With environmental compliance in mind, a general contracting company was required to find a solution to turn turbid, coal-laden water into clean, clear water prior to discharge into a neighboring trout stream. Not doing so meant facing the possibility of stiff fines and a project shutdown for violations of National Pollutant Discharge Elimination System (NPDES) Phase II regulations. Whiting Turner, the general contractor for the project, sought an innovative solution to filter water exiting the stormwater detention basin built for Boulder Business Park, a large-scale distribution and retail complex located just west of Allentown, PA.
The detention basin was incorporated into the project design to collect excess stormwater runoff from the large site. Providing a means to capture the excess runoff was important because, in addition to a 1 million-square-foot warehouse distribution complex, Boulder Business Park is also home to multiple-use office space. Construction on the complex began in 2003, and construction on the detention basin began in the spring of 2005.
To excavate the basin, workers with Whiting Turner blasted through rock layers to get down to grade. As the dust settled, they encountered groundwater, which filled up the basin. Whiting Turner began dewatering the basin, only to find that the water was contaminated with fine coal sediment seeping into the water from the surrounding deposits.
It soon was discovered that pumping the water through traditional geotextile filter bags did not capture the sediment, resulting in the coal-laden water being discharged downstream and imperiling a wildlife habitat. Inspectors with the Lehigh County Conservation District (LCCD), located in Allentown, quickly spotted the problem. As the lead agency for administering the Environmental Protection Agency’s NPDES Phase II permits, the LCCD reviews stormwater pollution control plans and inspects construction sites for compliance.
“The contractor was not using adequate best management practices [BMPs] when dewatering the excavated area of the proposed stormwater basin,” says John Bohman, lead resource conservationist for the LCCD. “As a result, sediment pollution occurred. We informed the contractor that the dewatering operation needed to stop until adequate BMPs were implemented.”
Facing a shutdown of the project due to noncompliance with Phase II and violating Pennsylvania’s Clean Stream laws, Whiting Turner brought in a biologist and environmental compliance specialist with Haines and Kibbelhouse (the H&K Group), a parent organization to several construction services companies throughout the country. The specialist worked with local site consultants and Lehigh Valley Site Contractors to evaluate the situation and devise a solution.
Contractors were advised to line the 200-foot channel connecting the detention basin to the stream with a geotextile fabric to prevent contamination of the channel bed. Next, suggestions were made to place both small and large stones in the channel to help filter the water. The small stones were 1.5-inch maximum diameter, while the large stones were 4-inch-diameter riprap.
However, even with these measures, the problem hadn’t been solved yet. Based on their experience, contractors knew that the stone filters and fabric would not be 100% effective and that they’d need additional measures to trap and treat the very fine sediment.
To devise a solution, contractors enlisted the help of Warren Cohn of ACF Environmental, a geosynthetic products supplier located in Norristown, PA. Cohn called upon Phase II experts from Profile Products LLC, a soil solutions company based in Buffalo Grove, IL, who suggested that Profile’s Terra-Tubes, fiber filtration tubes (FFT), would work in this challenging situation. Encased in heavy-duty, knitted tubes, Terra-Tubes are engineered composites of wood fibers, manmade fibers, and performance-enhancing polymers. Terra-Tubes have been proven through independent testing to be a highly effective stormwater treatment device designed to effectively trap, filter, and treat sediment-laden runoff. “Given the unique aspects of the water pollution, I thought that this would be an ideal application of the Terra-Tubes product,” says Cohn.
After lining the channels with the geotextile fabric, contractors staked two tiers of 6.5-foot FFT at intermittent intervals along the channel. Once the FFT were in place, the stone was carefully placed around the FFT and throughout the channel bed. Within three days of the stop-pumping request made by the LCCD, contractors had installed the geotextile, FFT, and stone and commenced filtration of the stormwater detention basin.
Cohn reports that the FFT have effectively eliminated the coal-laden water seepage problems via the product’s core functions of flow, filtration, and flocculation. The composite structure creates airspace and cavities to facilitate flow, which is critical to the filtration process. Unlike compressed fiber rolls and wattles, Terra-Tubes’ open structure contains far greater surface area and can trap more sediment. As water flows through the Terra-Tubes, flocculant crystals impregnated within the fiber matrix initiate coagulation or aggregation allowing the suspended soil particles to settle into collecting pools created by the tubes or directly on the surface of the soil or complementary erosion control devices.
The warehouse distribution center opened for business in late 2005. With no more coal-laden water endangering the wildlife of the surrounding area, “It is apparent how well they’ve performed when you see the black sediment that’s trapped in the tubes and see clear water leaving the site,” remarks Steve Zwilling of Profile.