Lyn Watts preparing to measure water levels, temperature, chemistry and streamflow of the surface water in the stream and the groundwater below it. Credit: Christine Hatch
Research

How to Rewild a Wetland (Hint: Focus on the Groundwater)

Using drones and thermal infrared imagery, UMass Amherst researchers show how best to restore wetlands—and why Massachusetts is leading the charge toward healthy ecosystems

Using a first-of-its-kind approach that entailed drones and infrared imagery, researchers from the University of Massachusetts Amherst investigated a series of former commercial cranberry bogs in eastern Massachusetts that are being restored. The team not only demonstrated how to best restore freshwater wetlands, but also showed that these wetlands are operating as self-sustaining ecosystems. The work was recently published in a pair of papers in a special issue of the journal Frontiers in Earth Science.

For generations, eastern Massachusetts has been the cradle of cranberry production in the United States, and currently has more than 14,000 acres under cultivation. Cranberries thrive in acidic peat bogs, which are the legacy of the last ice age. When Euro-Americans began commercially cultivating cranberries in the mid-1800s, they did so by drastically altering the natural freshwater wetlands in order to improve yields.

Collecting water quality parameter in the early post-restoration bog. Credit: Christine Hatch
Collecting water quality parameter in the early post-restoration bog. Credit: Christine Hatch
Lyn Watts holding a Real-Time-Kinematic Global Positioning System to log the precise location of the Ground Control Points (GCP’s) used to tie individual drone images together. Credit: Christine Hatch
Lyn Watts holding a Real-Time-Kinematic Global Positioning System to log the precise location of the Ground Control Points (GCP’s) used to tie individual drone images together. Credit: Christine Hatch
Hatch and Watts help rake cranberries for harvest at the UMass Cranberry station in Wareham, MA. These berries are heading to the Ocean Spray Cooperative. Credit: Christine Hatch
Hatch and Watts help rake cranberries for harvest at the UMass Cranberry station in Wareham, MA. These berries are heading to the Ocean Spray Cooperative. Credit: Christine Hatch
Lyn Watts flying the UAV at Foothills Preserve from a temporary construction bridge during restoration. Microtopography has been dug to the right of the straightened farm channel to help capture more of the abundant groundwater that surfaces here. Credit: Christine Hatch
Lyn Watts flying the UAV at Foothills Preserve from a temporary construction bridge during restoration. Microtopography has been dug to the right of the straightened farm channel to help capture more of the abundant groundwater that surfaces here.
UMassAir pilot and co-author Ryan Wicks conducts pre-flight check of the UAV under the full moon at dawn. Credit: Christine Hatch
UMassAir pilot and co-author Ryan Wicks conducts pre-flight check of the UAV under the full moon at dawn. Credit: Christine Hatch

“Instead of thick masses of peat,” says Christine Hatch, extension professor of earth, geographic and climate sciences at UMass Amherst, lead author of the paper on recovering groundwater and a member of the commonwealth’s Water Resources Commission, “these human-altered cranberry bogs look like a Kit Kat bar when you dig down into them.” That’s because chocolate-colored layers of peat are interspersed with wafer-colored layers of sand. Whereas peat acts like a sponge, soaking up and holding groundwater, the sand, deposited in inch-thick layers by cranberry farmers every few years over the past 15 decades, acts like a drain and can help get rid of excess water, as well as increase yields and suppress weeds and pests.

Such sand-filled bogs, which the authors call “anthropogenic aquifers,” perform very differently from natural ones. As small family-run cranberry bogs cease production, the question has arisen, what to do with them?One answer: return the bogs to their natural state—which is exactly what the state of Massachusetts is doing. “Massachusetts has recognized that wetlands are incredibly important resources,” says Hatch. “They’re the most biodiverse ecosystems we have. And they perform all sorts of ecosystem services, from managing floodwaters, to storing carbon and purifying drinking water. They’re also fantastic sites for recreation. The state has committed generous resources to restore these wetlands, which makes me proud to live in Massachusetts.”

How to Restore a Wetland

 

Shaded relief LiDAR terrain DEM showing the locations of samples, monitoring, and piezometers at Foothills Preserve (Town of Plymouth) and Tidmarsh Wildlife Sanctuary (Mass Audubon) on a shaded relief LiDAR. (Inset) Location of the two focus cranberry farms/restoration sites. Credit: Hatch et al., 10.3389/feart.2022.945065
Shaded relief LiDAR terrain DEM showing the locations of samples, monitoring, and piezometers at Foothills Preserve (Town of Plymouth) and Tidmarsh Wildlife Sanctuary (Mass Audubon) on a shaded relief LiDAR. (Inset) Location of the two focus cranberry farms/restoration sites. Credit: Hatch et al., 10.3389/feart.2022.945065

Though Hatch notes that it’s easier to restore what was once a wetland than to create a new one from scratch, it is still quite a complicated task to undo 150 years of landscaping. water moved through the old, peat-filled bogs incredibly slowly, it courses much more rapidly through the human-made anthropogenic aquifers, draining off and ultimately disappearing from the wetland ecosystem. Restoring the wetland means returning the groundwater to its slow pre-agricultural rate of flow and holding on to that water.

