The EPA has released load allocations for the primary pollutants that cause oxygen-depleted "dead zones" in the deeper waters of the Rappahannock and the mainstem Chesapeake Bay. These allocations are part of a Bay-wide pollution diet called a TMDL or "Total Maximum Daily Load".
The Commonwealth of Virginia is being required by EPA to develop plans in each of its Chesapeake river basins to reduce its loadings of nitrogen, phosphorus, and sediment to these levels by 2025.
For the Rappahannock, meeting the nitrogen reduction goals will present the greatest challenge. Nitrogen and phosphorous enter the river from a variety of sources, including fertilizer, animal wastes, airborne deposition, stormwater runoff, and wastewater discharges. The basinwide nitrogen load allocation is 5.84 million pounds per year.
For phosphorous, the load allocation is 9.0 million pounds per year.
The graphs below present actual monitoring data for both nutrients for the period 1988 to 2008. Much of the year-to-year variability is associated with rainfall levels, with rainy years generating greater loadings.
Of particular interest is the 10-year rolling average, which gives a better indication of long terms trends than individual year data.