The Bay Program is developing a new technique to estimate the amount of nutrients and sediment that enter the Bay each year.
Until now, there has been no complete picture of what goes into the Bay annually under real-world conditions.
The EPA’s Bay Program office makes annual model estimates of the amount of nutrients that enter the Bay under “normal” hydrology. Those figures, when nutrient control efforts are factored in, are used primarily to estimate nutrient and sediment control reduction progress under average conditions. But because normal rainfall years almost never happen, those estimates do not reflect what is actually entering the Bay.
The U.S. Geological Survey has a River Input Monitoring Network which generates data to calculate the amount of nutrients and sediment entering tidal rivers from the nine upstream waterways that feed the Bay. Altogether, that network monitors nutrient runoff from 78 percent of the watershed. That estimate, though, does not include runoff from the Coastal Plain areas east of Interstate 95, which goes directly into tidal waters. Because there are so many tidal waterways—and because of their back and forth movement—they are nearly impossible to monitor for nutrient loads.
The technique, which is still under development, uses nutrient and sediment loads from nontidal river monitoring, and combines them with model estimates of nutrient and sediment runoff from Coastal Plain areas, which are adjusted to reflect the conditions observed in the nontidal monitoring, such as the timing and amount of river flow which can affect nutrient and sediment transport. Also added to the figures are estimates of nutrient discharges from point sources, such as wastewater treatment plants located in the Coastal Plain.
The degree of certainty is greatest for the annual loads from nontidal rivers and point source discharges, and is lowest for the estimated Coastal Plain nonpoint source loads. However, the Bay Program officials believe that these are the best possible estimates of total load to the Bay.
The estimates indicate that last year about 470 million pounds of nitrogen, 26.9 million pounds of phosphorus and 6.86 tons of sediment entered the Bay.