A former Tyrone resident and presently a professor in the Department of Earth and Environment at Franklin and Marshall College in Lancaster, Dr. Dorothy J. Merritts made an interesting hydrology discovery a few years ago while scouting research sites for her students.
At Franklin and Marshall, Merritts teaches geomorphology, hydrology, and environmental geology, but when the 1976 Tyrone Area High School graduate was examining photographs of the silty banks of the West Branch of Little Conestoga Creek, she realized something was different – the silt was laminated and deposited in layers.
After the 50-year-old professor consulted with a sediment expert, Robert C. Walter, who eventually became Merritts husband, the two agreed that the sediments were not deposited by typical stream processes but instead, were pond sediments. The finding brought the two colleagues to collaborate on a research project on the region’s waterways in Lancaster County.
Their work was published in the journal Science and a story was featured in The New York Times, among numerous other publications. In the Times article, it was reported that “their work challenges much of the conventional wisdom about how streams in the region formed and evolved. The scientists say 18th- and 19th-century dams and millponds, built by the thousands, altered the water flow in the region in a way not previously understood.”
Merritts said that the legacy of all of the early dams in the region, and around Pennsylvania, still affect the state’s waterways. Those dams and millponds pose a threat to the Chesapeake Bay.
“There are two things about the bay that researchers and policy-makers worry about – fine grained sediment and nutrients,” stated Merritts. “Robert and I showed that there’s a previously unrecognized source of fine grained sediment. Until now, it was commonly regarded that the majority of sediment in streams was eroded from farms and construction sites.”
When the old dams deteriorated, failed during storms, or were removed around the state, the water was freed to flow more swiftly. Streams began incising channels through the beds of silt, then the fine material eroded rapidly, sending tons of sediment downstream. Much of the sediment carried agricultural chemicals like nitrogen and phosphorus.
Her husband started to conduct chemical analyses when the couple made their discovery of the storage of so much “legacy sediment,” and he found high levels of phosphorus in it. The Chesapeake Bay Tributary Strategy aims to reduce nutrient pollution permanently, such as nitrogen and phosphorus, consistently throughout the bay’s watershed.
Merritts noted that phosphorus attaches to clay, and nitrogen doesn’t attach like phosphorus does – but organic matter in the fine sediments can trap the nitrogen. She said when the banks of the old ponds become eroded, it’s not just silt and clay but also phosphorus and some nitrogen making its way into the water.
“We found that no one knew about this and it was not being regulated,” said Merritts.
In Lancaster County, Merritts and Walters estimated that more than half of the 180,000 tons of sediment flushed out of the Conestoga and into the Susquehanna River each year originates from legacy sediment, and not new sources of sediment being washed off farms or development sites.
The couple also thinks that the legacy sediment each year carries with it around 135,000 pounds of phosphorus. That number is a little more than two percent of the phosphorus reduction goal for the entire 64,000-square-mile Chesapeake Bay watershed.
Pennsylvania’s agricultural industry collectively is the largest contributors of nutrients to the bay’s tributaries, discharging 46 percent of the nitrogen and 58 percent of the phosphorus flowing into the bay from the several states in the watersheds. Around 20 percent of the problem is coming from sewage treatment plants, and around 17 to 20 percent of the problem comes from state forest land.
Merritts and Walters discovery could alter those numbers.
“Sewage plants around here are very interested in our work to seek credits through the state’s nutrient trading program,” said Merritts. “There’s hope though in the next decade or so for sewage authorities, because many can’t afford the upgrades – it could take a long time to process all of this new information through the policy process.”
The nutrient trading program gives sewer plants the opportunity to sell nutrient credits if the facility goes beyond its requirements. All sewer and industrial facilities must stay below the new caps set on nutrients.
Tyrone’s wastewater treatment facility upgrades are estimated to cost ratepayers $3.7 million in total, but $800,000 of the upgrades are required over the next few years. The rest of the upgrades will be done as it becomes feasible for the borough.
Merritts’ discovery reaches further than the Chesapeake Bay’s problems. She said that the legacy sediment in deep layers could impact streamside forest planting, because the roots of the trees may never reach shallow groundwater where nitrogen is absorbed to survive.
Deep legacy sediment could also be unsuitable for infrastructure such as roads, bridges, and sewer lines, due to the possibility of erosion.
Merritts’ discovery of legacy sediment will have a major impact on how scientists look at stream restoration and the chemical pollution affecting waterways flowing into the Chesapeake Bay. She had many positive influences while growing up in Tyrone through school, church, and local organizations.
“I hesitate to name specific individuals, because so many teachers had a positive influence,” said Merritts. “I was in band, chorus, and many other organizations, and benefited from all of them.”
She added, “The surroundings in Tyrone were especially important, as I enjoy the outdoors, hiking, and field work, and to this day I still do a lot of field work every week. I loved archaeology and anthropology, and enjoyed hiking the hills around Tyrone to hunt for old glass bottles, fossils and minerals.”
Merritts is an author of two textbooks and more than 40 scientific articles, and the editor and contributing writer for numerous scientific books. In 2004-05, she was the Flora Stone Mather Visiting Distinguished Professor at Case Western Reserve University in Cleveland, OH. She was also the recipient of the Association of Women Geoscientists’ Outstanding Educator Award in 2006.
“I had excellent teachers at Tyrone, and I was fortunate to have influential adults in my life from the Wesley Methodist Church and other local organizations,” said Merritts.
