BAY TOTAL MAXIMUM DAILY LOAD (TMDL)
Bay TMDL Target Loads
These draft target loads are “allowable loads,” meaning the amount of nitrogen or phosphorus that may enter each major river basin on an annual basis. A smaller allowable load means there is a need for an increase in reduction efforts.
For the Chesapeake Bay tributary basins in Virginia (James, York, Rappahannock, Potomac/Shenandoah and the Eastern Shore), the annual working target load for nitrogen is roughly 59 million pounds and roughly seven million pounds for phosphorus. These draft target loads have been developed so jurisdictions can begin developing watershed implementation plans to meet the Bay-wide TMDL.
The PSC agreed to these loads with the understanding that they are very broad targets that will change as additional information is gathered by EPA, the states and stakeholders over the coming months. Final loads will likely be different than these initial working targets. The current target loads were developed looking primarily at one of the main water quality “criteria” – dissolved oxygen in the Bay’s mainstem. There are a number of factors that will likely result in revisions to these target loads including:
- Upgrading the EPA Chesapeake Bay watershed model from Phase 5.2 to 5.3. When the model has been updated in the past, existing pollution loads have often been revised. That may or man not be the case this time. The model is based on monitoring data and is used to guide and evaluate reduction efforts.
- Filter feeder inclusion in the WQ model. EPA is proposing to include the impact of filter feeders, such as oysters and menhaden, in the model calculations. It is uncertain how large an impact this change would have.
- Submerged Aquatic Vegetation - water clarity target load analysis. While generally thought to be primarily a sediment issue, it is believed beds of these important grasses in certain segments of the watershed are also impacted by nutrients.
- Atmospheric deposition. These loads come from within and outside the Chesapeake Bay watershed. EPA has the lead on quantifying these loads.
- Loading reductions needed to meet local water quality criteria for SAV, dissolved oxygen or chlorophyll a. While nutrients from watersheds, such as the York and the James, have less impact on the Bay mainstem's dissolved oxygen levels than those from other Bay basins, there are local water quality issues that could affect final allowable loads. For example, the James River has a separate chlorophyll a water quality standard. Meeting that standard may affect reduction efforts in the James, hence the overall Virginia allowable loads.
Taking these and other considerations into account, the Bay Program will update these working target loads by April 30, 2010.
Click here for the EPA letter on basin target loads.
Table and barcharts of N and P target loads for all seven Bay jurisdictions.
Nitrogen and phosphorus target loads for Virginia’s major basins.