WATER POLITICS

It wasn’t until this past June that most people in Tianjin realized that the northern Chinese port city was sinking.

On May 31, fissures appeared on a road in Tianjin’s Jinnan district. Several residential buildings in the surrounding neighborhood had also started to lean, with water seeping through cracks in the walls. Over the next few days, nearly 4,000 residents had to be relocated, local media reported.

Experts later identified the cause of these problems as land subsidence — the gradual settling or sudden sinking of a piece of land — and linked it to a possible cavity in the earth 1,300 meters below the surface, local media reported.

Incidents of land subsidence date back to the 1920s in the seaside city. The phenomenon has also struck Beijing, some 150 kilometers (93.2 miles) northwest of Tianjin, as well as Shanghai.

Unlike earthquakes and tsunamis, land subsidence occurs silently. Yet the issue has only grown more pressing in China’s coastal cities, which have long been threatened by rising sea levels from global warming — not to mention climate change’s wide-ranging impact on residents and the economy.

China and India are considered the countries most threatened by land subsidence, according to a study backed by the United Nations Educational, Scientific and Cultural Organization.

As of March 2012, there were 50 cities in China considered to be partially sinking, according to a 10-year plan that national resource authorities issued to address the problem. Land subsidence is widespread, and has been found across northern China in provinces like Hebei, Liaoning, Jilin and Heilongjiang.

The major causes of land subsidence are construction and excessive groundwater extraction, with the latter resulting in part from northern China lacking enough water to meet the needs of its population. Government measures could help, including better regulation and infrastructure projects to bring more water from the country’s water-rich south.

The effects of the problem go beyond sinkholes and relocations. From 1974 to 2014, China suffered more than 300 billion yuan (nearly $41 billion) in economic losses due to land subsidence, according to the China Geological Survey (CGS).

Construction and groundwater extraction are the two primary causes of land subsidence, said Qin Siqing, a researcher at the Chinese Academy of Sciences’ Institute of Geology and Geophysics.

Shanghai’s experience offers a glimpse of how heedless exploitation of groundwater can lead to subsidence. The city began pumping water out of the ground in 1910 at a rate of up to 350,000 tons per year, according to official geological data. Just over a decade later, a tidal station at the mouth of the adjacent Huangpu River was found to be sinking.

In the decades since, Shanghai has reported a number of building collapses, including an incident in 2003 in which several buildings surrounding the construction site of a section of metro Line 4 collapsed or started to lean, according to the official newspaper of the China Meteorological Administration.

The newspaper quoted an anonymous expert saying in 2014 that Shanghai had sunk by more than 2 meters since 1921, while its average elevation relative to mean sea level was only 4 meters. “If Shanghai sinks another two meters, it will immediately fall into the ocean,” the expert told the newspaper.

The struggle with subsidence can be traced back to a different problem — China has a lot more water available in the south than in the north. Northern China is home to 50% of the country’s 1.4 billion population, but has only 20% of its water resources, according to public information.

Balitai East Road in Balitai Town in Tianjin. Photo: CCTV

This has led to the exploitation of groundwater resources in most parts of the north, exacerbating the subsidence problem.

It is impractical to ban groundwater pumping altogether, but it is feasible to reduce it, said Li Wenpeng, a senior engineer at the CGS’ geological environment monitoring institute.

Li told Caixin that water transfer projects, such as those designed to divert water from southern rivers to regions in the north. These projects have helped control land subsidence by alleviating the groundwater over-exploitation problem, he said.

Policymakers should make use of the growing amount of public data on land subsidence to avoid high-risk areas when planning how land gets used, said Zheng Wenjing, who earned a doctorate in earth sciences from Singapore’s Nanyang Technological University.

Zheng pointed to the city of Nanjing as a success story. While it is common for cities along rivers or coasts to suffer subsidence, this is not the case in the Jiangsu provincial capital, which is located on the lower reaches of the Yangtze River, she said.

Citing satellite data, she said the earth under Nanjing has actually been rising overall. This is probably due to the city government’s decade-long effort to tackle the issue. In 2013, Nanjing banned the direct extraction of groundwater for ground-source heat pump systems in certain buildings in densely built areas.

In 2019, the city’s natural resources department also unveiled a plan, which runs through 2025, requiring builders to conduct detailed surveys of construction sites, especially soft-soil sites.