Use of Wetlands for Stormwater Treatment
by Johnny Martin, PE & Tim Reid, PE
Moffatt & Nichol Engineers, Raleigh NC

Moffatt & Nichol Engineers was contracted by the Town of Emerald Isle to model the drainage characteristics of the southern tip of the coastal barrier island and find an acceptable solution to treating floodwaters that occur after major storm events. This was necessary due to changes in environmental regulations governing how the Town could remove the floodwaters. The study area is comprised of interior dunes and troughs that direct all runoff toward a single outfall located on the southwest end of the island. The entire area is heavily developed and the natural trough system has been interrupted by the development of roads across this natural trough. Fecal coliform is the major contaminant of the stormwater due to residential wastewater treatment systems that are mainly septic tanks with shallow leach fields. The complexities involved with hydraulics and environmental issues on a coastal barrier island along with the physical infrastructure constraints provided the challenge to design a viable solution.

Discussions with environmental agencies and residents gave us the parameters required for an acceptable solution. These parameters are: improve water quality, install with a minimum of disruption to the community, permitted by environmental agencies, little or no opposition from the community or other agencies or groups, and no adverse affects on the beaches or sound.

Only the solutions that meet these requirements would be studied in detail. The possible solutions included gravity feed through pipe systems, ocean outfall, aquifer storage and recovery (groundwater injection), packaged treatment station, land based infiltration / treatment, and the do nothing alternative. After evaluating each of the alternatives using the criteria listed above, the land/based infiltration/treatment system was chosen as the only solution that met all of the stated parameters.

The use of constructed wetlands/infiltration basins is a recognized method for treatment of stormwater in North Carolina, and therefore, the use of existing wetlands is the option with the best chance of being permitted in this case. This type of treatment system allows for many of the contaminants in stormwater effectively to be removed from the flow by natural means. The vegetation helps to remove phosphorous and nitrogen while the vegetation also slows the flow to allow sediments to settle. Downward percolation through the soils also removes additional contaminants and enteric bacteria. Fecal coliform removal efficiencies exceeding 90% have been reported from constructed wetlands. The multiple filtering of the runoff through sand filters and vegetated bays and then through the marsh grasses located at the edge of the property prior to entering the sound should yield similar results.

This option had no opposition from the Division of Water Quality or The Division of Shellfisheries. The Division of Water Quality also expressed a desire to work closely with the Town if this option was utilized to serve as a model solution for other coastal communities. The North Carolina Coastal Federation also preferred this alternative since it would require the acquisition of large tracts of coastal property which would preserve open space and wildlife habitat. Also, since large areas are required to effectively treat the storm water, there was a good chance that the Town could derive a secondary low impact use such as a park at the site. The transfer mechanism to get the stormwater to the treatment area consisted of a series of lift stations and forcemains.

As for design concerns, the proposed project was designed to temporarily store runoff in existing shallow, vegetated pools and depressions that will allow nutrient uptake and removal of urban pollutants. The infiltration area also had to be large enough to accept large quantities of water without adversely affecting nearby property owners (through surface flooding or amplification of groundwater levels). The treatment area would also need to be high enough to eliminate the possibility of a direct discharge during a storm surge. To answer these and other questions, Moffatt & Nichol Engineers developed a coupled surface-water, groundwater model. This model allowed the engineers to view the affects of surface and sub-surface flow on the proposed design. This also provided the guidance required to adequately size the small dikes and other perimeter elements to prevent impacting the nearby residents.

Another important factor to consider during design is that both the current plant and wildlife species may be impacted by the altered hydroperiod. These effects must be studied and quantified to verify that impacts will be minimized. In our case, the design allows a more complete wetland system to be developed and maintained due to the ability to hydrate the system throughout the year. The project is currently waiting funding for construction.