The federal "pollution diet" imposed on Virginia and other bay states is supposed to reduce the phosphorus, nitrogen and sediment dumped into the Chesapeake Bay and its feeder rivers.

Computer models based on a suite of reduction practices estimate that diet is mostly working.

But real-world monitoring data shows a different story, according to the Environmental Integrity Project, a nonprofit anti-pollution group based in Washington.

Measurements of pollution loads that actually make their way into the estuary indicate nitrogen appears to be declining at a slower rate than models predict, and phosphorus and sediment actually appear to be increasing, the group says.

The most likely reason, according to the EIP, is runoff from farmland.

"We know more about some of the other big sources because they're required to monitor their discharges and required to achieve a certain waste load allocation," explained Abel Russ, an attorney with the EIP and author of its "Murky Waters" report. "We know very well that they have made the reductions they're required to make. We don't really know how well agriculture is doing — it's hard to monitor as a non-point source, so we don't have the same degree of certainty."

Farmers in Virginia and other bay states are employing best management practices, or BMPs, to reduce farm pollution stemming mostly from chemical fertilizer, manure and erosion, Abel said. BMPs include fencing off livestock from streams, planting cover crops or reducing tilling to curb erosion and siting green buffers next to waterways.

But, while it's relatively simple to measure discharges from a point source such as a wastewater treatment plant, it's much harder to measure runoff from swaths of cropland or concentrated feed lots. And when such measurements are made, Russ said, evidence indicates BMPs aren't performing as well as models predict.

The result is more nutrients and sediment dumped into waterways and, ultimately, the bay than models anticipated, making it less likely that bay states will meet their ultimate goal of a restored bay by 2025.

Accountability

Wilmer Stoneman III, environmental specialist with the Virginia Farm Bureau Federation, agreed that hard data on farm runoff isn't easy to come by.

"That's generally because most farmers are harnessing our environment to provide food, fiber and fuel," Stoneman said. "So when you work with a natural system, it is hard to monitor because it doesn't come from a pipe."

But Virginia is about 62% forested, he said, with agriculture representing less than a third of its land use. The state also has a deer population of about 1 million — nearly the same number as cattle in the state — that generate another manure stream.

"I think we've been painted with a pretty broad brush for being accountable for all the sources that don't come from a pipe," Stoneman said. "I don't think there's any substantiated data that shows one way or another that agriculture isn't doing the right thing for the environment."

Russ would agree — the key recommendation in the report is better monitoring of agricultural land, ideally at the level of small watersheds, to build an accurate database.

Other recommendations are for farmers to reduce chemical fertilizer in a way that doesn't harm crop yields, and for universities or states to monitor the effectiveness of BMPs.

The report also calls for bay states to build a "margin of safety" into their nutrient trading programs, as the EPA is calling for. So far, said Russ, Virginia is the only state to adopt the practice.

Under a nutrient trading program, if a farmer reduces his nutrient load by, for instance, fencing his livestock from a stream, earning him about 10 pounds of nutrient credits, he can sell those credits, perhaps to a developer. But to make sure there are no net increases in nutrient loads by miscalculation, the developer will have to buy 2 pounds of credits for every pound to be offset — called a 2:1 uncertainty ratio.

Phosphorus Problem

According to the report, agriculture is the largest single source of bay pollutants. In 2013, it contributed 42% of the nitrogen, 57% of the phosphorus and 59% of the sediment delivered to the bay, the report says.

In a companion report called "Poultry's Phosphorus Problem," the EIP notes that poultry farms on the Virginia and Maryland portions of the Eastern Shore are a key source of phosphorus entering the bay.

According to the U.S. Department of Agriculture, the region's 1,339 chicken farms produced more than 500 million broilers in 2012 — as well as more than a billion pounds of manure containing more than 30 million pounds of phosphates, the report says. That manure typically ends up spread over farm fields or pasture.

The lower Delmarva Peninsula is already saturated with phosphorus, the report says, so much of the manure ends up in the bay through runoff.

Too much phosphorus and nitrogen cause massive algae blooms or red tides in the bay. The decomposition of algae consumes oxygen in the water and creates dead zones that are lethal for marine life. Sediment clouds the water, keeping sunlight from reaching underwater grasses, and silts over the hard water bottom essential for oysters, crabs and other creatures.

The U.S. Environmental Protection Agency imposed the pollution diet, or Total Maximum Daily Load, in 2010 on Virginia, Maryland, West Virginia, Delaware, Pennsylvania, New York and the District of Columbia. Each state must meet two-year milestones, with specific targets for 2017 and the ultimate goal of a sustainable bay by 2025.