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Bay Journal: Legacy Sediment Behind Old Mill Dams Could Help Heal Abandoned Mine Land

By Ad Crable, Chesapeake Bay Journal

Fertile sediment bottled up behind old mill dams in Pennsylvania is a relentless source of nutrient and sediment pollution in local waterways that flow toward the Chesapeake Bay.

But it could become a prime ingredient in restoring another scourge in the state: abandoned mine land.

Documented as a sediment and nutrient pollution problem for the Bay about 15 years ago, legacy sediment is topsoil that ran off the land long ago from farm fields and logging areas throughout the Bay region and piled up behind mill dams, many of which date back to colonial times.

As the dams have been torn down or abandoned, the now-elevated streams seek their original course, cutting through the soft earth that buried them and sending the soil and attached nutrients into the water.

Also, the gantlet of mill ponds buried and altered healthy stream channels that once were a braid of connected streams with floodplains buffering nearby land from rising water.

In Pennsylvania, the 126-square-mile Chiques Creek watershed is one of the largest sediment and nutrient polluters in Lancaster County, itself one of the Bay’s top sources of such pollutants with one of the highest concentrations of easily erodible legacy sediment.

More than 400 old mill dams have been mapped in Lancaster County alone.

Primarily surrounded by farmland and urban development with scant forest cover, the Chiques watershed has 48 dam sites that have been identified through satellite imagery.

All but nine of the dams are gone, resulting in slow but persistent pollution from the easily erodible soil in the former mill ponds.

About 70 million pounds of sediment from banks up to 8 feet high erode each year, washing toward the Susquehanna River and on to the Bay, according to the Susquehanna River Basin Commission.

Sediment makes the water murky, smothering fish habitat and blocking sunlight for underwater grasses. It also carries nutrients, which contribute to harmful algae blooms and the Bay’s oxygen-starved “dead zone.”

Partners in the Chiques Creek Legacy Sediment Removal Project would like to turn that problem into a solution. If the project secures enough funding, as much as 283,000 cubic yards of legacy sediment stranded on streamside terraces would be dug up at 10 sites.

The nutrient-rich soil would be trucked or taken by train to spread on abandoned mine land, where vegetation struggles to grow in the acidic soil.

With an estimated price tag of about $10 million, the project is being assembled by the U.S. Department of Agriculture’s Natural Resources Conservation Service, the nonprofit Water Science Institute, and the Lancaster County commissioners and soil conservation district.

NRCS has already provided $800,000 from its small watershed improvement program to study the idea’s potential.

“I personally feel we will never solve the nitrogen and phosphorus and sediment problem unless we address some of these legacy sediments,” said Denise Coleman, the NRCS state conservationist in Pennsylvania. “A lot of the farmers today are doing all the right things as a farming community, but we still have huge amounts of sediment loading.

“We’re testing this [approach] because we have this problem all up and down the Mid-Atlantic and the Chesapeake Bay.”

The proposed project could cut the annual 70-million-pound sediment loading in half. In addition, the watershed, with its 200 miles of streams, has long been flood prone.

Restoring floodplains and long-buried wetlands could reduce flood threats considerably, according to the Water Science Institute.

Habitat in and along the streams could benefit, too. Cooler water often results after legacy sediment is removed, aiding fish habitat. And scientists have found that, in many cases, long-buried native seeds sprout again.

“If done correctly, these things bounce back with new growth like Chia Pets with benefits for wildlife habitat,” said Joe Sweeney, executive director of the Water Science Institute.

The partners have spent two years documenting the problem and planning creative solutions. The full-scale project would evaluate the impacts for the creek, its tributaries and the mine land.

Officials say the costs of transporting the soil to mine sites would be weighed against the environmental benefits to determine if the strategy makes economic sense.

By far, transportation costs are the biggest barrier.

“If we can find someone who wants that legacy sediment, it really lowers that cost and it’s not just a waste product,” Sweeney said.

Project leaders are encouraged by the increased funding available in the NRCS small watershed improvement program. Plus, there is a huge pot of money coming to Pennsylvania for abandoned mine land cleanup under the federal Infrastructure Investment and Jobs Act.

The legislation authorized $6.4 billion for such purposes in Pennsylvania, Maryland, Virginia and West Virginia.

“We are very, very, very interested in supporting dredge or legacy sediments to mine lands,” said John Dawes, executive director of the Foundation for Pennsylvania Watersheds, a group that works on improving mine lands and treating acid mine drainage.

“There are 119,000 acres of scarred mine lands in the Pennsylvania portion of the Bay watershed. They’re like moonscapes. They produce acid mine drainage every time it rains, and they kill 2,000 miles of streams.

“The highest and best use of those lands would be reforestation. But I’m an agriculturist. So to put that back into grazing land or usable farmland would be just fantastic also,” he said.

Coleman, of the NRCS, said that although reclaiming abandoned mine land would be a priority, project managers would consider other uses of the sediment, such as repairing industrial brownfields, improving fertility of farm fields, capping landfills or selling it as a soil additive, possibly with biochar added to increase carbon sequestration.

With a high content in silica, the material might even be used for making cement.

[For more information, visit LandStudies’ Legacy Sediment and Floodplains webpage, DEP’s Floodplain Restoration webpage.]

(Photo: The remains of a mill pond on Chiques Creek in Lancaster County, PA, have filled over time with water and vegetation. Shown at the site are Joe Sweeney (rear) of the Water Science Institute, Matt Koffroth of the Lancaster County Conservation District and Heather Smeltz of the Natural Resources Conservation Service by Ad Crable.)

(Reprinted from Chesapeake Bay Journal.)

Resource Links:

-- Mark Gutshall, LandStudies, Talks About Legacy Sediments, Multiple Benefits Of Floodplain Restoration - Video

-- Governor’s 2023 Environmental Excellence Award: Dover Twp., York County Fox Run Floodplain Restoration

-- EPA: Stream Restoration Project Shows Benefits Of Removing Legacy Sediments [Big Spring Run, Lancaster County]

-- USGS: Effects Of Legacy Sediment Removal On Nutrients And Sediment In Big Spring Run, Lancaster County

NewsClip - Legacy Sediments:

-- Franklin & Marshall College Students, LandStudies Research Ancient Climate Through Stream Restoration

Related Articles - Legacy Sediments:

-- Franklin & Marshall College Launches Chesapeake Bay Watershed Initiative Based On Legacy Sediments Research [PaEN]

-- Water Science Institute, Partners Celebrated Big Spring Run Legacy Sediment Stream Restoration Project In Lancaster County [PaEN]

-- Water Science Institute Storymap Shows How Old Mill Dams, Land Use & Legacy Sediments In Lancaster County Affect Water Quality [PaEN]