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.”
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