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Filtering Through the Landscape's History
A springtime thunderstorm rumbles across Lancaster County, pouring sheets of fresh rainwater onto the fertile countryside. The rain runs down hills and into valleys, through creeks and streams as it makes its way toward the Susquehanna River. Soon, the river spills the rainwater into the Chesapeake Bay, the largest estuary in the United States.
As Dorothy Merritts and Robert Walter '75 have discovered, the water has taken part of the landscape's history along for the ride.
Faculty members in the Department of Earth & Environment, Merritts and Walter have been studying Lancaster County stream water as part of a research project since 2003. They determined that tens of thousands of mill dams once existed in Pennsylvania, Maryland and other mid-Atlantic states, forming slackwater ponds that trapped sediment over a period of centuries. As aging dams break, this mud is released into the waterways. The sediment has lowered the water quality in the Chesapeake, which was declared an impaired water body in 2000 by the United States Environmental Protection Agency (EPA).
Officials from the EPA, Pennsylvania Department of Environmental Protection and United States Geological Survey have taken an interest in the professors' research, working with F&M students on a long-term project to test the possibility of restoring the natural ecological function of the landscape. They visited Lancaster last week to take water samples with 25 students in Walter's class, The Dynamic Earth, in an exercise dubbed "The Sample Slam."
Students in Robert Walter's class, The Dynamic Earth, take a break from their work at Big Spring Run during last week's "Sample Slam." The students collected water samples to test the possibility of restoring the natural ecological function of the landscape.
"After every rainstorm, our streams run like chocolate milk," Walter says. "We've come to recognize that the greatest stream pollutants in the Chesapeake are high suspended-sediment loads and high nutrient loads. That presents all sorts of water-quality issues."
Merritts and Walter embarked on the research seven years ago when then-student Lauren Manion '03 returned from the field after observing a 20-foot-high cut bank in the west branch of the Little Conestoga River.
"If the bank was caused by a meandering stream over the course of time, you would see a very characteristic set of features, from gravel all the way down to silts," Walter says. "We saw none of that. We only saw finely laminated mud. Standing water is the only way to produce that."
Merritts and Walter soon found the source of the standing water: the obscure remnants of an old stone dam, 100 yards upstream. The dam had trapped sediment—mostly soil erosion from land clearing by early American settlers—in a mill pond for centuries. When the dam failed, water cut down through the sediment like a knife through butter, Walter says.
Since 2003, the professors have located nearly 400 similar dams in Lancaster County. But Merritts says the landscape was an entirely different type of ecosystem prior to human settlement.
"Valleys in this area once consisted of beautiful wetlands, watered by ubiquitous freshwater springs," says Merritts, who uses sediment layering to construct the history of buried landforms. "We don't see these today. Wetlands were buried by sediment eroded from farms, and now appear in a thin, black layer of soil beneath many feet of mud."
Merritts and her students extract seeds from the buried black soil to determine what types of plants grew in the wetlands. They find between 20 and 50 seeds in a single teaspoon of soil, mostly those of the tussock sedge—prolific in the region's wetlands centuries ago.
Wetlands act as water filters, lowering pollutants in larger bodies of water such as the Chesapeake. That is one reason why the EPA, which has spent millions creating wetlands, has taken such an interest in the work of Merritts and Walter. The professors are nearing the end of a two-year monitoring period of Big Spring Run, where the "Sample Slam" took place last week.
"We're testing a new kind of stream restoration, one that we think will result in substantial water quality improvements," Walter says. "As much of the mill pond mud as possible will be removed, restoring the landscape's ecological function."
In the process, they are creating a new chapter in the landscape's history.