Crumbling concrete, equipment repairs and restrictions on the movement of goods and services are all too familiar for U.S. Army Corps of Engineer employees who deal with the challenges of aging infrastructure on a daily basis.
What may be lesser known is the impact of aging infrastructure on water quality in the Pittsburgh District. Like many large cities that built sewage systems in the late 1800s and early 1900s, Pittsburgh built combined sewer systems that combine sewage and surface runoff.
During periods of high rainfall, the sewage pipes are inundated with surface water and overflow into local bodies of water. These discharges are called combined sewer overflows (CSOs). In addition to CSOs, many of the sewer lines that run from Pittsburgh houses, built nearly 100 years ago, utilized clay pipe that may be broken or crumbling after years of use and may or may not carry all of the sewage to where it is supposed to go.
Because of these two pathways, during high rainfall events a measurable amount of pollutants --nutrients being one of them -- is transported via surface and ground water to Pittsburgh’s rivers.
Recently the district’s water quality unit joined efforts with Emily Elliott’s lab at the University of Pittsburgh (http://www.pitt.edu/~eelliott/index.html), which is researching the sources and loads of nutrients to regional water bodies.
Utilizing GPS technology, water-quality sonde measurements, flow cells, peristaltic pumps and an ultraviolet nitrate sensor, the Army Corps and experts from the university were able to map nitrate concentrations in the three rivers around Pittsburgh.
The survey occurred on Oct. 24, 2016, just after about 2.5 inches of rain had occurred over the region between Oct. 20-22, resulting in increased gage heights on both rivers and a CSO advisory from Allegheny County Sanitary Authority, or ALCOSAN.
The nitrate measurements provided interesting spatial trends. The Monongahela River had nitrate levels nearly double that of the Allegheny River. Two factors support these findings: the increased population in the Monongahela River basin as compared to the Allegheny and the amount of CSOs that drain into the Allegheny and Monongahela River main stems, 149 and 245 respectively.
The Elliott lab also collected water samples for nitrate isotope analysis, which allows the researcher to pin point the source of the nutrient, e.g. fertilizer, animal waste, or atmospheric in the case of nitrate.
In low concentrations, nitrate is not problematic and is necessary for primary production. But with the increased amounts found in the Monongahela River during this survey, it is indicative of the pathways by which nitrate may be carried into waterways.
This is especially important in light of the occurrence of the large algae bloom that occurred on the Ohio River in the summer of 2015 because it identifies the nutrient sources and pathways by which these algae blooms are supported. In addition, it highlights the need for renovating the existing sewer infrastructure of Pittsburgh and the beneficial contributions of “green infrastructure” that take up nutrients and slow / reduce the flow of surface water to waterways.
There are still many questions about nutrient pollution, algae blooms and the processes by which they are supported. However, with additional studies like this one, those monitoring water quality on local rivers are able to understand a little more.