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Coal combustion in the US, largely in power plants, generates ~130 million tons of ash each year. Some ash is collected from the bed of combustion chambers (bottom ash), while other ashes are collected as fly ash from smokestacks and from air pollution control devices, including flue gas desulfurization (FGD) fixtures. Currently only a small portion of coal ash is utilized as an additive to cement and in road construction. The remainder is often stored dry in large piles exposed to rainfall or in a wet slurry in unlined lagoons near the point of production.
Seepage from coal-ash ponds has been in the news lately, with a special feature on the CBS weekly news program, 60 Minutes. Recently attorneys from the Department of Justice filed federal Clean Water Act criminal charges against Duke Energy for illegal discharges and inadequate maintenance at several of its coal ash storage sites in North Carolina. In response, Duke Energy seeks to get permits for its coal ash lagoons that would authorize their leakage as part of the normal course of business. The EPA has recently issued new guidelines to control pollution from coal ash ponds, but these are widely regarded as inadequate for the toxic contaminants, such as mercury and arsenic that leak from them.
What do we know about coal ash?
Coal is predominantly composed of carbon, which is released as carbon dioxide (CO2) during combustion. Some of the other volatile components in coal, for example sulfur, boron, and mercury are captured in fly ash, whereas a variety of metals and other non-volatile constituents of coal are concentrated in bottom ash. Arsenic, cadmium, chromium, lead, mercury, and selenium are of particular consequence to humans and wildlife for their toxic properties. Coal ash also contains significant concentrations of radioactive materials that can release radon.
Whereas bottom ash is typically alkaline (pH > 7), fly ash and scrubber sludge can be acidic (pH < 7), depending upon the concentration of sulfur. The pH of waters draining through ash piles determines the solubility of various constituents and thus their concentration in outflow waters. Water-extractable fractions of boron, barium and chromium are greatest in alkaline waters, whereas cadmium, lead and mercury are enhanced in acidic conditions. The concentrations of arsenic and selenium are affected by both pH and the amount of oxygen in the outflow waters. Arsenic is adsorbed on certain iron minerals in oxic environments.
Coal ash lagoons have caused tremendous environmental damage through catastrophic failures of their unlined earthen dams and pipes, as they did in Kingston, TN and at the Dan River in North Carolina. But every day, coal ash lagoons also cause continuous environmental impacts via ongoing discharges of contaminated waters that contain elements leached from the ash. Examining a number of lakes and rivers upstream and downstream of coal ash lagoons in North Carolina, Ruhl et al. (2013) found evidence of contamination from waters draining from many coal ash lagoons. The study looked at Duke Energy’s Asheville, Riverbend, and Mayo power stations and found concentrations of selenium, arsenic, cadmium and other trace elements above EPA health standards in the outflow waters from these facilities.
When outflow waters mix with the river waters that receive them, the concentrations of some elements are attenuated by reactions with sediments. This may reduce the content of contaminants in surface water, but produce undesirable concentrations and forms of some elements in sediments. Arsenic and selenium are of particular concern. Arsenic is converted to more toxic forms in suboxic conditions (low redox potential) of many sedimentary environments. For example, in oxygenated waters, arsenic occurs as As5+ that will adsorb on iron oxide minerals. In suboxic waters, the iron minerals and As5+ are reduced—the latter to As3+ which is soluble and more toxic to wildlife. Selenium shows the opposite reaction; it is more available and toxic in oxygenated waters (Ruhl et al. 2013).
Microbial reactions in organic sediments can also transform mercury to methylmercury, which is of far greater toxicity than the ionic forms of mercury that are typically found in outflow waters. Mercury, captured from stack gases as part of air pollution abatement measures, can now enter the environment by leaching from coal ash piles and lagoons, where flue-gas residues are disposed.
Toxic Effects of Coal Ash Seepage
Most of the attention to contaminants from coal ash has focused on arsenic, selenium and mercury, which have toxic effects on humans that have been recognized since Biblical times. The effects of these elements on aquatic organisms, which include growth abnormalities and tumors, have been reviewed at length.
Selenium is of special interest, since it is an essential trace micronutrient at low concentrations, but toxic to animals at high concentrations. Plants concentrate selenium in their tissues, where it is transferred to higher trophic levels. Large accumulations of selenium were seen in fish in the Hyco Reservoir in North Carolina, exposed to outflow from a coal-fired power plant. The history of selenium poisoning of fish in Belews Lake, North Carolina is well known: beginning in 1974, outflow from coal-ash ponds resulted in selenium accumulations in fish, abnormalities in their growth, and eventually a massive loss of various fishes from the lake. Diversion of the effluent has allowed the system to recover partially, and selenium concentrations in fishes have declined.
In focusing on these elements, we should not overlook cadmium and chromium, which are found at lower concentrations in coal-ash outflow waters, but with well established toxic and carcinogenic properties, especially for Cr6+.
Most of this discussion has focused on surface waters, which are more obvious and easily amenable to scientific study. However, waters passing beneath coal ash piles and lagoons can transfer chemical contaminants to groundwater. Here, interactions with mineral constituents can reduce the concentrations of some constituents, but long-term contamination of groundwaters is both likely and difficult to mitigate. Already North Carolina’s Department of Environment and Natural Resources has issued a $25 million fine for Duke Energy’s pollution of the groundwater at its Sutton Power Plant. There are 13 more instances of groundwater pollution across the state.
Electricity from coal is dirty business. These ash piles and lagoons should never have been placed in situations where they could contaminate natural waters. Contaminants such as arsenic and mercury are well-known toxins and should be regulated as hazardous waste—a more restricted level of management than the EPA has proposed. Costs for the management of coal ash should not be externalized on certain locations near the piles and lagoons, but rather borne by all those who use electricity generated in coal-fired power plants.
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