Eastern Shore’s nutrient woes exacerbated by location, topography
Slow-moving groundwater hampers effects of reduction efforts while proximity to Bay hastens runoff.
The intense poultry farming on the Eastern Shore, combined with the peninsula’s flat topography and proximity to tidal waters, has produced water quality problems that will likely plague the Chesapeake Bay for decades, a new report concludes.
While the Eastern Shore covers just 7 percent of the Bay watershed, it receives nearly twice the nitrogen and phosphorus applications per square mile as the rest of the watershed, according to a report released by the U.S. Geological Survey in March.
The amount of nitrogen and phosphorus, both nutrients, would be difficult to deal with anywhere, but a host of factors — from slow-moving groundwater to the Eastern Shore’s soils — work together to make the pollution problems associated with them even more difficult to resolve.
Figures scattered throughout the report illustrate that stark reality. Phosphorus concentrations in the Pocomoke River, a watershed filled with poultry farms and natural sources of phosphorus, are among the highest in the nation.
The amount of nitrogen seeping into Eastern Shore groundwater and eventually reaching the Bay is greater than the amount of nitrogen that reaches waterways from groundwater along the rest of the Coastal Plain from North Carolina through Virginia, Delaware and New Jersey. (The Coastal Plain, in general, is the area east of Interstate 95).
“Nitrogen and phosphorus commonly occur in streams at concentrations that may adversely affect aquatic ecosystems and have increased in recent decades,” the report said. In much of the Eastern Shore, nitrogen concentrations in groundwater are high enough to pose a human health risk as well.
The report, which summarized recent work by the USGS and other scientists, sought to explain why, decades after the region began implementing nutrient reduction programs, progress remained so elusive on much of the Eastern Shore.
The root cause of the region’s water quality problems is agriculture, which has greatly expanded and intensified over the decades — driven in large part by the region’s huge poultry production — now accounts for about 90 percent of the nitrogen and phosphorus used on the Eastern Shore. Between 1950 and 1980, applications of nitrogen fertilizers increased sixfold, to 60 million kilograms a year.
Phosphorus fertilizers similarly increased, though they have more recently decreased, the report said. But the reduction in phosphorus fertilizer has been offset, at least in part, by vast amounts of phosphorus-rich poultry litter applied to fields. Poultry production on the Delmarva Peninsula has increased from 4 million broilers in 1919 to nearly 600 million in 2007. Nearly a quarter of the phosphorus reaching the Bay from manure originates from the Eastern Shore, the report said.
Applications of nitrogen and phosphorus from fertilizers and manure have exceeded crop needs since at least the 1980s, according to the USGS.
Acre for acre, Eastern Shore application rates are similar to those in other areas with intensive animal agricultural operations, but its proximity to the Bay means more of those nutrients reach the Chesapeake.
The impacts of other places with intense animal agriculture, such as Pennsylvania’s Lancaster County and Virginia’s Shenandoah Valley, are diluted in part because they are parts of larger watersheds, which contain more forests and other low-runoff land uses. Also, being farther upstream, there is more opportunity for the nutrients to be removed before they reach the Chesapeake.
In contrast, agriculture is not only the dominant land use on the Eastern Shore — half of its land is farmed — its network of streams and drainage ditches minimizes the opportunity to remove nutrients before they contribute to algae blooms, low dissolved oxygen levels and poor water clarity.
The soils and topography further aggravate the problem.
Phosphorus tends to attach to sediment particles, which gradually move downstream during severe storms. But the Eastern Shore is so close to the Bay that its phosphorus is delivered more quickly and efficiently during storms than phosphorus from other parts of the watershed. Meanwhile, the flatness of the terrain, coupled with high application rates of poultry manure, has led to a huge buildup of phosphorus in soils that continues to run off the land.
“Besides the phosphorus that’s been eroded off the land or is moving down the streams, you’ve got the source, which keeps being refueled,” said Scott Ator, a USGS hydrologist and co-author of the report.
