The Chesapeake Bay's cleanup may be delayed "several decades" by the slow pace at which farm pollution is being flushed from ground water on the Delmarva Peninsula, a new study says.
The research by the U.S. Geological Survey also suggests pollution control efforts on Eastern Shore farms may need to be increased in order to achieve hoped-for water quality improvements.
Using a computer model to simulate ground-water flows, USGS scientists found that when nitrogen from fertilized farm fields on the Shore soaks into the ground, it takes 20 to 40 years on average for the nutrient-laden water to make its way underground into streams and rivers. Ground water now oozing into bay tributaries is likely carrying pollution picked up by rainfall decades ago, researchers said.
"It’s just going to take time for changes at the land surface to show up in the streams," said Ward E. Sanford, a USGS hydrologist in Reston, Va. and lead author of the study, which was published in Environmental Science & Technology.
Nitrogen and phosphorus are the two plant nutrients in sewage and fertilizer that foul the bay, feeding its algae blooms and "dead zones." Both are washed off the land into streams whenever it rains, but nitrogen dissolves in water, so it also soaks down into the soil, ultimately reaching the water table.
Bay scientists have long realized that there's a "lag time" between when nitrogen seeps down into the ground and when it resurfaces in a stream or river. Most had estimated the delay was a decade or even more, but the new study suggests it's longer.
Sanford said the lag time likely explains why nitrogen levels in the Choptank and Nanticoke rivers have risen rather than declined, despite expanded efforts to control runoff from the many farms in their watersheds.
"People, including managers and political leaders, in general under appreciate the length of time that will be required for pollution loads to decline as a result of management actions," said Donald F. Boesch, president of the University of Maryland Center for Environmental Science.
Indeed, the computer modeling on which the Environmental Protection Agency has based its bay "pollution diet" does not factor in the ground-water lag at all, Sanford said.
And the study indicates that the ground-water lag may require even greater efforts be made to reduce farm pollution than previously projected in order to achieve the bay cleanup goals, Sanford said.
"Enough is known to include lag times in the models so that they better represent reality," UM"s Boeach said in an email. "This should be a priority."
But Boesch also suggested that ground-water lag may not fully explain the failure of nitrogen levels to drop as much in streams as expected. It may be, he contended, that "the management practices are not as effective as we thought. As we progress toward restoration goals, we need to better sort this out."
Studies have shown that certain nutrient-absorbing practices, such as planting "cover crops" in winter, reduce the amount of nitrogen that soaks into the ground water, said Ken Staver, associate research scientist at the University of Maryland's Wye Research and Education Center. But other farm pollution practices haven't been proven as effective yet, Staver said, and there's no large-scale monitoring to tell if pollution-control measures on farms are really reducing nitrogen leaching into the ground water.
A further complication, Sanford said, is that information isn't publicly available on what pollution controls are on what farms, so it's not possible to match them up with water-quality measurements in a given stream. USGS is now working with the U.S. Department of Agriculture, he explained, on a study that aims to bridge that information gap on a single bay tributary, the Chester River.
There are indications that farm pollution control efforts may be having an effect, Sanford said. While the study, found nitrogen contamination in Shore ground water continues to increase, he said, it's at a much lower rate and appears to be leveling off.
However, until further studies are complete, he added, it's not clear that the slowing of nitrogen buildup in ground water is actually from pollution control efforts.
Sanford's co-author was USGS scientist Jason Pope in Richmond, Va.