Chesapeake Bay watershed states continue to make progress toward their cleanup goals, but the region – held back by lagging Pennsylvania -  is far off the pace needed to achieve nitrogen reduction targets set for the end of 2017, according to figures released Monday.

The figures, which update state nutrient and sediment reduction efforts through 2015, also show that the vast majority of nitrogen reductions — roughly three-quarters — have come from updating wastewater treatment plants.

That’s good news because those reductions produce quick — and reliable — results. But it also means that more nutrient control efforts are needed where they have historically been problematic, primarily in agriculture and stormwater.

The story appeared slightly more positive for reducing phosphorus, suggesting the region had already attained its 2017 objectives. But Bay program officials cautioned that upcoming revisions to the computer models used to estimate the Bay’s nutrient pollution may cast those gains in doubt.

Overall, “we continue to make progress toward the ultimate reduction goals for nutrients and sediment,” said Nick DiPasquale, director of the Environmental Protection Agency’s Bay Program Office.

Those goals were set by EPA in the 2010 Chesapeake Bay Total Maximum Daily Load, which called for reductions in the amount of nitrogen, phosphorus and sediment that could enter the Bay each year in order to make Bay water healthier for fish, crabs and even worms that burrow into bottom sediment.

States are supposed to take all steps needed to reach those pollution limits by 2025, with actions sufficient to achieve 60 percent of the reductions to be in place by the end of 2017. States report on actions they take to the EPA each year, which then uses sophisticated computer models to determine whether they are on track to meet goals.

The latest modeling shows that:

• Nitrogen loads flowing from the watershed to the Bay have fallen 7 percent since 2009, from a baseline then of 260.2 million pounds to 241.5 million pounds last year. The 2025 goal is 192.4 million.

• Phosphorus loads have declined 20 percent over the same period, from 19.23 million pounds in 2009 to 15.36 million pounds. The goal is 14.46 million pounds by 2025.

• Sediment pollution also has dropped 7 percent, from 8.68 billion pounds in 2009 to 8.04 billion pounds last year. The goal for 2025 is 7.34 billion pounds.

Reducing nitrogen has proven difficult, and many officials privately acknowledge the region will not meet its interim goal for that nutrient. The latest modeling shows that controls have reduced nitrogen loads by about 19 million pounds since 2009, but another 22 million pounds would be needed by the end of next year to stay on track.

The majority of the shortfall stems from Pennsylvania, which is lagging badly. State officials — facing threats of action by the EPA — recently announced plans to “reboot” their Bay-related efforts. But other states are also falling behind in some areas, especially stormwater, where most are reporting increased runoff pollution.

The greatest success has come from wastewater treatment plant overhauls. Those have slashed nitrogen levels by roughly 75 percent, from 52.2 million pounds in 2009 to 38 million pounds last year. That far surpasses the interim pollution reduction goals set for sewage facilities, and it nearly achieves the 2025 target set for them of 37.9 million pounds.

Wastewater treatment plants are more easily regulated than many other nutrient sources, and several states have poured hundreds of millions of dollars into upgrading their facilities. Further, DiPasquale said, those upgrades are in many cases achieving greater pollution reductions than anticipated.

James Davis-Martin, Bay coordinator for the Virginia Department of Environmental Quality and chair of the Bay Program’s water quality implementation team, said the wastewater plant upgrades also provide more certain, and measurable, nutrient reductions than those from stormwater or agricultural runoff control measures, which often yield highly variable results

It makes sense, Davis-Martin said, “to implement the most reliable, longest term, longest lived practices first. It sets you up for success as you continue down the road toward 2025.”

The picture for phosphorus is a bit murkier, though.

The new figures show the region has reduced phosphorus by 3.87 million pounds since 2009, already exceeding the interim 2017 goal. But that’s tempered by the fact that actual water quality monitoring shows phosphorus levels increasing in many streams and rivers.

DiPasquale noted that the current computer models do not reflect new science that shows increased runoff from certain agricultural areas with phosphorus saturated soils. An updated version of the model, which is now under development, is expected to simulate the effect of over-application of phosphorus-rich animal manure on farm fields, and how that affects water quality in rivers, and the Bay.

“While the numbers in terms of the overall progress reporting look good…we know that we are seeing phosphorus in certain areas of the watershed in water quality monitoring data actually increasing, so we know we don’t have a truly accurate picture of what’s going on in the model,” the EPA official said.

An overview of Bay nutrient trends and supporting data can be found here.