It’s not enough to simply pull out all the drainage pipes and fill the ditches that farmers have laid and dug over the generations. “You have to deal with all that sand,” says Hatch. “In the perfect scenario, we’d dig it all out, down to the untouched deposits of solid peat,” she continues, “but that’s cost-prohibitive and risks disturbing decades’ worth of pesticides that growers have sprayed over their bogs.”

(A) The anthropogenic aquifer at Foothills Preserve is bounded by the cranberry mat at the surface and the peat at the base. (B) A visual description of a sediment core through the anthropogenic aquifer yields a kit-kat bar of alternating sand-and soil layers. Credit: Hatch et al., 10.3389/feart.2022.945065
(A) The anthropogenic aquifer at Foothills Preserve is bounded by the cranberry mat at the surface and the peat at the base. (B) A visual description of a sediment core through the anthropogenic aquifer yields a kit-kat bar of alternating sand-and soil layers. Credit: Hatch et al., 10.3389/feart.2022.945065

Hatch and her colleagues conducted their research at two sites near Plymouth, where they cored the soil, collected water samples, monitored the location of the groundwater and the speed at which it moved and measured water temperatures and levels. Armed with this data, they discovered that it’s not necessary to remove the sand from the bog for it to return to its pre-agricultural state. It’s only necessary to move it around, mixing it into the layers of peat, enough. But how much is enough?

Mapping success

 

• Comparative UAS imagery from pre-restoration (top row) to post-restoration. From left to right: (left) plain light (RGB image), (center) thermal infrared, and (right) surface expression of groundwater (black = groundwater and gray =mixing zone). This figure shows an area where the channel was reconstructed from a series of ditches into a meandering channel. The red box indicates a single groundwater seep whose flow direction changed with the movement of the stream channel. Credit: Watts et al., 10.3389/
Comparative UAS imagery from pre-restoration (top row) to post-restoration. From left to right: (left) plain light (RGB image), (center) thermal infrared, and (right) surface expression of groundwater (black = groundwater and gray =mixing zone). This figure shows an area where the channel was reconstructed from a series of ditches into a meandering channel. The red box indicates a single groundwater seep whose flow direction changed with the movement of the stream channel. Credit: Watts et al., 10.3389/fenvs.2022.946565

The answer to that question hinges on how much and how slowly groundwater moves through the bog. In order to track the and measure the movement of groundwater, Hatch and her graduate student, Lyn Watts, lead author of the paper on mapping groundwater, as well as co-author Ryan Wicks, of UMass Amherst’s UMassAir, took to the air during the pre-dawn hours in the dead of winter during 2020 and 2021.

Post-restoration UAS-derived thermal infrared imagery 14 February 2021. Ground surface is cold (blue). Much of the surface has been altered with microtopography, allowing more free expression of warmer groundwater (red and orange) areas, which flow into the reconstructed meandering channel. Credit: Watts et al., 10.3389/fenvs.2022.946565
Post-restoration UAS-derived thermal infrared imagery 14 February 2021. Ground surface is cold (blue). Much of the surface has been altered with microtopography, allowing more free expression of warmer groundwater (red and orange) areas, which flow into the reconstructed meandering channel. Credit: Watts et al., 10.3389/fenvs.2022.946565

Watts, an ace drone pilot, flew a UAV equipped with an infrared camera capable of seeing heat. Since groundwater remains at a nearly constant temperature year-round, she was able to “see” how the groundwater, which was warmer than the frozen surface water, moved through the former cranberry bogs, and to map its flow across the entire system of wetlands at the study sites.

What Watts discovered was that the groundwater was spending more time moving through the restored bog, just as it would in its pre-agricultural state, and that this increased residence time allowed the bog to “fill up” with enough water for it to pool at the surface.

“We show that, at these restored bogs, groundwater is remaining in the area, not moving off of it,” says Watts. “This means that restoration is successful, and the bogs will quickly return to self-sustaining ecosystems.”

Looking beyond Massachusetts

 

Because the geology of eastern Massachusetts is similar to that throughout much of the Northeastern U.S., Hatch and Watts’s work is broadly applicable. “Our research can help restoration designers and engineers to more deliberately plan their efforts,” says Watts.

“Restoring buried wetlands to their previous ecological glory has a very high success rate,” says Hatch. “That success depends on getting groundwater to stay in the system. We’ve shown how to do that, and our research can help us conserve one of our most treasured ecosystems.”

 

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To support improvements at the UMass Cranberry Station in Wareham, in February 2021 the Baker-Polito Administration matched a $2 million commitment from the Amherst campus with $5.75 million in state capital funding for a total project cost of $7.75 million, supported by the Legislature. The work will provide infrastructure upgrades, including the design, construction, retrofitting and outfitting of enhanced laboratory space. The research supported by this funding will help Massachusetts’ cranberry industry continue to thrive as an important sector of the agricultural economy in the commonwealth.