To be certain, these are challenging times for stormwater mitigation.
Water quality—as regulated by EPA and the Clean Water Act—is only as good as the approaches being used to achieve it. Those approaches should consider water quality not as a piecemeal approach, but a unified effort within watersheds, as recommended by the National Research Council.
That’s important when municipalities, design engineers, and private developers are trying to comply with the National Pollutant Discharge Elimination System Phase II and total maximum daily load requirements against the backdrop of limited space and limited budgets, which often call for creative design solutions.
Development brings an increased amount of impervious surfaces, and the natural flow of stormwater runoff is being altered as it enters streams and groundwater supplies. This also has brought about a greater amount of pollutants that are swept up and delivered in the flow, creating water-quality degradation that in urban streams is reaching levels with disconcerting human health, aquatic life, recreation, and economic implications. Aging infrastructure amplifies the challenges.
In New York City, StormChambers, manufactured by Hydrologic Solutions, have been used in conjunction with green infrastructure to eliminate a combined sewer overflow (CSO) problem at a metal recycling facility. Sims Metal Management Municipal Recycling (SMR) in the Bronx successfully recycles hundreds of tons of metals and plastics daily for the industrial and commercial sectors. The facility would often flood during rain events, with thousands of gallons of stormwater runoff heavily laden with metals, hydrocarbons, suspended solids, and other pollutants heading into the adjacent Bronx River.
Now, stormwater travels through a system of native plant meadow swales; an infiltration conveyance system composed, in part, of StormChambers; an evaporative wall; a created wetland; and a doubled-stacked underground detention/infiltration system consisting of 240 StormChambers and 15 StormChamber SedimenTraps that collectively collect, clean, and safely infiltrate the stormwater.
This was the first zero-discharge industrial retrofit of a material handling facility ever built in New York City, or the state, that was able to handle all runoff from a 10-year-or-larger storm through such an integrated system.
“In New York City, even one inch of rain can create a huge problem because all of the stormwater runoff goes into the sewer system, and the sewer system eventually ends up in the wastewater treatment plants,” says Professor Joshua Cheng of the Environmental Sciences Analytical Center at Brooklyn College of the City University of New York. “If the wastewater treatment can’t handle it, it gets released into the water bodies that are around the city and that includes all of the animal and human waste—whatever it is that is in there—that is going to the CSO.”
New York City does not have the capacity to handle one-inch rains, he points out, adding that the idea of the StormChamber system is to relieve the amount that goes into the sewer system and divert the water into green infrastructure.
Paul Mankiewicz is a hydrologist and consultant to New York City on stormwater and related issues and is executive director of the not-for-profit Gaia Institute in the Bronx.
The institute couples ecological engineering and restoration with the integration of human communities in natural systems.
For Mankiewicz, the SMR project exemplifies the greater need to find solutions for urban stormwater management, especially in areas such as New York City that still have CSOs.
For the SMR project, Mankiewicz wanted an underground stormwater infiltration system that would be able to capture large amounts of stormwater and help remove pollutants while being strong enough to sustain the impact of 18-wheelers loaded with steel. It also had to be cost effective and meet regulations.
He chose StormChambers from among the options because the technology offers open-bottomed, arch-shaped plastic chambers made of 50% recycled HDPE, each with 115 cubic feet of storage (with 6 inches of stone above and below). They exceed the AASHTO H-20 Wheel Load Rating requirement by more than four times (more than 32,000 pounds per square foot), with only 18 inches of compacted soil.
Not only are plastic chambers less expensive than pipe, but they require much less installation time and labor. A single row of StormChambers can provide a conveyance capacity approximately equivalent to 42-inch-diameter pipe. Other benefits over pipe for conveyance include cost, groundwater recharge, peak flow attenuation, a longer lifespan, efficient maintenance, and water-quality enhancement with effective removal of phosphorous, nitrogen, suspended sediment, hydrocarbons, heavy metals, and fecal coliform bacteria.
The biomat that forms on the stone and soil exposed to the stormwater does most of the water-quality enhancement. Microorganisms naturally occurring in the soil will metabolize just about anything, but their populations are very low. When a food source is made available, the numbers of those microorganisms that metabolize it increase exponentially until they eliminate it, and then their populations die back to pre-existing levels, similar to septic drain field functioning.
Chamber technology also offers significantly improved infiltration abilities, with open-bottom chambers providing more than 1,000 times the open area in direct contact with the surrounding stone and soil than does perforated pipe, thus avoiding the potential for clogging and reduced infiltration.
In an effort to ensure the removal of metals and hydrocarbons from stormwater to keep it away from the Bronx River, the concrete work surface was tilted a couple of degrees away from river, so instead of water flowing east, it flows west toward the street, Mankiewicz explains.
There, it enters a long, heavily mulched infiltration meadow of native plants, which captures some of the sediment and removes some of the heavy metals and nutrients. The stormwater is then conveyed via a single row of StormChambers to a manmade aquifer consisting of the stacked StormChamber system and gravel sitting above the groundwater.
Four of eight solar pumps move water into the head of a wetland system adjacent to the scale of the recycling facility. The system circles around a railroad spur. From there the water runs into a wetland along the property line, and when that fills up, it drops back down into the groundwater and is filtered again before slowly being released to the Bronx River.
The other four solar pumps also withdraw some of the stormwater from the StormChamber system, but they discharge it over a 3,000-square-foot evaporative green wall to help reduce the amount of runoff through evapotranspiration.
This helps to reduce the amount of water stored in the StormChamber system to maximize available storage capacity when storms occur in rapid succession. It also creates an aesthetically pleasing effect. The wall is covered with ferns, mosses, and many other moisture-loving plant species. The sheet flow over the wall adds to the visual appeal.
Mankiewicz’s choice of StormChambers met the design requirements for strength because the chambers could be stacked three deep with 30 feet of cover, if needed. The chambers were large enough to accommodate the volume of stormwater that would pass through the system and, according to Mankiewicz, also were 30% more cost effective than other options.
Pollutant removal was achieved, in part, by constructing a large enough humic and rhizosphere filter to remove and capture metals and hydrocarbons in the runoff from the metal, plastic, and glass material handling zone.
The StormChambers also functioned to further remove pollutants and, through infiltration, provided for zero discharge of stormwater.
The system has the capacity to capture a 10-year storm—about a half million gallons of runoff—but because silt and clay fill was removed and replaced with a sandy structural material, the actual capacity may be as much as a million gallons of additional runoff, Mankiewicz notes.
The system intercepts 6.4 million gallons of runoff containing metals, hydrocarbons, and suspended solids each year from the 6.4-acre site. Nitrogen, phosphorus, and potassium are intercepted, utilized, and recycled by growing plants and the StormChamber biomat system.
Changes in the water chemistry in the StormChambers as well as in the groundwater following a storm event are being monitored, with an eye to long-term trends in water and soil quality changes. Current findings show that no pollutants are going into the groundwater or river, but instead are being degraded by the underlying biomat to nontoxic byproducts of microbial metabolism and captured by the soil column in keeping with the design.
