The attached charts (.pdf files below) present the model-estimated amount of nitrogen and phosphorus reaching the Bay (also called “loads”) from various sources by major river basin and by political jurisdiction within each basin.
- 2003 Estimated Nitrogen Loads By River Basin (118 kb .pdf)
- 2003 Estimated Phosphorus Loads By River Basin (114 kb .pdf)
- 2003 Estimated Sediment Loads By River Basin (112 kb .pdf)
The figures also show the model-estimated amount of change from 1985, the baseline for measuring nutrient reductions. Negative percentages represent decreases, positive percentages represent increases.
These estimates come from the Bay Program’s Watershed Model, the tool used to estimate progress toward meeting nutrient goals. Like a giant accounting program, the model estimates the amount of nutrients expected to runoff from different land uses (nonpoint sources), then subtracts from that number the nutrient and sediment reductions expected from the implementation of various runoff control practices. The model then estimates the amount of nitrogen and phosphorus that would be expected to reach the Bay under “normal” river flow conditions from various land uses.
(Point sources, though, are actual figures from discharge monitoring.)
In reality, the actual loads to the Bay from runoff sources in 2003 were much higher than those shown here. That’s because 2003 was an unusually wet year, and high levels of rainfall drive more nutrients off the land and into waterways.
Also, the current model does not address “lag times” for nutrients that move through groundwater. Half of the nonpoint source nitrogen reaches streams through groundwater. On average, it takes about 10 years for groundwater to reach the stream, although the range is from hours to decades. As a result of this delay, nearly half of the nonpoint source nitrogen control actions taken today will be felt in the Bay only gradually, as the stored nitrogen slowly drains from the groundwater.
It is best, therefore, to think of these numbers as estimates of nutrient reductions that will occur in the Bay at some time in the future under “average” rainfall conditions.
What is more important than the absolute numbers is the relative size and direction of nutrient changes for various land uses in various places—are they going up or down, and by a lot or a little?
- The model estimates that under normal rainfall conditions, nitrogen loads have decreased by 62 million pounds since 1985 (18 percent) while phosphorus loads have decreased by 8 million pounds (29 percent).
- The greatest single source of nitrogen reductions since 1985 has come from the agricultural sector, which accounts for about 60 percent of all nitrogen reductions. The greatest single source of phosphorus reductions since 1985 has come from “point sources” such as wastewater treatment plants, which account for an estimated 58 percent of all phosphorus reductions.
- Sharp reductions in nitrogen from point sources are seen in rivers where much of the flow from wastewater treatment plants has been treated with biological nutrient control technology. Point source loads are increasing in rivers where few plants use that technology, as population and waste flows increase.
- Estimated nitrogen loads are increasing from septics as more development takes place beyond the reach of sewer systems.
- Estimated nutrient loads from agricultural activities are declining because of the implementation of nutrient control practices and a decrease in agricultural land.
- Estimated loads from “mixed open” lands are increasing largely because the amount of land in that category has grown sharply. Mixed open land generally represents land that is transitioning from agriculture to low-density development.
- Phosphorus loads from point sources have declined sharply, in large part as the result of phosphate detergent bans enacted during the 1980s.
Where’s The Air?
Air pollution contributed at least a quarter of the nitrogen entering the Bay, but it doesn’t look that way on these tables. That’s because the “atmospheric” nutrient sources shown here represent only direct atmospheric deposition on river surfaces in the watershed. The rest of the atmospheric load is distributed among the land uses it falls upon.
(In addition to what is shown here, about 20 million pounds of nitrogen lands directly on the Bay and its tidal tributaries. The Bay Program goal of capping nitrogen inputs at 175 million pounds a year does not include the direct deposition to the Bay and tidal tributaries.)
It’s A Dirty Job, But It Has To Be Done
Cleaning up the Bay means more than just curbing nutrient inputs. It now means keeping dirt out as well.
The Bay Program in 2003 set its first-ever sediment reduction goals aimed at keeping silt and clay on the watershed—and along shorelines—so it doesn’t kill grass beds by muddying the Chesapeake’s water. Dirt, like algae blooms, clouds the water, blocking sunlight from reaching important underwater grass beds.
In certain shallow water areas, sediment is now thought to play as important a role as nutrients in reducing the sunlight available for underwater grasses. Restoring those grass beds and the shallow water habitat they provide is considered a key part of bringing back a healthy Bay ecosystem. Sediment also smothers habitats for oysters and other bottom-dwelling creatures and can clog fish gills.
The sediment goals are aimed at helping the Bay Program meet its 185,000-acre underwater grass goal by clearing the water in places where nutrient control alone may not be enough to do the job.The sediment goals are being set in two stages.
The first step calls for reducing from 5.05 million tons to 4.15 million tons annually the sediment that washes into the Bay from the major tributaries. That reduction is aimed at helping to clear the water in the Upper Bay and the upper reaches of tidal tributaries—the areas where sediment from the rivers is thought to have its greatest impact.
The figures on this page reflect those goals established in 2003.
The second step involves shorelines. In areas below tidal freshwater areas, where sources such as shoreline erosion and resuspension by waves are thought to be more important, reductions are being set on a case-by-case basis to restore historic levels of grass beds.
Controlling sediment brings new challenges to the Bay cleanup effort. Shoreline erosion rates have increased over the past century as water levels have risen; combating that can be particularly expensive.
Reducing sediment from the rivers can be problematic as well. Research suggests it often takes decades for sediment to move down the rivers and into the Bay. That means control actions taken today may not yield results until far into the future.