With only three and a half years left to meet its nutrient reduction goal, the Bay Program appears headed for a split judgment.

Computer model estimates show the states are on track to meet their goal for phosphorus, but unless cleanup efforts are significantly accelerated, they appear likely to miss the nitrogen goal by a wide margin.

The findings are not particularly surprising, as officials from the Bay states and federal agencies involved in the cleanup effort have recognized for some time that reducing levels of nitrogen - which is harder to control than phosphorus - is a daunting task. But information produced in recent weeks represents the first time the size of the shortfall has been predicted.

The 40 percent nutrient reduction goal, established in 1987, has been the cornerstone of the Bay restoration effort, as excess amounts of nutrients are considered to be the Bay's chief water quality problem.

Right now, it is estimated the Bay states will miss their 74 million pound nitrogen reduction goal by 21 million pounds - a 28 percent shortfall. The deadline for meeting the goal is Dec. 31, 2000.

While the figures may be revised somewhat in the coming months as information used in the calculations is refined, officials at the EPA's Chesapeake Bay Program Office say it is unlikely the numbers will change substantially.

State and federal officials are scrambling to find ways to overcome the shortfall, though they admit the size of the gap makes it difficult to close.

"We need to take necessary actions to close the gap and it's going to be a close call as to whether we can put together that much of an effort in time," said Bill Matuszeski, director of the EPA's Chesapeake Bay Program Office.

Time is a problem because major nutrient reduction efforts - such as upgrades at wastewater treatment plants - often take years to complete.

A special team of state and federal officials has been combing through state nutrient reduction plans and other documents looking for practices that might help close the nitrogen gap.

"There are no silver bullets left out there, or if there are, we're still hunting them," said Tom Simpson, of the Maryland Department of Agriculture, who is heading the effort.

The group has identified a few potential quick fixes, such as curbing the increasing use of urea - which is 46 percent nitrogen - in the winter to melt ice in favor of other chemicals. Other possibilities include getting homeowners to use less fertilizer on their lawns, encouraging agriculture practices that reduce nutrient runoff - even composting dead chickens. All efforts identified by the group so far would - at best - cut nitrogen by an additional 10 million pounds a year.

The most effective way to close the gap, Simpson said, was to speed up implementation of the "tributary strategies" that Bay states have been developing to guide nutrient reduction efforts on each major river feeding the Bay. In many cases, though, that would require increases in state spending which would take time to be approved.

"Some of the fixes are getting time constrained," Simpson said, noting the closeness of the 2000 deadline.

It is also unclear whether full implementation of the tributary strategies would meet the goal. Strategies completed for Pennsylvania's portion of the watershed and Virginia's portion of the Potomac basin fell short of the goal. The Bay Program computer model has not yet estimated how close the tributary strategies come to reaching the nutrient reduction objective.

Progress toward the nutrient reduction goal is calculated by the Bay Program's Watershed Model, which estimates nutrient inputs from various activities - both natural and human - from the Chesapeake's 64,000-square-mile watershed. The model simulates the movement of those nutrients down the rivers, calculating how much is absorbed en route, and estimates how much actually reaches the Bay.

Information about sewage treatment plant upgrades, fertilizer use, implementation of runoff control techniques on farms, deposition from air pollution, population growth and other factors are included in the model.

For nitrogen, it shows that while 353 million pounds of nitrogen entered the Bay in 1985 - the base year for measuring nutrient reduction efforts - that amount had been reduced to 326 million by 1995. Expected nutrient reductions through the year 2000 would reduce that to 300 million pounds, short of the 279 million pound goal.

But the model indicates that had no action been taken, nitrogen levels in the Bay would have increased to 362 million pounds by the turn of the century.

For phosphorus, the picture is more encouraging. About 27 million pounds entered the Bay in 1985, but that has been reduced to 20 million by 1995. It is estimated to fall to about 17.7 million pounds - about 1 million pounds less than the 18.6 million pound goal.

Had no action been taken, the model estimates that the amount of phosphorus entering the Bay would have increased to 29 million pounds by 2000.

Phosphorus has generally proven easier to control than nitrogen. Much of the reductions achieved result from the phosphate detergent ban enacted in all the Bay states during the late 1980s and upgrades at wastewater treatment plants. Also, because phosphorus tends to bind to soil particles, agricultural practices aimed at controlling sediment runoff can also be highly effective at curbing phosphorus runoff.

In contrast, technologies to control nitrogen discharge at wastewater treatment plants have come into wide use only in the past few years. Also, nitrogen is more water soluble than phosphorus, so nitrogen runoff from farmland is more difficult to control than phosphorus. In addition, large amounts of nitrogen - roughly a quarter of the total reaching the Bay - stems from air pollution, much of which originates outside the watershed.

New practices, such as the planting of nitrogen-absorbing forest buffers and changes in other agricultural practices have started to come into use in recent years, but the full impact of such efforts will not be realized until after the turn of the century.

The Chesapeake Executive Council - consisting of the the governors of Maryland, Virginia and Pennsylvania, the mayor of the District of Columbia, the administrator of the EPA and the chairman of the Chesapeake Bay Commission, a tri-state legislative advisory panel - set a 40 percent nutrient reduction goal in the 1987 Bay Agreement.

Excess amounts of nutrients spur algae blooms in the Bay that block sunlight to important Bay grasses, which provide habitat and food for fish, blue crabs, waterfowl and other species. When algae die, they sink to the bottom and decompose in a process that depletes the water of oxygen needed by many aquatic species.

Phosphorus tends to be most important in spurring algae blooms in freshwater portions of the Bay, while nitrogen stimulates algae growth in salt water areas.

The 40 percent reduction goal was based on computer modeling that indicated such a reduction would result in significant water quality improvements in the Bay. In fact, officials credit reductions achieved so far with water quality improvements seen in some rivers, as well as a gradual increase in the amount of underwater grasses in the Bay.

After the reduction goal is met, the Bay states have pledged to maintain nutrient discharges at those reduced levels, despite anticipated population growth, which will add more wastewater to treatment plants and result in the development of more land, which can increase runoff and generate more air pollution.

Maintaining that cap could prove to be a daunting task. Already, the computer model projects that anticipated phosphorus control measures should overshoot the phosphorus reduction goal by 2 million pounds, but half of that reduction will be offset by increased population growth, which will increase discharges from wastewater treatment plants.

"We're fighting growth every step of the way," noted Lewis Linker, modeling coordinator for the EPA's Bay Program Office.

The Bay Program estimates that population in the watershed will grow from about 15.59 million in 2000 to 17.76 million in 2020. In 1985, the base for measuring the nutrient reduction, the population was 13.5 million.