As Aquifers Are Depleted, Areas Along The East Coast Of The US Are Sinking

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A study by researchers at Virginia Tech and the US Geological Survey using the latest satellite data shows that parts of America’s east coast are sinking, a process that geologists call subsidence. The culprit appears to be caused by withdrawing too much water from the aquifers beneath those coastal areas. Not surprisingly, the situation is most apparent near the coastal cities that dot the Eastern Seaboard — Boston, New York, Baltimore, Washington, DC, Roanoke, Savannah, Jacksonville, and Miami, to name a few.

Out of sight, out of mind. That’s the way most of us approach long term problems. A vicious puma rushing toward us gets our attention right away, but worrying about something that might not happen for a decade or more? Yeah, we’ll get back to you on that. The mindset is best captured by the bumper sticker that says, “How can I be overdrawn? I still have checks!” We can’t see aquifers, so the problem is invisible to us. We turn on the faucet; water comes out. End of story.

Aquifers, Subsidence, & Infrastructure

Here is the introduction to the Virginia Tech/USGS study (footnotes deleted):

Coastal regions, where most mega-cities are located, are on the front lines of climate change impacts and associated uncertainties. The coincidence of population migration toward low-elevation coastal areas and continued accelerating sea level rise will increase the future vulnerability of coastal communities worldwide. The impact of SLR-amplified hazards on coastal communities, such as flooding and erosion, dominates in global climate change discussions, with other coastal hazards, such as land subsidence (the lowering of land elevation) relegated to the background.

Land subsidence, however, is a pernicious and growing problem on a global scale with more immediate hazards to coastal areas and often presents more pressing and localized challenges. In many nations, land subsidence barely registers as an issue of public policy. In nations, where the adverse effects of subsidence are recognized, the slow, gradual, and unapparent land sinking motion explains the lack of urgent policy interventions and subsidence governance.

The resulting delayed response increases the exposure of coastal residents, especially in light of the yearly elevation gain in sea levels due to climate change and elevation loss due to subsidence. Recent considerations of the combined effect of SLR and subsidence indicate that subsidence increases the threat to coastal communities from SLR and may even triple estimates of potential flooding areas over the next few decades.

On the East Coast of the United States, the high density of population and infrastructure networks, coupled with SLR and land subsidence hazard, increases the exposure of the population, properties, and assets in the region. The functionality, mobility, social comfort, and economic growth and development of society depend on civil infrastructure networks.

Aging stock, extreme weather events, and differential land subsidence negatively impact infrastructure networks’ safety. The 2021 American Society of Civil Engineers report card for airports, schools, roads, bridges, dams, and levees in the United States found that these infrastructures were in “mediocre” or “poor” condition, with only railways in “good” condition.

A similar infrastructure assessment in 14 coastal states on the US east coast (Florida to Maine) indicates an overall “poor” condition. The ASCE report estimates that US$786 billion, US$125 billion, and US$45.2 billion are needed for the backlog of roads, bridges, and railways maintenance. This complacency toward maintenance in a high-hazard-prone coastal area increases the susceptibility of infrastructure to failure.

Draining Aquifers Leads to Soil Compaction

“You have a hazard that is becoming worse every day with sea level rise,” lead author Leonard Ohenhen, Ph.D. candidate at Virginia Tech, told the New York Times. The global average sea level has been rising around 3.3 millimeters per year since the early 1990s, according to satellite readings from NASA. Based on longer records from tide gauges, we know that rate is accelerating, said Kenneth Miller, a professor of earth and planetary sciences at Rutgers University. Local subsidence, or sinking land, makes the threat of sea level rise worse in some places than others.

The researchers say a dominant cause for subsidence is groundwater depletion. A layer cake of overlapping aquifers extends from New Jersey to Florida along the coast, providing a reliable source of water for drinking, irrigation and industrial uses. Though the region gets regular rainfall, deeper aquifers below clay or bedrock can take hundreds or thousands of years to recharge once water is pumped out. Surface aquifers can be prone to pollution and saltwater intrusion.

Once water is removed, soils can compress and collapse, causing the land surface to sink. Cities built on drained marshland or fill are especially vulnerable to compaction. Of course, many cities are built on just such drained and filled land. Boston’s Back Bay, home to many of that city’s iconic brownstone buildings, was once a part of Boston Harbor, for instance.

Other forces can also influence vertical land movement: Sediments can build up at river deltas and naturally compress under their own weight. And as the weight of enormous glaciers that once extended down to New Jersey lifted after the last ice age, a seesaw effect is causing bedrock across the mid-Atlantic and South to sink by about 1 millimeter per year.

A few millimeters of sinking a year can seem gradual, but the effects can be extreme: storm surges can suddenly wash away the soil from beneath the roads, or floodwaters can fill basements and cut off emergency routes. Each inch the land slumps towards the water table can make floods significantly, catastrophically worse.

“That’s the thing about sea level [rise]. It’s slow and it’s insidious and continuous,” said Kenneth Miller, at Rutgers. That is, until the next big storm hits. “We’ve been fortunate in the past 11 years since [Hurricane] Sandy that we’ve not seen an event like that. We obviously are very likely to see similar events in the next ten years or so.”

Patches of land that sink or rise more quickly than adjacent areas can also do outsized damage, cracking foundations and destabilizing structures over time. The authors of the study found these distortion hot spots near Cape Canaveral, the suburbs and exurbs of Boston, and across Delaware and Maryland on the Delmarva Peninsula.

“We definitely have seen an increase in the number of incidents related to subsidence as it relates to flooding in the last decade plus,” said Sandy Hertz, director of the Office of Climate Change Resilience and Adaptation at the Maryland Department of Transportation. The state expects to see an additional 2.5 feet of relative sea level rise by 2100, according to a study published last year.

An effort among multiple federal agencies to coordinate on these regional climate risks has grown in recent years. This detailed survey of Atlantic Coast land motion was part of a broader U.S.G.S. study on a suite of coastal hazards. It adds to a vast trove of new data for decision-making around coastal risk, including where people live and rely on critical infrastructure, and where the water is now versus where it may be in 10, 20, 50 years, or beyond.

“Water does not obey geographic boundaries, and neither does land subsidence,” said Hertz. “We really need to take a shared approach to resiliency to protect not just the emergency evacuation routes, but the significant infrastructure and destinations that we have across the East Coast.”

The Takeaway

Somehow, humans have to train themselves to think about long term problems. Some places in the US are putting restrictions on new housing developments because groundwater sources are being depleted. Phoenix, Arizona, is one such place, and yet the moratorium on building new structures does not extend to commercial and industrial activities.

If aquifers can take decades or even centuries to be replenished, people should incorporate that information into their water use planning. It’s possible that new technologies for desalinizing seawater may help meet the need for fresh water but so far desalinization is an expensive way to address the need for potable water.

The normal way people address issues like over-pumping from aquifers is to wait for them to run dry and then run around trying to find someone to blame. It would be better to address the issue before it gets critical. Perhaps this report from Virginia Tech and USGS will start a conversation about how to address the issue in a timely and sensible fashion — emphasis on perhaps.