Complicating the phosphorus situation is that sediment particles can only hold a limited amount of the nutrient, after which phosphorus flows more freely into surface waters and even groundwater. USGS scientists speculate that phosphorus-saturated soils have led to increased phosphorus levels in the Choptank River in recent years, even as the amount of sediment, which typically carries phosphorus, has decreased.
“Because of the amount of phosphorus that’s already in the soils over there, any newly applied phosphorus is going to be less likely to find an attachment to a soil particle and might be more likely to run off,” Ator said. “At the same time, you also might have increased dissolved phosphorus transport.”
It will likely take decades for crops to reduce phosphorus in the soil to the point where it will no longer run off. And, the report noted, the anoxic — oxygen starved — water found in many Eastern Shore rivers naturally recycle phosphorus when it reaches the bottom, returning it to the water column where it can again generate algae blooms.
“We don’t have the good news story yet, if there is one,” said Judy Denver, a USGS hydrologist and co-author of the report.
The story for nitrogen is not much better. In many other areas of the Bay watershed, natural conditions in soils and shallow groundwater promote the removal of nitrogen through denitrification — a process in which the nutrient is transformed to a harmless gas. But those conditions are less common on the Eastern Shore, especially in the most intense agricultural areas.
Unlike phosphorus, which is typically tied to particles, nitrogen readily absorbs into water. The flatness of the Eastern Shore, especially in the most intensive agricultural areas, means that most of the water — and the nitrogen it carries — sinks into the ground and travels through groundwater, rather than running off the land surface.
The region’s flatness also means groundwater moves slowly, often less than a meter a day — or the length of three or four football fields in an entire year. More than half of the nitrogen traveling through groundwater on the Eastern Shore takes more than 13 years to reach a stream. Only 15 percent of the groundwater in the rest of the Bay’s watershed moves that slowly.
As a result, nitrogen applied to the fields decades ago continues to work its way through the groundwater and into the Bay — and will continue to do so for years to come. The USGS has calculated that a 13 percent reduction in nitrogen applications would be needed now just to keep overall nitrogen levels in Eastern Shore streams from increasing through 2050. A 40 percent reduction today could reduce nitrogen reaching streams to levels seen in 1980, but it would take until 2050 for those improvements to show up.
While the Eastern Shore presents a unique set of challenges compared with other watersheds, scientists say there are some slivers of hope.
When poultry litter applications were ended on Green Run, a small tributary of the Pocomoke River, and replaced with chemical fertilizers that can be applied with more precision, nitrogen levels in the stream dropped, though there was little change in phosphorus. Another small project in Sussex County, DE, produced similar results, Denver said.
“The trouble is, it’s very hard to see that on a regional basis because there’s so much variability in the land use,” Denver said.
Soil conditions, topography and other factors make effective nutrient control difficult in many places, especially in the short term. “It is almost a case-by-case situation for putting the best practices you can on a particular property,” she said.
USGS scientists say the information in the report, coupled with the increasing availability of high-resolution mapping technology, may help to better locate nutrient control practices, such as wetland creation or forest buffer planting, to areas where they will be most effective.
For instance, to achieve quicker water quality improvements, the planting of cover crops — which absorb nitrogen before it reaches groundwater — could be emphasized in areas where groundwater travel times are shorter.
In some places, it may be important to promote the planting of stream forest buffers to intercept phosphorus runoff, even if they are in areas that have little impact on nitrogen. Replacing manure with chemical fertilizer will likely be important to reduce phosphorus in many areas, the report suggests.
“The science in this report is going to promote, I think, stronger innovation for precision conservation and precision targeting of practices,” said Scott Phillips, USGS Chesapeake Bay coordinator. “I think it also might promote the greater need to look at ways to take manure and turn it into a commodity that can be used in other places, whether it’s within the watershed, or nationwide.”
The report, “Understanding Nutrients in the Chesapeake Bay Watershed and Implications for Management and Restoration — the Eastern Shore,” is available on the USGS’ Chesapeake Bay website, http://chesapeake.usgs.gov.