Why San Antonio is slowly, but steadily, sinking

• While we're not sinking as fast as others, San Antonio's infrastructure is the most at risk of major cities. Subsidence can crack roads, destabilize buildings and make low-lying areas more flood-prone.

What they did: In a peer-reviewed study published this month in Nature, researchers analyzed six years of satellite radar data in the 28 most populous U.S. cities.

• They found that 25 of the 28 cities are subsiding, affecting more than 33 million people — over 10% of the U.S. population — who live on sinking land.
• The cities are sinking by 2 to 10 millimeters — or 0.08 to 0.39 inches — per year, the study found.

State of play: Subsidence has long been associated with sinking cities such as Jakarta, New Orleans and Houston — the fastest-sinking of the 28 studied — but the research shows it's a widespread issue across the U.S.

The big picture: Eight cities, including San Antonio, account for more than 60% of the U.S. population that's experiencing subsidence.

• More than 1 million people in San Antonio are exposed to the sinking, which is happening at a slower rate of about 1mm per year.

Threat level: San Antonio has the highest risk to buildings of all the cities analyzed — 1 in 45 are at high or very-high risk to the effects of subsidence, meaning structures can fracture and crack; or, in a worst-case scenario, even collapse.

• Austin had the second highest ratio, with 1 in 71 buildings at high risk.
• 82% of the U.S. buildings at very high risk are in San Antonio (1,515), Austin (706) and Houston (376).
• Risk to buildings can also depend on factors like construction practice and materials.

Zoom in: The study lists human causes of subsidence that are common in a city like San Antonio experiencing rapid population growth — namely, groundwater extraction and urban development.

• Pumping more groundwater than can be replaced "can have a direct relationship with what happens on the surface," Leonard Ohenhen, lead author of the study and a researcher at Columbia University, told the New York Times. "You can cause the ground to sink significantly."
• Drought can also exacerbate the risk of subsidence.

What they're saying: These factors are "potentially accelerating subsidence rates and transforming previously stable urban areas into vulnerable zones," the study's authors write.

• "The latent nature of this risk means that infrastructure can be silently compromised over time, with damage only becoming evident when it is severe or potentially catastrophic," researchers wrote. "This risk is often exacerbated in rapidly expanding urban centers."

What's next: Researchers urge cities to factor subsidence into zoning, infrastructure upgrades and flood planning.

• They also call for long-term ground monitoring and public outreach to ensure communities are prepared for slow but damaging shifts.

The bottom line: "We should start talking about those solutions right now," Ohenhen told the Washington Post.