The saying, 'what goes up must come down,' certainly holds some weight (no pun intended), but now appears to influence water quality too.

A recent study shows that cleaner air above the Potomac watershed, including that of the Washington DC metropole may account for the recent improvements in Chesapeake Bay water quality. A team of researchers from the University of Maryland Center for Environmental Science suggest that there is a link between improved water quality of freshwater systems in the Upper Potomac River Basin and improved air quality as a result of the Clean Air Act, which has seen a reduction in nitrogen pollution contaminating land and freshwater systems in the Potomac watershed.

According to lead author, Keith Eshleman, a professor at the University of Maryland Center for Environmental Science:

"The recent water quality successes in the Chesapeake Bay restoration are apparently driven more by air quality regulation rather than by water quality control efforts. These air quality regulations were intended to address human health issues and acid sensitive streams. No one thought you would have this positive impact on water quality. It was totally unanticipated."

Chesapeake Bay is the largest estuary in the US. It is also one of the most polluted, suffering from high nutrient loads and hypoxic conditions due to oxygen depletion resulting from excessive nitrogen inputs. Much focus, and credit, has been given to improved land-based strategies, such as wastewater treatment and agricultural practices, to reduce nitrogen pollution in freshwater systems. Yet the research team found that the improvement in water quality in the Upper Potomac River Basin, covering an area of approximate 12,000 miles that extends across the District of Columbia, Pennsylvania, Virginia, West Virginia and Maryland, can be attributed to improved air quality, or more specifically, due to a reduction in atmospheric nitrogen deposits which have been curbed as a result of the Clean Air Act, introduced in 1973 and amended in 1990.

"Most best management practices--like a riparian buffer or retention pond--only impact a relatively small area," said Eshleman. "You can think about the Clean Air Act as a best management practice that affects every square meter of the watershed."

Nitrogen in the atmosphere — largely arising from emissions produced when fossil fuels are burnt — is eventually deposited on land or on surface waters. If the amount of nitrogen that is deposited is higher than the amount of nitrogen that plants and trees need for growth, soils may become nitrogen saturated. This surplus nitrogen can enter freshwater systems, where it can result in algal blooms that negatively impact aquatic life in freshwater and marine ecosystems.

The scientists have been analyzing water quality data from streams and rivers in the Upper Potomac River Basin since 1986. They have discovered that water quality throughout the watershed has improved universally. In particular, the scientists noted a decline in atmospheric nitrogen deposition since 1996 — the same period that emission limits were placed on coal-fired boilers. The researchers also noted that nitrate concentrations in the Upper Potomac river began to decline soon thereafter, and continued to do so through to 2012. As a result, nitrogen saturation within the watershed was quickly reversed. The study suggests that in the future, water quality within the Potomac watershed is likely to improve further as cleaner energy sources such as renewables and natural gas replace coal-fired energy plants.

Journal Reference

Keith N. Eshleman, Robert D. Sabo. Declining nitrate-N yields in the Upper Potomac River Basin: What is really driving progress under the Chesapeake Bay restoration? Atmospheric Environment, 2016; DOI: 10.1016/j.atmosenv.2016.07.004