore, by using fly ash in concrete rather than landfilling it, the potential for the metals to leach into the environment is reduced. Concern about higher incidence of Radium-226 in fly ash than in cement: An EPA study suggests that the slight increased risk imposed by the greater exposure was offset by the reduced exposure to radon gas, which is less likely to escape fly ash’s glass sphere structure and the less permeable nature of high volume fly ash concrete.
“There are tradeoffs to anything; there’s no perfect material,” Hendeen points out. “Fly ash, in a way, is still solving a symptom. In one sense, yes, you’re using up this waste material. In another way it’s justifying the burning of coal as a fuel source. Until we find better ways to produce energy, it is a good use of the byproducts.”
Coal plants are eager to get rid of the highly toxic ash. In Minnesota, which ranks 26th in the country for coal waste with 1,544,110 tons of waste reported, the ash is stored in massive containment ponds. In other states the ash is allowed to be piled up in giants mounds, as was the case in the recent environmental disaster that sent nearly one billion gallons of highly toxic coal ash sludge spilling over a six-mile area near a Tennessee power plant.
Fly ash in Agriculture
Less scrutiny has been given to agricultural applications of coal fly ash. In many parts of the country, food crops are allowed to be grown in soil amended with fly ash. The ash stabilizes the soil and has shown to increase yields. But a study conducted at Indiana State University concluded that plants grown in fly ash concentrates of 5 to 20 percent of soil absorb toxic metals.
According to Dr. Sandra Brake, an Associate Professor of Geology and one of the authors of the study, “if the fly ash is amended in soils, some of the trace metals may be in a bio-available form that can be taken up by some plants.”
While the State of Minnesota does not allow fly ash to be used in crop production, the Minnesota Pollution Control Authority (MPCA) has approved the use of fly ash from Otter Tail Power Company’s Hoot Lake facility in Fergus Falls, Minn., to be used as a soil stabilizer in livestock pens, as an aggregate for private roads and as a base pad in feed storage. Up to two inches of ash can be spread over an area to treat the top six inches of soil.
“Depending on the type of fly ash, it can contain high levels of trace metals, some of which are toxic to animals,” said Dr. Brake. “It depends on how the fly ash is used and whether the animals would be ingesting the material.”
Geoffrey Strack, a senior engineer at the MPCA, said the agency requires the fly ash be tested before being dispersed for alternate uses.
“We do have them test the material for the leachability of the elements from the fly ash,” said Strack. “We also limit liquids access to the aggregate, such as having a roof over a feed pad.”
Research into agricultural uses for fly ash began with a study conducted at North Dakota State University (NDSU) during the late 1990s using coal fly ash from the Hoot Lake facility.
“We started the experiment in bison feedlots and it worked so well we applied to the MPCA,” said Jeff Olson, Hoot Lake plant manager.
The MPCA then asked NDSU researchers to look into the environmental impacts of fly ash application. The study concluded that the ash, “if used properly, is not a hazard to the environment when used for soil stabilization at the addition concentrations used in the sites for this project.”
But the MPCA’s subsequent approval seems to contradict the way fly ash is handled at its source. While proper application of fly ash for agricultural purposes is the sole responsibility of the property owner, Minnesota coal plants are governed by some of the strictest regulations in the nation. Coal plants must store fly ash in enormous lined containment pools with earthen dams that are required to be inspected every eight years.
These stringent regulations were put in place after the state was confronted with its own coal ash disaster. On July 28, 1993, a 770,000 cubic yard ash heap at an LTV Steel plant near Lake Superior collapsed after heavy rains, sending the material down a hillside and covering Highway 61 before reaching the lake’s waters.
Olson says the fly ash from Hoot Lake is routinely tested internally by Otter Tail Power, and so far actual use of fly ash has been limited due in large part to landowners’ lack of understanding about the product.
“The majority of the problem is the farmers haven’t found a way to transport it.”
Fly ash must be transported in a vehicle that is both lined and covered with a layer of sand to prevent it from being inadvertently dispersed into the environment.
The Environmental Protection Agency has never regulated the use of fly ash, leaving that to the states, but that policy seems likely to change. On March 4, Senate Resolution 64 was introduced calling on the agency to use its authority under existing law to inspect coal waste facilities and establish a rule making policy following federal guidelines. EPA Administrator Lisa Jackson has already begun the process.