The Potomac River 1940-2019

For over a century the U.S. Army Corps of Engineers operates Washington Aqueduct and provides drinking water to Washington, D.C., Arlington County and Falls Church. The Aqueduct routinely samples the Potomac River at Great Falls, its source of water for basic water parameters and several pollutants and metals. Their data goes back decades upon decades. Since 2001 the Aqueduct has been putting the water quality reports online, but until now the older data has been unavailable.

The Water quality staff at Interstate Commission on the Potomac River Basin (ICPRB) recently reviewed and proofed and digitized decades of water data collected from from fading paper records and checked for accuracy to provide a look at Potomac water quality over time. Then the ICPRB updated the dataset to include the data from 2000 to 2019. Using this data they analyzed the changes in temperature, hardness, pH, total solids, chloride, nitrate, and sulfate over the last 80 years since ICPRB was formed in 1940. The water quality data speaks of trends and conditions in the watershed and the condition of Potomac River itself.

Below, I have excerpted some of the findings from the report Potomac River Water Quality at Great Falls: 1940-2019 or you can read the report in its entirety at this link. or you can watch this short video

“Statistically significant increases in temperature since 1940 occurred in the Potomac River. Overall, average temperature increased 6.0 degrees F. It is too facile to simply attribute the 80-year increasing trends to global warming. A more likely factor was the rapid population growth and attendant land and economic changes during this period that substantially altered the watershed upstream of Great Falls. By the early 20th century, the forests that once cooled surface waters had been logged or replaced with open agricultural lands. Forests were recovering in the middle of the 20th century, but agriculture was giving way to development and, with more people, urban “heat islands” were appearing (e.g., Sprague et al. 2006, Jaworski et al. 2007)…Regardless of the cause(s), rising temperatures in the river are a concern. Warming seasons disrupt the life cycles of aquatic organisms, and very hot summers can kill them.”

“Water pH rose from 7.6 to 7.9 between 1940 and 1970 and then stabilized. This rise in pH represents a roughly 50 percent decrease in hydrogen ion (H+) concentrations. The timing of the rise suggests parts of the Potomac watershed were in the process of recovering from the destructive agriculture practices and large-scale logging of the late 18th and early 19th century (Sprague et al. 2006). A contributing factor could very well be the Great Appalachian Valley bisecting the Potomac watershed. The Valley is underlain by carbonate (“karst”) geology, which would tend to reduce the acidity of waters flowing through or across it.”

Chloride concentrations at Great Falls rose significantly between 1940 and 2019, with the highest measured values occurring in winter and early spring. Weathering of rocks and sediment are natural sources of chloride in rivers, but high concentrations also come from winter road salting, fertilizer runoff, and oil and gas production. Road salting during snow and ice storms is now considered the largest source of chlorides to the Potomac and its tributaries in the Washington, D.C. region (e.g., Porter et al. 2020). “

“There has been an increase in chlorides in summer and autumn.This may indicate that groundwater holds chlorides deposited during winter and slowly releases them to the river as baseflow during drier months. Evaporation from the river surface during warm weather could also concentrate chloride in the water.”

“The 80-year trends presented show a myriad of environmental impacts to the Potomac River, many of which are relate to population growth and unsustainable uses of the basin’s land and water. Efforts to mitigate these impacts were begun after 1940. They include state and federally mandated upgrades to drinking water and wastewater treatment plants, bans on phosphate detergents, and reductions in coal-burning power plant emissions. A more holistic understanding is emerging of the entire hydrologic system and the landscape that supports it.”