Water from large precipitation events, such as tropical storms Irene and Lee in 2011, circulated in New England surface and groundwater for four to five years, longer than previously thought, hydrogeologists from the University of Massachusetts Amherst say.
More water probably soaked into the ground during those two storms because the soil was already wet. If the ground had been drier, the water would have been more likely to run off, researchers said.
“Because New England has been having wetter and wetter conditions over the last 30 to 40 years, events like this allow more water to be going through the soil,” said hydrogeologist David Boutt, lead author of the recently published study in Geophysical Research Letters.
Using the isotopic signature of water, Boutt and his team traced surface and groundwater in Western Massachusetts over seven years, beginning in 2011. They discovered that a lot of stormwater was quickly stored as groundwater during August and September 2011, when Irene and Lee battered the Eastern United States and caused major flooding.
The study is considered to be the first multi-year analysis that traced stream and groundwater isotopes following major flood events, UMass Amherst said in a statement.
The soil was already saturated with water when Irene and Lee hit, which allowed the stormwater to move quickly into the soil, pushing the water that was already in it down, toward streams. That’s why there was more flooding, he said.
“An event like this produced much more flooding than we would anticipate, just based on precipitation, and one of the reasons was because the soil was wet before tropical storms Irene and Lee,” Boutt said.
Boutt said it was “a long-standing misconception” among hydrogeologists that water would pool or “glide over” the ground if the soil was already saturated with water. His research suggest that instead the wetter soil allows for more water to infiltrate, or soak into, the soil.
The increased infiltration could lead to increased flooding and rising water tables — the level at which the soil is permanently saturated with water, Boutt said.
“This could mean water in basements, or even septic tank failures, which can lead to worsening water quality,” he said.
Boutt said the study could affect how scientists look at such events in the future.
The new research “impacts everything that we know about streamwater chemistry, temperature distribution, even fish and other ecosystems,” he said.