Inside a shed in the middle of a grassy field last week, workers hammered away at a steel rod sticking out of a hole bored into the ground.
The device, called an extensometer, doesn’t look like much on land. But once installed, it will extend just over 1,400 feet underground, from this shed in the town of West Point to the bottom of the Potomac aquifer, the source of eastern Virginia’s groundwater.
With this huge rod, scientists can measure something tiny: “sub-millimeter changes of motion,” said Greg Connock, a geologist with the U.S. Geological Survey in Virginia.
In other words: sinking land.
Called land subsidence, the process has been happening in Hampton Roads for thousands of years in response to natural geological forces. But it’s been accelerated by humans’ withdrawal of groundwater, and is a key factor in our region’s high rate of sea level rise and flooding.
The sinking may sound slow, but it’s happening more quickly here than most other places on the East Coast.
Recent research out of Virginia Tech has found land in southeastern Virginia is sinking twice as fast as sea levels are rising, and that the issue poses a threat to regional infrastructure.
Over time, those millimeter-level changes add up, Connock said.
“Maybe not tomorrow or next year, or even 10 years, but decades from now, how is that variability in the land subsidence potentially going to impact infrastructure in the eastern part of the state?”
Figuring it out, he said, starts with getting the best data possible.
Discovering land subsidence
Sinking land wasn’t really on geologists’ radar in Virginia until the 1970s, said David Nelms, who served as state groundwater specialist with the USGS for more than three decades before retiring several years ago.
During routine surveys, the agency noticed a pattern across the Virginia coast, he said.
The land elevation was lower than it had been during previous surveys in the 1930s. And the data mirrored patterns of groundwater withdrawal, indicating a connection between the two, Nelms said.
In the late ‘70s, the USGS installed the region’s first extensometer in the city of Franklin, where it still operates. Data from that helped confirm that groundwater withdrawal contributed to land sinking, Nelms said. They added another instrument in Suffolk in the ‘80s.
The Potomac aquifer is part of an underground coastal system that stretches from North Carolina to New York.
Humans have been taking water out of the aquifer faster than it can naturally replenish, Connock said. The lower water pressure then makes sediments shift and compact, lowering the land’s surface.
But natural forces are also at play.
Some 16,000 years ago, parts of North America were covered by massive ice sheets. The earth is still adjusting to the retreat of all that ice.
Nelms said subsidence remained mostly on the back burner for most of his career. USGS lost funding to operate its extensometers in Franklin and Suffolk, meaning there’s a years-long gap in data.
But sea level rise driven by climate change has renewed scientists’ interest — and brought a sense of urgency.
Part of Hampton Roads’ high rate of relative sea level rise is sinking land. If the land sinks by a foot and sea levels rise by a foot, for instance, the net change in sea level is two feet.
“It’s kind of morphed into something really important, especially in our area,” Nelms said. “You want to understand what’s causing things.”
“A picture that’s unfolding”
The extensometers run by the USGS here in Hampton Roads are the only ones operating on the East Coast, Connock said. The newest device being installed in West Point is the fourth.
When workers dug up the surrounding field as part of construction, they encountered plenty of evidence of its former, ancient life as an ocean. Connock said that included roughly 20-million-year-old shark teeth and seashells.
The extensometer rod and the casing surrounding it weigh roughly 50,000 pounds combined.
By touching the bottom of the aquifer, Connock said the instrument will record evidence of sediment compaction that leads to sinking.
The old extensometers in Suffolk and Franklin are back online. About a decade ago, the agency also installed one at the Hampton Roads Sanitation District’s Nansemond Treatment Plant in Suffolk.
Officials are keeping an eye on an ongoing project there they hope could help reverse local subsidence.
The sanitation district treats millions of gallons of our sewage to drinking water standards, then injects it back into the Potomac aquifer.
The goal is to provide a sustainable source of water. But officials think reducing sinking land by replenishing the aquifer could be a secondary benefit.
Nelms said they notice slight changes in the land whenever water is injected or withdrawn. Whether it can make a long-term difference remains to be seen.
Connock hopes geologists can give decisionmakers the data they need to make “meaningful change.”
The issue isn’t going away anytime soon, Nelms said.
“Everything’s moving,” he said. “It’s a picture that’s unfolding, but it’s been unfolding for quite a long time.”
