Dry Weather Helps Lower Pollution in Chesapeake Bay

According to tributary measurements and computer modeling the amount of nitrogen, phosphorus and sediment pollution flowing into the Chesapeake Bay fell significantly between 2014 and 2015. The U.S. Geological Survey (USGS) and the U.S. Environmental Protection Agency’s Chesapeake Bay Program Office attribute this drop in pollution mostly to dry weather that produced below-normal river flows, but believe that regional efforts to reduce this pollution under the EPA mandated “pollution diet” also contributed to the reduction. Ongoing studies do show that “best management practices”—including upgrading wastewater treatment plants and reducing runoff from farmland—have lowered nitrogen, phosphorus and sediment in our regional waterways. However, the effect of weather is much stronger.

The Chesapeake Bay Program tracks nitrogen, phosphorus and sediment pollution in the Chesapeake Bay and its tributaries as it monitors the states’ performance under their plans to comply with the EPA mandate. According to data from the USGS and Chesapeake Bay Program between 2014 and 2015, total nitrogen loads fell from 290 million pounds to 217 million pounds. Phosphorus loads fell from 17.7 million pounds to 9.9 million pounds. Sediment loads fell from 7.2 billion pounds to 2.9 billion pounds.

That is a huge reduction and sounds great, but a lot of this improvement is due to a low lower than average rainfall. There is a direct relationship between rainfall and nitrogen, phosphorus and sediment pollution. The more and harder it rains the more soil and nutrients are washed into rivers. The USGS attempts to normalized the flow data before they look at the long term  trends in the data. However, "normalization" could distort the data if river flows are changing. The USGS monitors nitrogen, phosphorus and sediment loads entering the Chesapeake Bay from the nine largest rivers in the watershed. Together, pollution loads computed at all nine River Input Monitoring (RIM) stations reflect pollution loads delivered to the Bay from almost 80% of the watershed. They use an algorithm to estimate the total nitrogen, phosphorus and sediment loads delivered to the Bay in a given water year.

The long-term trends in nitrogen are improving at six of the nine monitoring stations, including those on the Susquehanna, Potomac, James and Rappahannock (the largest rivers in the watershed). Long-term trends in phosphorus and sediment, however, are more variable: phosphorus is improving at three monitoring stations and getting worse at five monitoring stations, while sediment is improving at three stations and degrading at four. It is puzzling why despite all the efforts by the Chesapeake Bay States that sediment and phosphorus are getting worse overall.

As Scott Phillips, Chesapeake Bay Coordinator, for the USGS, pointed out “While the lowered amount of pollution entering the Chesapeake Bay in 2015 is encouraging, the trends of nutrients and sediment over the last decade in the major rivers flowing into the Bay show mixed results. There will need to be improving trends in all of these rivers to support improvement in the Bay’s health.”

While continued improvements in water quality will take time, the EPA says the ecosystem is beginning to respond to protection and restoration efforts. Unfortunately, not fast enough for the regulators. In June, the EPA released its two-year milestone evaluations of how the states are progressing towards meeting the requirements of the “pollution diet.” The EPA mandate to the states calls for all needed pollution control measures to be in place by 2025, with measures that would achieve 60% of pollution load reductions in place by 2017. This is all measured by computer simulations which show that the states are behind. According the the EPA evaluation, measures are in place to achieve 31%of the nitrogen reductions, 81% of the phosphorus reductions and 48% of the sediment reductions necessary to reach the mandated targets.