Water from Extreme Storms Like Irene Takes Years, Longer than Expected, to Flow Through Forest Soils into Streams

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AMHERST, Mass. – In a surprise finding from the first multi-year analysis to trace stream and groundwater isotopes after major flood events, hydrogeologist David Boutt and geoscientist
Qian Yu, with state geologist Stephen Mabee at the University of Massachusetts Amherst, offer evidence that floodwater spent much longer moving through southern New England surface- and groundwater systems than expected after Tropical Storms Irene and Lee in 2011.

A major assumption in most isotopic studies is that these trace markers are stable over time in surface and groundwater on an annual basis, lead author Boutt and colleagues note. But they detected isotope signatures in water attributed to those two extreme storms still percolating through hydrological systems in New England four and five years after the events. They now suspect the process could be even longer, “much more than we thought before,” Boutt says. Details appear in the current issue of Geophysical Research Letters.

Their analysis addresses a long-standing misconception in the hydrologic community about forest soils and whether they are so permeable that they’ll accept water indefinitely, Boutt says. To a hydrologist, this means the pathways allow new water to filtrate more easily, he adds.

It’s the same phenomenon seen in home houseplants; if the soil in the pot is bone dry, a watering will be readily absorbed and may even pool on the surface. But if the soil is damp already, any water added will run quickly through to the bottom. The fact that Vermont and Massachusetts soils were already very wet in 2011 allowed floods from Irene to be as huge as they were, he explains, with the new precipitation moving quickly through wet soils to streams.

He adds, “What we’ve identified is that fluctuations in the subsurface water are not just happening on an event scale or an annual scale; these happen over many years. This is new, and a lot of people will be surprised that four or five years after these storms, we can still see signatures of this water in streamflow. It turns out that these big events have an outsized impact, and seeing how long they can last is eye-opening.”

Boutt says, “This water enriched with heavy isotopes is a signature that provides direct evidence that during August and September we had a lot of water become stored in groundwater in a short time, more than we’ve ever seen. It illustrates the speed of the transit. Now, it is slowly diluting, mixing in and declining in concentration over time, a process we’re still watching today.”

He adds, “One of the big implications is that last year, in 2018, we set all these records for rainfall, so we may see more water infiltration to soils and increases in the elevation of the water table during times of year when the water table should be low and declining.

Further, “I imagine that similar scenarios played out in the hurricane of ‘38, especially if the right antecedent soil conditions were in place; observers probably saw a similar response in groundwater. As southern New England continues to be inundated by precipitation, this study suggests that we are going to have increased impacts of tropical storms and heavy rains in the form of flooding, rising water tables, and saturated soils,” he notes.

To evaluate the response of stream‐aquifer systems to such extreme precipitation events, they traced 743 surface and groundwater isotopes over seven years, including pre-Irene samples, in drainage areas ranging from 0.1 to more than 800 square km (~300 sq. mi). Results of this analysis, which was supported by the National Institute of Water Resources, show the long-lasting impact of these storms on isotopic composition of shallow groundwater, they report.

Boutt and colleagues say their results suggest that hydrogeologists should re-think the impact of large events on stream-flow generation processes, residence time estimates and interpretation of surface water isotope composition and trends. This impact is not only important to human communities but to the health of the watershed and its biogeochemistry, Boutt adds. “This is another insight in how watersheds process water, and it suggests we should be looking at this in a really new way.”

The year 2011 was, at the time, the wettest calendar year on record for western Massachusetts, the researchers point out. August and September that year are the wettest consecutive two-month period in the 123-year precipitation record. The ground was already saturated when the storms came, which led to increases in soil moisture and high infiltration rates.

Hydrogeologists once thought that major floodwaters would clear through local surface- and groundwater systems in a short time, a single year, Boutt notes. New isotope-tracing techniques allow scientists to use unique signatures in rainwater to pinpoint its origin, to map the water’s residence time in the ground and its progress back to the surface.