By Grace Chan
Image: A Right of Way Bioswale. (Source: NYC Department of Environmental Protection)
Grace Chan on the problem of urban stormwater pollution and the high tech way New York City is measuring and mitigating the problem through its Green Infrastructure Program.
As cities around the world continue to grow, one of the major urban planning challenges they have in common is sanitation.
From waste management to the quality of our water, most people think of sanitation issues as primarily access to indoor plumbing and underground pipes, but it is actually much, much more than that. Today I will be discussing the water runoff created by storms, and its role as a major source of pollution in urban areas today.
A Uniquely Urban Problem
In nature, a large portion of precipitation is absorbed by soil, which is then stored in the earth as groundwater that slowly seeps back into the waterways. This process sustains life in the area and maintains the normal volume and flow rate of our waterways. ]In a city covered by asphalt and buildings there are less permeable surfaces for water to infiltrate. In a storm the water has less places to go, stressing both the piping systems and wastewater treatment plants that are designed to handle the runoff, resulting in combined sewer overflow, or CSO.
CSOs consist of untreated sewage waste and rainwater that gets discharged straight into a waterway. The untreated water carries with it human waste, chemicals and heavy metals that can cause health problems for people who come into contact with it as well as negatively effect a city’s marine ecosystem.
This is primarily an issue in cities that have sewer systems dating to the 19th and early 20th centuries, such as New York City.
A Brief History of Sewage in NYC
When the Dutch settled what would come to be New York City in the 17th century, the local inhabitants simply dumped their refuse onto the streets and into waterways. Human and animal waste, garbage and food scraps went directly into gutters, backyard outhouses, ponds and streams. As the city’s population grew, the waste that accumulated produced virulent forms of microbacteria that resulted in multiple deadly cholera outbreaks.
It wasn’t until the mid-19th century that the city began building sewage piping underground, much of which remains in use to this today. Known as a combined sewer system, this form of piping system transfers both excess stormwater and untreated sewage waste to one of the 14 wastewater treatment plants in NYC. These plants treat a combined total of 1.4 billion gallons of wastewater discharged by 8 million residents and workers every day, making it one of the biggest municipal water systems in the world.
Why Does it Matter?
So why does this matter to modern New Yorkers? It matters because when left untreated, stormwater runoff can create a supportive environment for harmful microbacteria to live in, which can find its way onto beaches or any place where people have access to the water. Chemicals and heavy metals found in CSOs have been found to be carcinogenic to humans, causing a variety of serious health issues.
When taken together this is an issue that touches on a number of important environmental and public health challenges, which is why New York City is working hard to address this problem with innovative technological solutions.
The Bioswale Project
To help reduce pollution and collect useful information about our urban environment, New York City’s Department of Environmental Protection (DEP) is utilizing a tool known as a “bioswale”.
A vegetated landscape armed with remote sensing technology, Bioswales are great examples of smart green infrastructure, allowing plants and soil to naturally filter out harmful chemicals and heavy metals while collecting actionable baseline data to infor
m city planning efforts. They also have the effect of slowing down the flow of stormwater runoff, decreasing the stress on NYC’s combined sewer system during rainfalls and reducing pollution caused by CSOs.
To the untrained eye bioswales can appear to be ordinary patches of greenery, however if you look closely you will notice two indented section of the curb which allow water to move in and out of the bioswale. The inlet is where runoff from the street first gets diverted into the bioswale, which travels through specially modified soil that holds runoff water and filters out pollutants. Once the bioswale soil is saturated, excess water then flows through the outlet into a catch basin at the end of the street.
Informing Urban Planning Through Data
My job was to help the DEP, in partnership with the New York Restoration Project, monitor a bioswale adjacent to the Gil Hodges Community Garden in the Gowanus neighborhood of Brooklyn. This was accomplished using remote sensing equipment known as piezometers and collecting soil samples. A piezometer is a probe that measures liquid pressure and is located in the bioswale to monitor the movement of water. These sensors provide useful data on the amount of water collected in the bioswale during rain events due to the change in pressure on the transducers in the probe.
Two rain gauges were also installed on a nearby roof to measure the amount of rainfall in the area to compare to the bioswale sensor data. This raw data would then be downloaded from both sets of sensors and uploaded into a DEP database known as Vista Data Vision for processing and analysis.
Besides monitoring the movement of water the DEP also wants to know what is in the soil, so soil samples are taken from both the inlet and the outlet side of the bioswale and brought to Brooklyn College for further analysis of chemicals, such as gasoline, and heavy metal concentrations, such as lead and nickel, following EPA protocols.
While bioswales are high tech ways to understand our urban environment, reduce stormwater flow and filter out harmful substances, they are fortunately also aesthetically pleasing, adding a little nature back into the amazing concrete forest we call New York City.