Project Profile: Airport Runway in Alaska Requires Innovative Stormwater Treatment
Petersburg, AK, is a fishing village in the southeastern part of the state. Located on Mitkof Island, situated roughly 120 miles from Juneau to the north and Ketchikan to the south, the town of around 3,000 has maintained its quaint feel and its Norwegian heritage. The Petersburg James A. Johnson Airport (PSG) needed to upgrade its runway safety area (RSA) from the existing 200 feet to a Federal Aviation Administration (FAA)-compliant 1,000 feet. Additionally, the RSA would be widened to 500 feet along its entire length. A total of five new cross-runway culverts would be installed and drainages redirected to pass through the newly installed culverts.
The initial phase required excavation down to depths sometimes more than 40 feet below the runway surface through soils predominant in glacial till overlaid with peat and muskeg up to 20 feet deep. Because of the closure of the airport, the project required 24-hours-a-day work during the initial snow melt and the beginning of the rainy season. Numerous erosion and sediment control measures were used to minimize sedimentation from impairing drainage basins that lead right through the town and into the narrows between Kupreanof and Mitkof Islands. Kiewit Pacific Contractors was diligent in working with the Alaska Department of Transportation (ADOT) to find multiple methods of preventing erosion and dealing with sediment on the project. One method of erosion prevention was to pump the snowmelt and rainfall from one side of the project to the other and eliminate runon. To accomplish this, Power Prime pumps from Rain for Rent were used in tandem.
Because of the high volume of bypass water, precautions were taken to prevent scour at the discharge points. Large native boulders were placed on a geotextile liner as a means of energy dissipation before the water drained into the basin.
Because the area has an average 234 days of precipitation, annual rainfall of over 110 inches, and 61.5 inches of snowfall, the volume of moisture along with large amounts of clay material made the likelihood of very turbid stormwater runoff extremely high. ADOT, in coordination with the FAA, the Alaska Department of Environmental Conservation, and the Alaska Department of Fish and Game, set up a pilot treatment best management practice (BMP) using a natural biopolymer chitosan (pronounced kahy-tuh-san) in a biofiltration model through a muskeg bog. The muskeg bog was located in an area that was permitted by the US Army Corps of Engineers to eventually be filled as part of the runway extension project.
The stormwater collected during the culvert installations was pumped up to 5,000 feet to a treatment system located at the end of the runway. The treatment system included four treatment trains to reduce system performance concerns and to enhance the data collection. Two products were used for comparative purposes, StormKlear GelFloc and ChitoVan LC. Chitosan was chosen because of its proven track record on glacial clays, complete biodegradability, low toxicity, and residual monitoring capability in the field for any excess polymer. Because of the remote location, the expense of transporting samples, and the construction schedule demands, residual testing at outside laboratories was unfeasible and impractical. A reliable field test needed to be available to ensure no environmental impact would be caused from the pilot system.
The treatment system consisted of four valves (one to each treatment train), followed by sock chambers (8-inch PVC in Y shape for access during sock changes) and a sprinkler head for high agitation of the polymer prior to arrayed directional discharge. Mini weired ponds were added to prevent scour of the bog.
A semicircular silt fence installation was used to control any surface water runoff on top of the bog. The system had significant results: Intake water contained more than 5,000 nephelometric turbidity units (NTUs) at times, but that measurement dropped to 279 NTUs at the discharge from the lower weir pond, 92 NTUs at the silt fence, and about 40 NTUs after surface flow had passed through the organics about 20 feet farther along.
Alex Zimmerman, CPESC, of CSI Geosynthetics in Vancouver, WA, consulted with ADOT on the project and the permitting of the pilot treatment system design and operation. Southeast Alaska conditions challenge the best contractors; with the island’s sole airport closed during the culvert replacement, delays of any kind were not an option.
All residual chitosan monitoring and system operation was conducted by John Barnett, CISEC, an environmental impact analyst with ADOT’s Southeast Regional Office in Juneau. “Residual chitosan at the outlets of the treatment system remained below detection levels, and turbidity reductions were generally quite impressive”, notes Barnett. “I was surprised at the overall effectiveness of chitosan socks with water temperatures that were consistently close to zero degrees Celcius. Although it was still very effective in reducing turbidity, the colder water may have reduced the solubility of chitosan, thereby reducing the risk of detectable residuals. I was also pleased to see very little deviation in background
dissolved oxygen or pH.”
Art Dunn, CPESC, ADOT permit compliance reviewer, says, “Chitosan will be an extremely valuable tool in our ongoing effort to comply with the Clean Water Act in this extremely challenging environment.”
The residual testing methods for chitosan allowed field verification of the retention of muskeg bog soils of sediment and polymer. With more than 90 water samples taken during three major dewatering events over a three-week period, treatment effectiveness in a coldwater environment was verified under conditions considered extreme at best and impossible at times.