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Study Shows Fossil Fuel Use has Outsized Impact on River Sediment

According to a new study, fly ash (leftover particles from coal-burning) constitute around 37% to 72% of all particulate organic carbon carried by the Yangtze River in China, or about 200,000 to 400,000 tons of carbon annually.

Study Shows Fossil Fuel Use has Outsized Impact on River Sediment
A coal ash disposal site in China. Image Credit: © Zhao Gang/Greenpeace.

The first-of-its-kind study demonstrates just how huge an effect fossil fuel consumption has on Earth. Apart from pumping carbon dioxide into the atmosphere, burning coal dumps equal amounts of particulate carbon into the Yangtze River as natural processes do.

The study results were reported in the Proceedings of the National Academy of Sciences (PNAS) on May 17th, 2021.

About one-fifth of the world’s coal consumption occurs along this river. We knew that would have an impact on the river; we just had no idea how big an impact it would be.

Gen Li, Postdoctoral Scholar and Research Associate, Caltech

Li is the lead author of the PNAS study. The Yangtze River, which is the third-largest river in the world, cuts east across central China from the Tibetan plateau to the sea at Shanghai.

At present, China is the largest consumer of coal in the world, burning 2,500 megatons of coal in 2008, when this study’s samples were collected, and more than 4,000 megatons of coal in 2020.

Despite an increase in the total amount of coal burned in China from 2008 to 2020, the country also improved the regulation, recollection and storage of fly ash in that period; at present, nearly 80% is retrieved immediately.

Coal fly ash is the byproduct of burning coal in the form of unreacted particulate—the fine grains that become sooty smoke instead of burning.

Made of minerals and fossilized particulate organic carbon on the same scale as silt or clay particles, those particles are harmful when inhaled and are also heavy and settle out of the air.

A major portion of it is trapped for use in industry as an additive to cements and concretes, where it enhances workability and overall strength. In addition, it is used in agriculture as a fertilizer. But a portion of it escapes and eventually collects in rivers and washes downstream with the normal sediment that erodes out of riverbeds.

This is a new angle for carbon emissions that we haven’t tracked before. The fact that human processes are producing about as much as natural processes in this region shows just how much of a problem this is.

Woodward Fischer, Study Co-Author and Professor of Geobiology, Caltech

As a next step, the researchers look to continue the investigation of the role of coal fly ash in other large river basins located near dense populations—for instance, the Mississippi River. At the same time, they will analyze sediment cores taken offshore from the Yangtze River outflow to check whether they can find coal fly ash there.

We’re just beginning to appreciate how rivers are massive conduits for carbon at the earth surface. By studying them further, we can quantify, evaluate, and, with future planning, potentially mitigate inimical human impacts on the carbon cycle.

Woodward Fischer, Study Co-Author and Professor of Geobiology, Caltech

Caltech's co-authors are Michael Lamb, professor of geology, and Xingchen (Tony) Wang, formerly a postdoctoral researcher at Caltech and now an assistant professor at Boston College.

Other co-authors are A. Joshua West of USC; Valier Galy of Woods Hole Oceanographic Institute in Massachusetts; Hongrui Qiu of Rice University in Texas; and Ting Zhang, Shilei Li, Gaojun Li, Liang Zhao, Jun Chen, and Junfeng Ji of Nanjing University in China.

This study was financially supported by the National Key R&D Program of China, Caltech Geology Option Postdoctoral Fellowship, Foster and Coco Stanback, Caltech’s Terrestrial Hazard Observation and Reporting Center, and the Resnick Sustainability Institute.

Journal Reference:

Li, G. K., et al. (2021) Coal fly ash is a major carbon flux in the Chang Jiang (Yangtze River) basin. Proceedings of the National Academy of Sciences of the United States of America.


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