Bay’s geography means that not every pound of pollution needs a pound of cure
Scientists working to come up with nutrient reductions that will get the job done yet are fair
When a pound of nitrogen washes off the streets of Bowie, MD, and into the Patuxent River, almost every ounce will end up in the Chesapeake's tidal waters.
When a pound washes into the Susquehanna River from Binghamton, NY-hundreds of miles upstream from the Bay -only about 60 percent will make it to the estuary.
Yet nutrients from those widely separated areas have strikingly different impacts on the Chesapeake's oxygen-starved "dead zone."
Every ounce of nitrogen that makes it out of the Susquehanna has three times the impact of deep water oxygen concentrations in the Middle Bay as the same amount originating above the Patuxent fall line.
Such confounding issues now confront policy makers as they begin setting new nutrient reduction targets for each state, and each tributary: What is a fair share?
Because of the way nutrients move through the Bay's 64,000-square-mile watershed, and the way they move within the Chesapeake itself, the relative impacts of people far upstream who may never see the Bay can in some cases exceed those of people who may fish its waters every week.
In the past, nutrient reduction goals assigned to states have been based on a mixture of science and compromises. But the new goals will be placed into legally binding cleanup plans, known as Total Maximum Daily Loads, and therefore may be more subject to legal challenges. State and federal officials are searching for a process that assigns nutrient reductions that are both fair, and defensible.
"Because it is a TMDL, we have to be able to explain to a judge why and how we made specific decisions," said Rich Batiuk, associate director for science with the EPA's Bay Program Office. "If you say, 'Someone didn't like this, so we gave them a break,' your logic completely breaks down. We want to remove these artificial judgements."
The greatest amounts of nutrients enter from, in order, the Susquehanna, Potomac and James rivers. But a pound of nutrients from each river impacts the Bay and its tributaries differently.
To develop a fair way of distributing nutrient reductions, Bay Program modelers have developed a method to show the relative impact that a pound of nutrients from each river-and even different parts of rivers-affects various parts of the Bay and its tidal tributaries.
Doing that requires dissecting the complex relationship of geography and water quality.
The issue starts in the watershed. The amount of nutrients that make it to the Bay and its tidal tributaries depends on, as a real estate agent may put it, location, location and location.
Generally, the vast majority of nutrients that enter a waterway below the fall line make it into the Bay or its tidal tributaries, largely because they don't have far to travel before reaching tidal waters.
The fall line, which parallels Interstate 95 around the Bay, is the geological boundary where the Coastal Plain meets the Piedmont, an area usually marked by rapids or falls, hence the name. In many Bay tributaries, the fall line is near the upper extent of the Chesapeake's tidal influence, where water is pushed upstream during high tides.
Above the fall line, the journey to tidal waters is generally much longer, and may begin with the nutrients traveling through many miles of small, winding streams. Nutrients in small streams are more likely to be used by aquatic life, or-in the case of nitrogen-removed from the stream altogether through denitrification, a natural microbial process.
As a result, less than 20 percent of the nitrogen entering streams in the mountainous headwaters along the watershed's western edge typically reaches tidal rivers.
That changes when big rivers are involved. Large rivers act like a highway rapidly moving water, and the nutrients it carries, downstream. That leaves little chance for biological processes to remove nutrients.
That's especially true for the Susquehanna, which supplies half of the freshwater to the Chesapeake. The large size of the river and many of its tributaries creates a system that quickly shunts nutrients downstream.
As a result, 60 percent or more of the nitrogen that enters the Susquehanna at the New York-Pennsylvania border reaches the Bay. In comparison, less than half of the nitrogen in streams feeding the Shenandoah makes it to the tidal Potomac-even though it's much closer to tidal waters than New York-because the nutrients spend more time winding through a network of smaller streams
"The Susquehanna's a bigger river," said Gary Shenk, a modeler with the EPA's Bay Program Office in Annapolis. "When you're going into a big river, nutrients really don't have enough time to be removed on the way down. Smaller rivers are more efficient at removing nutrients while larger rivers act more like pipelines, carrying the water and nutrients along without as much loss."
The journey through the watershed provides only half of the story of how geography impacts water quality. Movement in tidal waters is equally important.
Lighter freshwater generally flows toward the Bay's mouth along the Western Shore near the surface, while heavier ocean water generally makes its way northward along the Eastern Shore. Tides further mix the water.
