Abstract
Fly ash consists of mainly silt-size spherules that form during high-temperature coal combustion, such as in steam locomotives and coal-burning power plants. In the eastern USA, fly ash was distributed across the landscape atmospherically beginning in the late 19th century, peaking in the mid-20th century, and decreasing sharply with implementation of late 20th century particulate pollution controls. Although atmospheric deposition is limited today, fly ash particles continue to be resedimented into alluvial and lacustrine deposits from upland soil erosion and failure of fly ash storage ponds. Magnetic fly ash is easily extracted and identified microscopically, allowing for a simple and reproducible method for identifying post-1850 CE (Common Era) alluvium and lacustrine sediment. In the North Carolina Piedmont, magnetic fly ash was identified within the upper 50 cm at each of eight alluvial sites and one former milldam site. Extracted fly ash spherules have a magnetite or maghemite composition, with substitutions of Al, Si, Ca, and Ti, and range from 3–125 µm in diameter (mainly 10–45 µm). Based on the presence of fly ash, post-1850 alluvial deposits are 15–45 cm thick in central North Carolina river valleys (<0.5 km wide), ~60% thinner than in central Illinois valleys of similar width. Slower sedimentation rates in North Carolina watersheds are likely a result of a less agricultural land and less erodible (more clayey) soils. Artificial reservoirs (Lake Decatur, IL) and milldams (Betty’s Mill, NC), provide chronological tests for the fly ash method and high-resolution records of anthropogenic change. In cores of Lake Decatur sediments, changes in fly ash content appear related to decadal-scale variations in annual rainfall (and runoff), calcite precipitation, land-use changes, and/or lake history, superimposed on longer-term trends in particulate pollution.
Highlights
Fly ash emissions from coal burning became regulated in the USA after the Clean Air Act of 1970 and were dramatically reduced with requirements of electrostatic precipitators by 1975; significant use of precipitators in the USA began in the 1960s [8]
Our findings have shown that post-1850 alluvium in the North Carolina Piedmont study is about 60% of the thickness of such deposits in central Illinois for the same valley width (Figure 6)
The use of magnetic fly ash is here shown to be a simple and effective method for identifying the thickness, and sediment accumulation rate, of post-1850 alluvial or lacustrine deposits. This methodology can help to provide context to research associated with land-use change, ecological change [37], atmospheric pollution, accelerated soil erosion, archeological sites, and sediment provenance
Summary
Industrial fly ash, fine particles released to the atmosphere from high temperature coal burning, have been successfully used as a geochronological tool for recent alluvial or lacustrine sediments worldwide [1,2,3,4,5,6]. The presence of fly ash in sediments is an excellent marker because it only occurs in deposits that post-date industrial coal-burning technologies, generally after 1850 in the eastern USA [1,4,7]. A rapid increase in carbonaceous fly ash particles, at about 1950 in lake sediment cores worldwide, has been suggested as a potential global stratigraphic marker for the Anthropocene [4]. Tracking unintended releases of stored coal ash is of importance because several trace elements (e.g., boron, arsenic, sulfate, selenium) in coal ash leachate can lead to groundwater toxicity [12] and can be detrimental to aquatic life [13]
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