Setting science-based water quality criteria and designated uses is one thing; figuring out how to actually meet those cleanup goals in the water is another.

To do that, the Bay Program turned to computer models backed up by decades of monitoring and research. Using its Watershed Model, which estimates the amount of nutrients flushed out of the Chesapeake’s drainage basin and into the Bay, officials calculated the levels of nitrogen and phosphorus reductions that might be achieved under various levels of pollution reduction efforts.

The results were fed into the Bay Program’s Water Quality Model, a complex computer program that simulates how nutrients affect dissolved oxygen levels (as well as other water quality characteristics) throughout the Bay.

The model confirmed some things that, to a degree, were already recognized. First, big nutrient reductions would be needed to meet the new water quality criteria.

Modeling showed that if annual nutrient loadings were decreased to 175 million pounds for nitrogen, and 12.8 million pounds for phosphorus, the dissolved oxygen criteria would be met for most of the Bay.

Second, when it comes to assigning nutrient reductions to rivers, location is everything. Nutrient reductions in different parts of the watershed do not have equal impacts on the Bay. Low levels of dissolved oxygen are disproportionately caused by flows from the Susquehanna River, although other rivers from the Potomac River north contribute significantly to the problem. But rivers south of the Potomac have less impact on the Bay’s dissolved oxygen levels.

Using that information, a formula was developed to assign nutrient loads to different basins and states.

Among the elements:

  • A minimal level of nutrient control implementation was required everywhere, both to help the Bay and protect tidal river water quality.
  • Lower levels of effort were required for rivers south of the Potomac because of their reduced impact on dissolved oxygen.
  • Tributary basins with the highest impact on Chesapeake Bay tidal water quality were assigned the greatest level of effort.

States without tidal waters—Pennsylvania, New York and West Virginia—were provided some relief because they do not directly benefit from improved water quality in the Chesapeake and its tidal tributaries, but they still have significantly greater levels of effort than southernmost tributaries.

The result was the nutrient allocations shown in the tables at the right.

The 175-million-pound nitrogen allocation also assumes 8 million pounds of reductions from new clean air regulations being proposed by the EPA.

If those reductions do not materialize, the 8 million pounds would have to be offset by the states.

The models show that the planned nutrient reductions may not fully attain the dissolved oxygen criteria in the deep portion of the mid-Bay or water clarity criteria in shallow areas where sediment may be particularly important. State and federal officials chose not to pursue greater reductions at this time.

In part, this is because the models are not precise tools and may not accurately predict future water quality.

As a result, officials will re-evaluate the nutrient reduction goals in 2007, after state water quality standards regulations are finalized and existing computer models are refined, to determine whether nutrient reduction goals should be adjusted to meet the new standards.