Pitt geologists find substantial sewage inputs of nitrogen in
Nine Mile Run
PITTSBURGH—Aging sewer systems are spilling a considerable amount
of nitrogen into urban watersheds, diminishing both the quality of
water and ecosystems’ habitats. However, many studies documenting
the impacts of nitrogen on urban environs have not properly
estimated the contribution of leaky sewer systems—until now.
Using water samples from the Pittsburgh-based Nine Mile Run
watershed, a Pitt research team reveals in the current issue of
Environmental Science & Technology that an estimated 10 to 20 tons
of reactive nitrogen from sewage flows into Pittsburgh’s Monongahela
River every year from the six-square-mile watershed. That means that
up to 12 percent of all sewage produced by residents living in the
Nine Mile Run watershed area leaks from the sewers and is
transferred to the stream, negatively affecting stream water
quality.
“This is a very complicated problem,” said Marion Divers, principal
author of the paper and a Pitt PhD candidate who conducted the study
under the supervision of Pitt assistant professors of geology and
planetary science Emily Elliott and Daniel Bain, who were coauthors
of the paper. “You build a sewer system once, put it underground,
and unless there’s a catastrophic failure, you may not have a reason
to dig it up and make sure it’s not leaking. Now sewers across the
United States and in Pittsburgh are aging, and as these systems grow
older, more sewage is leaking into groundwater and streams.”
While living organisms need nitrogen to build essential proteins,
leaky sewer systems, the burning of fossil fuels, and overuse of
chemical fertilizers have contributed to an overabundance of
nitrogen in U.S. rivers and streams. Too much nitrogen can deplete
the water of oxygen, with results as threatening as those seen in
the Gulf of Mexico Dead Zone, where marine life doesn’t have the
oxygen necessary to live.
“Leaky sewers are simply not something most people are interested
in until they begin to smell it in the stream or see things like a
particular fish disappear from the stream,” said Bain. “Based on the
results from our Nine Mile Run study, this paper forces the wider
urban ecology community to more carefully consider this sewage
problem.”
In order to accurately measure nitrogen’s impact on Nine Mile Run,
the Pitt team had to first determine how much was coming from leaky
sewer systems. Over a two-year period, the researchers collected
water samples biweekly from the small stream located in Pittsburgh’s
East End during both rainy and dry time periods with intensive
sampling during one summer storm. Nitrogen concentrations were
measured in the samples, and the researchers used this data to
estimate sewage contributions to nitrogen in the stream’s water.
Notably, the results highlighted that sewers in this study basin are
leaking consistently, even during dry weather conditions. While the
apparent volumes of sewage are concerning, the study also reaffirms
the substantial ability of urban systems to hold onto this nitrogen,
despite the heavily impacted stream channel and the predominance of
paved areas.
“This suggests a pervasive influence of leaking sewers—even during
periods without rainfall. This is in addition to the raw sewage
contributions during wet weather from combined sewer overflows that
are currently the subject of mitigation efforts in Pittsburgh,” said
Elliott. “Our report highlights the importance of assessing nitrogen
leakage from sewers into our waterways, particularly as sewer
systems age across the United States.”
The paper, “Constraining Nitrogen Inputs to Urban Streams from
Leaking Sewers Using Inverse Modeling: Implications for Dissolved
Inorganic Nitrogen (DIN) Retention in Urban Environments,” appeared
online Feb. 17 in Environmental Science & Technology. This research
was supported by the University of Pittsburgh Kenneth P. Dietrich
School of Arts and Sciences, the Pennsylvania Water Resources
Research Institute, the Geological Society of America, and the U.S.
Steel Foundation.
SOURCE: University of Pittsburgh
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