Slow-moving Eastern Shore groundwater could delay full impact of nitrogen reductions for decades
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Fertilizers placed on Delmarva fields when Dwight D. Eisenhower and John F. Kennedy were presidents — and even earlier — are offsetting current efforts to reduce nutrients from the Eastern Shore because much of that nitrogen is still traveling through slow-moving groundwater, a new study concludes.
The study, by scientists with the U.S. Geological Survey, found that much of the nitrogen placed on the vast swaths of Eastern Shore farm fields takes longer to reach the Bay than nitrogen from fields farther away in other parts of the Chesapeake watershed because groundwater typically moves significantly more slowly under the Delmarva Peninsula.
The study, recently published in the journal Environmental Science & Technology, found that on average, nitrogen inputs to Eastern Shore streams from groundwater are continuing to rise. It estimated that a 13 percent reduction in nitrogen application was needed now just to maintain the current nitrogen levels entering the rivers in 2050.
As a result, stated the paper by USGS scientist Ward Sanford, “several decades will be required to see the full effects of current and future [best management practices]. The magnitude of this time lag is critical information for Chesapeake Bay watershed managers and stakeholders.”
The notion that such groundwater “lag times” exist between when nutrients are applied to the land and when they affect water quality is not new. Earlier work by the USGS in other parts of the Bay watershed showed that, on average, about half of the nitrogen applied to fields, lawns and other land uses made it to streams within a year, either directly running off the land or moving through shallow groundwater. About 25 percent reaches streams within 10 years, with the remainder takes between 10 and 50 years.
But the new study found that on the Eastern Shore, only about a quarter of the nitrogen applied to land makes it to streams within a year. The median time to reach streams is 20 – 40 years.
“That doesn’t mean nothing is going to happen for 20 years and all of a sudden you will see a response,” Sanford said. “It means half of the response will have occurred before that time, and the rest will come after that time.”
Significant amounts of Eastern Shore nitrogen take more than half a century to reach the Bay, Sanford’s study found. About a third of the nitrogen entering the Choptank — the largest river on the Eastern Shore — is more than 50 years old, according to the study. The percentages are slightly higher for the Blackwater, Wicomico and Pocomoke rivers.
Nitrogen applications on agricultural lands began increasing after World War II, reaching their peaks in the 1960s and 1970s before they started to taper off. Much of the nitrogen reaching streams today reflect the large applications made decades ago.
The nitrogen reductions being sought in the Chesapeake Bay Total Maximum Daily Load for the Eastern Shore is about 25 percent. Because so much “old” nitrogen is working its way through the system, the paper said that level of reduction in the amount of nitrogen reaching the groundwater would only achieve about a 12 percent reduction in the amount of nitrogen ultimately reaching streams by 2050.
The reason for the slow movement on the Delmarva is that water infiltrates its sand and clay soils slowly, and the shore’s flat topography provides little push to move water through the system. In contrast, fractured rock and steeper slopes found in the Piedmont and mountain areas of the watershed allow water — and the nitrogen it carries — to move more rapidly through aquifers.
“There is very little topographic relief on the Eastern Shore, which results in very slow movement of groundwater between where it recharges in a watershed and where it is discharges into the streams draining the area,” said Scott Phillips, USGS Chesapeake Bay Coordinator.
Water easily absorbs nitrogen, so how water flows over, and under, the surface dramatically influences how long it takes for the nutrient to reach the Bay.
The scientists examined the concentrations of chlorofluorocarbons and radioactive particles in the groundwater samples drawn from springs in the Delmarva Peninsula to determine its age. Since the 1940s, different levels of CFCs have entered the atmosphere each year and the ratio of tritium to helium in particles has varied as the result of nuclear tests. Those concentrations are reflected in the rain, which absorbs the elements, giving scientists a way to age the water when it emerges from springs.
Knowing how fast the water flowed provided the information to develop a model that estimates how long it will take nitrogen applied to the land in different areas to reach Eastern Shore streams.
Although the results generally show that water quality improvements to the Eastern Shore creeks and rivers will be slow, Phillips said the results can be used to prioritize nutrient reduction efforts toward areas where local water quality results will be seen more rapidly. In fact, the study said nitrogen levels reaching streams would be even higher today if not for conservation actions taken during the last two decades.
“Theoretically, we can use these results to focus on areas that have younger groundwater travel times and implement best management practices,” Phillips said. “We will hopefully see water-quality improvements more quickly than in areas where groundwater takes decades or longer of travel time.”
The Chesapeake Bay TMDL calls for all practices needed to clean up the Bay to be implemented by 2025, but officials recognized that issues such as lag times mean that actual clean water goals will not be met until sometime after that. The new report suggests that the delay may be longer than earlier thought, particularly for tidal rivers and parts of the Bay where water quality is directly influenced by nitrogen loads from the Eastern Shore
“As 2025 approaches, we must remind the public that lag times exist and ask for their patience in seeing a healthy Bay,” said Nick DiPasquale, director of the EPA Bay Program Office in a recent blog post. “Because through patience—and vigilance—the Bay will be restored.”
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