It might not feel like it, but the ground under our feet is sinking.
Using satellite data, researchers were able to precisely map rates of subsidence, the gradual sinking of land, in dozens of major U.S. cities, including Columbus.
The subsidence rates are small, just a few millimeters per year, but the research raises some big questions about how this issue could impact cities’ infrastructure and development in the long term.
Professor Manoochehr Shirzaei heads up the Earth Observation and Innovation Lab at Virginia Tech. He co-authored the study, published recently the journal Nature Cities.
Matthew Rand: When we talk about subsidence in cities like Columbus, how much sinking are we really talking about, and how is this happening?
Manoochehr Shirzaei: So, the majority of the land subsidence that you experience in Columbus is driven by a natural process. We call it glacial isostatic adjustment.
So, the northern part of the America was covered by thick layer of ice about 15,000 years ago. Since then, that thick layer of ice has melted and land in Canada is rising, and the perimeter of that area—which is part of United States, specifically northern part and eastern part—are subsiding, and we are talking about one to three millimeters [per year] of the steady subsidence happening in Columbus, Ohio.
On top of that, in part of the region, we have human activities like groundwater extraction, which is the main component of the human activities driving the land subsidence that we observe today.
Matthew Rand: How does groundwater extraction work and how does it end up contributing to this phenomenon of ground sinking in cities like Columbus?
Manoochehr Shirzaei: Aquifers, specifically confined aquifers are where the good water is stored under the ground. So, we use that water for drinking, agriculture, economic activity and that's a precious water.
In addition to that valuable service, that water holds the ground above the aquifer steady. So, when we extract the water from the aquifer, specifically a confined aquifer, we empty those porous spaces and those porous spaces no longer can hold the weight of the material above them and they begin to collapse inward. As a result, land above them subsides.
Matthew Rand: How much and how fast is the ground sinking in Columbus, compared to the other cities you looked at in this study?
Manoochehr Shirzaei: So, Columbus land subsidence is notable compared to many other cities in the country, but it's not near what we have at the fastest-sinking cities such as Houston and San Antonio in Texas.
Those cities experience subsidence due to human activity, which is groundwater, but also oil and gas exploration—a different type of fluid, but the same process.
But also, they have been affected by compaction of the sediments in the coastal region. So, in that sense, Columbus, Ohio is not at as critical a condition compared to those. Nevertheless, the rate of land subsidence in Columbus is also one of the fastest that we saw in the country.
Matthew Rand: Columbus, as we know, is growing fast, with lots of new construction and infrastructure. What kinds of things—roads, buildings, utilities and the like—are most vulnerable if this subsidence trend continues?
Manoochehr Shirzaei: So, the good news is that land subsidence that we monitored and we detected are slow, relatively speaking. Therefore, there is no immediate threat to any infrastructure.
However, if these rates continue over the time, and we do not implement any adaptation and mitigation strategy to slow down or stop the land subsidence, in a decade or two, we expect to see significant impact on infrastructures, particularly roads, bridges, and building foundations due to this land subsidence.
