Air pollution in China has led to higher rates of asthma, heart disease, and type 2 diabetes, and—according to The Lancet—it contributed to 1.1 million premature deaths in 2015 alone. But now researchers think they have found an unanticipated risk from air pollution that can kill instantly: landslides. A new study suggests that acid rain from pollutants from burning coal can slowly but inexorably weaken certain layers of rock underlying mountain slopes to the point of failure.
The new work is “thought provoking,” says geologist Alison Duvall of the University of Washington in Seattle, who was not involved with the study. “It’s opening our eyes to something we really should be considering.”
Landslides typically occur when an outside force—like rainfall or an earthquake—destabilizes the layers of rock and soil that make up a mountainside. Until then, friction holds the layers together; how strongly they cling to one another depends on the materials they are made of. But when the shaking of an earthquake or the lubrication—or weight—of rainfall lowers the friction between the layers, gravity takes over, and the affected area turns into a sliding zone.
The 2009 Jiweishan landslide in southwestern China, which killed 74 people and injured eight, had no clear trigger—no recent earthquake tremors or heavy rainfalls. But a pair of geoengineers suspected something else was in play: a slow but progressive change within a specific rock layer that loosened the mountain’s slope until it gave way. The culprit was none other than acid rain, which altered the chemical composition of the rock layer and made it more slippery, they report in the 1 January 2018 issue of Earth and Planetary Science Letters.
The source of that rain, say the researchers, was coal combustion—the main source of power in southwestern China. Burning coal releases sulfur dioxide, nitrogen dioxide, and other pollutants into the atmosphere. In rainwater, sulfur dioxide and nitrogen dioxide convert to sulfuric acid and nitric acid, which increase the acidity of rainwater. “Acid rain” is anything with a pH of 5 or below, and normal rainfall has a pH of 5.6. Between 1986 and 2014, the average pH of rainfall for the nearby city of Chongqing, China, varied between 4.3 and 5, weather records reveal.
“When the idea came to us, we surprised ourselves,” says study co-author Ming Zhang, an engineering geologist at the China University of Geosciences in Wuhan. To find out whether acid rain was indeed the problem, the team focused on one problematic sliding layer in the Jiweishan avalanche: a thin bed of black shale, which contains slippery clay minerals such as talc, in addition to fine organic material and calcite, which helps cement the shale together. If acid rain dissolved the calcite, the team reasoned, slippery talc would be the main mineral left behind.
In a simple laboratory experiment, they took a sample of black shale from the landslide site and immersed it in an acid bath with a pH of 3, a level more acidic than the rain to accelerate the test. As expected, the acid dissolved the calcite, leaving nothing but a weakened black sponge full of micropores. Acidic water reached the shale layer by percolating through the overlying limestone, the researchers suggest, aided by cracks created by mining operations.
But a second effect within the sliding layer may have played a bigger role. The acid rain supplied extra oxygen and nutrients, including nitrogen, potassium, and sulfur, which could have fertilized microorganisms living inside the shale. A DNA test on a shale sample revealed hundreds of genera of microorganisms, including ones capable of decomposing organic materials within the shale and weakening it. If these microorganisms were sufficiently fertilized, they could eat the organic materials at a faster rate. The same bacteria almost certainly were present in the shale before the avalanche, but heat from the slide erased the DNA signal of the existing microorganisms, says co-author Mauri McSaveney, an emeritus engineering geomorphologist at GNS Science in Lower Hutt, New Zealand.
“It’s an interesting and plausible hypothesis,” says geochemist Richard April of Colgate University in Hamilton, New York, who was not involved with the study. But he points out that naturally acidic rainwater has steadily chipped away at slopes for thousands of years, and that it would be difficult to imagine how acid rain could cause such accelerated weathering in just a few decades.
Zhang plans to collect more evidence in other regions around the world with the same geological and atmospheric settings to prove his hypothesis more directly. If his research holds, he says, this new factor could contribute to landslides in parts of the United States, Europe, and Asia.