Because the Rain for Rent system was able to operate in a flow-through model, it was the best model for this application. This project has now come to a close and shows how invaluable a mobile treatment process can be for a large, multi-year project of this magnitude. 
 
Alaska: Airport Runway in Alaska requires innovative stormwater treatment for compliance and preservation
 
Petersburg Alaska is a fishing village located in southeast Alaska. Located on Mitkof Island, situated roughly 120 miles from Juneau to the north and Ketchikan to the south, the town of around three thousand 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 ft to the FAA compliant 1000 ft. Additionally, the RSA would be widened to 500 ft along its entire length.   A total of five new cross-runway culverts would be installed and redirecting drainages to pass through the newly installed culverts. 
 
The initial phase required excavation down sometimes over 40’ below the runway surface through soils predominate in glacial till overlaid with peat and muskeg up to 20 feet deep. Due to closure of the airport the project window required 24 hour 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 runoff 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 Transportion (DOT) to find multiple methods of preventing erosion and dealing with sediment on the project. One method of erosion prevention was to pump the snow melt and rainfall from one side of the project to the other and eliminate run-on. To accomplish this, Power Prime pumps from Rain for Rent were used in tandem.
 
Due to 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 prior to draining into the basin.
 
With an average 234 days of precipitation in annual rainfall of over 110” and 61.5” of snowfall, the volume of moisture along with large amounts of clay material the likelihood of very turbid stormwater runoff was extremely high. The Alaska Department of Transportation (ADOT) in coordination with the FAA, Alaska Department of Environmental Conservation and Alaska Department of Fish and Game, set up a pilot treatment BMP using a natural biopolymer chitosan (pronounced kahy-tuh-san) in a biofiltration model thru muskeg bog. The muskeg bog was located in an area that was permitted by the US 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 5000’ 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, its 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. 
 
An overview of the runway is seen in the picture below. The treatment system is shown to the right of the runway in what looks like a baseball field.
 
The treatment system consisted of four valves (one to each treatment train), followed by sock chambers (8” 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. See picture.
 
A semicircular silt fence installation was used to control any surface water runoff on top of the bog. The system had significant results with intake water running over 5000 NTUs at times; dropping to 279 NTUs at the discharge from the lower weir pond to 92 NTUs at the silt fence and about 40 NTUs after surface flow had passed through the organics about 20 feet further. The picture below shows the vast difference in treated and untreated water. 
 
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 sole airport for the island 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 SE Regional Office in Juneau. John noted that “Residual chitosan at the outlets of the treatment system remained below detection levels and turbidity reductions were generally quite impressive. I was surprised at the overall effectiveness of chitosan socs with water temperatures that were consistently close to 0o C. Although 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 (DO) or pH”. Art Dunn, CPESC, ADOT Permit Compliance Reviewer stated ”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 over 90 water samples taken during 3 major dewatering events over a 3 week period, treatment effectiveness in a cold water
 
environment was verified under conditions considered extreme at best and impossible at times.
 
PSG 001 – intake, PSG 002 – weir pond discharge, PSG 003 – silt fence
 
Reverse angle of treatment system with coir logs used for small sediment pond with a liner. The initial deposition area proved effective at preventing the clogging of the muskeg and burial of the existing vegetation extending the life of the treatment area. Providing for an initial deposition area of chitosan treated discharge must always be considered, especially in frozen and freeze thaw conditions.
 
With salmon population endangered from environmental impacts and the town of Petersburg’s economy so closely tied to the seafood industry, it will take proactive approaches in stormwater management to prevent further harm and maintain the majestic environment that is Alaska, the final frontier.