The amount of nitrogen entering the Chesapeake has been underestimated by nearly 20 percent in the past, meaning the job of cleaning the Bay may be significantly more difficult than previously thought.

The new figures mean government officials and stakeholder groups may need to brace themselves when preliminary nutrient reduction goals are set this fall. The figures are likely to show that most areas have to do as much, or more, than in 2003, the last time nutrient reduction goals were set.

Since then, new research, and a new model, have resulted in many changes about how nutrients are accounted for in the watershed and the rivers that flow into the Bay. Most of those revisions increase estimates of the amount of nitrogen entering the Bay.

"We are reflecting better science, and it's saying it is going to be tougher," said Rich Batiuk, associate director for science with the EPA's Bay Program Office. "If it is going to be tougher, we need to face that as early as possible."

In late October, the EPA Bay Program Office plans to assign preliminary nutrient reductions for states and rivers. States will refine those figures, with final numbers being adopted next year.

Those reductions are needed to achieve water quality standards designed to allow striped bass, blue crabs and underwater grass beds to thrive.

Nitrogen and phosphorus spur algae blooms that deplete water of oxygen when they die. The algae also block sunlight from reaching underwater grass beds and other important bottom habitats.

The new figures are generated from a new version of the Chesapeake Bay Watershed Model, a sophisticated computer program used to estimate nutrient movements from the farthest headwaters to the Bay.

Some of the changes result from the updated model. A number of improvements from the previous version tend to move nutrients more rapidly into the Bay, especially if they originate in the Coastal Plain or near large rivers.

But much of the changes result from improved science reflected in the model. A recent review of best management practices used to control runoff from farms and urban lands showed that many were less effective than earlier thought.

Also, a new review of meteorological data showed that information used in the earlier model was actually about 5 percent drier than normal. The new model simulates a "wetter" watershed, which results in higher estimates of nutrient runoff as wetter conditions flush more nutrients off the land and into rivers.

The bottom line: The new model estimates that about 395 million pounds of nitrogen reached the Bay in 1985-generally the baseline from which nutrient reductions are measured-compared with 337 million using the old model.

The change is less dramatic for phosphorus: 28.4 million in 1985 using the new estimates versus 27.1 with the old model.

By 2002, the new model estimates that cleanup efforts had reduced nitrogen levels to about 315 million pounds. The old model estimated 277 million pounds.

The nitrogen goal needed to achieve a clean Bay, for now, remains unchanged: 175 million pounds a year.

That means that when the last cleanup goals were set in 2003, officials thought they had to reduce nitrogen pollution by about 100 million pounds a year. Using the new figures, the needed reductions were closer to 140 million pounds.

The story for phosphorus is a bit brighter. The new model put the 2002 estimate at 18.8 million pounds while the old model put it at 19.5 million.

And in a bit of good news, the revisions suggest phosphorus goals may be changed to 14.1 million pounds to meet water quality goals-not the 12.8 million previously estimated.

Final model estimates for 2008 were not available at Bay Journal press time. But nitrogen reductions throughout the watershed have averaged around 3 million pounds a year. If so, the amount reaching the Chesapeake today is likely higher than what was estimated in 2002.

That means, at least for nitrogen, that reductions assigned this fall to river basins and states could be even greater than those made in 2003.

"The summit hasn't changed," Batiuk said, "but it's going to be a steeper climb."