As a result, the water quality impact of a pound of nitrogen or phosphorus varies based on where it enters tidal waters. A pound of nitrogen from Virginia's Eastern Shore has more of an impact on Middle Bay water quality than a pound from the York or Rappahannock rivers. Although directly across the Bay, much of their water is heading toward the mouth of the Chesapeake and into the Atlantic. A portion of the Eastern Shore water, meanwhile, is heading north, farther up into the Middle Bay.
The Susquehanna and Maryland's Western and Eastern shores, have the greatest impact on chronic low-oxygen levels in the Middle Bay. But even there, the story is more complex. In many tributaries, the relative amount of nutrients reaching the Chesapeake's "mainstem" (the Bay minus its tidal tributaries) from above the fall line is much smaller than from below the fall line.
That's because a portion of the nutrients from above the fall line are used up in the tidal portion of the river-either by growing algae or being removed from the system by biological processes. Nutrients originating below the fall line are more rapidly swept into the Chesapeake by outgoing tides. In the Patuxent River, for instance, each pound of nutrients originating below the fall line has twice the impact on the Bay as a pound from above the fall line.
Factoring in the effects of geography can produce surprising results.
Nutrients coming from Pennsylvania's portion of the Susquehanna actually have more of an impact on water quality in the lower Potomac than nutrients from Pennsylvania's portion of the Potomac watershed, which is farther upstream from tidal waters.
By factoring together the amount of nutrient movement through the watershed and the effect of nutrient movement within the Bay, it's possible-on a per pound basis-to determine where nutrient control actions within the watershed will be most effective for restoring water in specific areas of the Bay and its tidal tributaries.
State and federal officials are trying to decide how to translate that information into new nutrient reduction goals. There is general agreement on the basic principle that areas of the watershed that contribute most to the Bay's water quality problems-on a per pound basis-should make the greatest level of effort.
Another principle is that all areas and sources within the watershed have to participate-otherwise some areas would be forced to implement unrealistic levels of nutrient control actions. Also, because effectiveness is determined on a per-pound bases, some areas where nutrient controls are highly effective, such as Virginia's Eastern Shore, contribute relatively small amounts of nutrients.
A basic framework for applying those principles is also emerging:
As a first cut, the Bay Program will use computer models this summer and fall to assign nutrient reduction targets to the six states' and the District of Columbia's portions of each major river basin. River allocations will also be split at the fall line of each river basin. In all, about 30 geographic areas will get nutrient reduction targets.
Those targets will be based on the level of nutrient reductions needed to improve oxygen levels in the deep water of the Middle Bay-the location of the infamous summertime "dead zone"-and the lower tidal Potomac River to acceptable levels.
Those nutrient reductions will likely resolve most Baywide water quality problems.
"Dissolved oxygen is a pretty neat integrator of a lot of difficulties in the Bay," said Lewis Linker, modeling coordinator for the EPA's Bay Program Office. "The dissolved oxygen problem is derived from the overabundance of algae problem which, in turn, reduces water clarity."
States will analyze those nutrient reductions to determine whether they also remove water quality impairments in all tidal rivers and embayments.
That's because leaning up "the Bay" refers not only to the main stem of the Chesapeake, but also its tidal tributaries and embayments. All of these tidal areas-main Bay and tidal tributaries-are divided into 92 segments, each of which has specific water quality standards for dissolved oxygen, chlorophyll a (a measure of algae), acres of underwater grasses and water clarity.
Meeting those standards in some tidal rivers may require efforts beyond those needed to clean up the mainstem of the Chesapeake Bay, so targets will have to be revised.
Finally, those revised targets will be run through the Bay Program's computer models to ensure that water quality standards will be met in all 92 segments once the nutrient load reductions are achieved. Those nitrogen and phosphorus numbers will then go into legally binding cleanup plans for each individual tidal Bay segment, known as a Total Maximum Daily Loads.
The TMDLs will be completed by the end of 2010 and will drive billions of dollars of spending for at least a decade, highlighting the importance of decisions in the coming months.
"We are trying to develop an objective nutrient load reduction decision-making tool that we can apply across the six-state watershed that people can understand," Batiuk said. "At the same time, we want to make the ultimate allocations the most cost effective and as equitable as possible."
- Category: Pollution
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