Wetlands for Water Treatment and Erosion Control
Constructed and natural wetlands at work
Found on every continent except Antarctica, wetlands are defined by the USEPA as “lands where saturation with water is the dominant factor determining the nature of soil development and the types of plant and animal communities living in the soil and on its surface.”
While they vary from region to region, as far as the Clean Water Act is concerned, the USEPA further defines wetlands as “those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas.”
Wetlands are sometimes called the “kidneys of the landscape,” removing pollutants from the water. They also serve as a flood-control measure, storing excess runoff.
While wetlands develop over time as the result of natural flow patterns, they also can be constructed. Designers can build wetlands and choose plant species to remove specific pollutants such as sediment, organic matter, or nutrients.
The main types of constructed wetlands include free-water-surface wetlands—shallow ponds with a water depth of no more than 3 feet, with various types of plants at and below the water line—and subsurface-flow wetlands, in which water is maintained at a constant depth below the surface of the growing medium, usually gravel, and is typically less than a foot and a half deep.
Stormwater treatment is evolving, says David Whitney, owner of Ecosolutions LLC in Burlington, VT, a company that designs constructed wetlands.
“Historically, engineers that were tasked with doing stormwater treatment would think of ponds,” he says. “There’s a lot of buzz now around using wetlands, but people need to realize there are a couple of different types of wetlands, so a natural wetland may not resemble a manmade or constructed wetland for stormwater treatment.
“It depends on the watershed in which it’s treating stormwater,” he continues. “In general, a natural wetland is one where you expect to see the water surface—bogs or mucky places with frogs and emerging vegetation. Those will take up the biggest footprint in the landscape.”
When installing wetlands for stormwater treatment, Whitney considers site size. “The more impervious the area, the bigger the footprint for your wetland treatment system.”
Water quality is the first consideration in doing stormwater treatment, Whitney says. “Ninety percent of storm events will fall under a certain volume. The theory is you can treat for the pollutants in that,” he adds. “Anything over that, we’re usually trying to capture flow and restrict discharge back into the natural environment so we can minimize the erosive and scouring potential back to the watershed.
“Constructed wetlands will not provide that type of treatment, the flow continuation,” he adds. “They are excellent for treating things like suspended solids and removal of any kind of biological contaminants found in stormwater runoff, like nutrients running off of a fertilized lawn or water coming off of a feedlot.”
But constructed wetlands may not be the best technology for large water volumes and equalization, says Whitney.
“You can design it with freeboards so they can back up, but it becomes a site consideration,” he adds.
Another consideration in designing wetlands is aesthetics, Whitney adds.
A Small Footprint in Steep Terrain
One project on which Whitney worked more than two years ago was the Jay Peak Ski Resort in northern Vermont. Subsurface-flow (SF) constructed wetlands were designed by Engineered Solutions Inc. in Burlington, VT.
In that project, a rapid expansion of condominium construction took place adjacent to ski trails on slopes of 15% and greater. In an effort to preserve the mountain landscape, while protecting the environment, resort developers sought to disturb as little land as possible on the hillside.
Also, part of the stormwater system was being designed as a retrofit for existing buildings not compliant with current regulations.
The construction sites presented challenges: limited tree clearing, existing buildings, steep terrain, high groundwater, and shallow
The approach the designers decided on would be Vermont’s first SF-constructed wetlands.
A conventional stormwater design of large ponds was ruled out because of safety concerns, as well as the amount of downslope fill required to reach existing grade; wetlands, in contrast, have no standing water. In addition, the smaller footprint of wetland stormwater treatment systems addressed site challenges created by the existing buildings.
The solution: constructed SF, small footprint wetlands for water-quality treatment of the runoff generated from the slope-side condominiums and dry detention ponds for temporary storage of extreme storm events.
The constructed wetlands designed for Jay Peak’s condominiums are no more than 4 feet deep with nearly vertical side slopes, allowing them to be cut into the hillside, significantly reducing the amount of required downslope fill.
Each wetland was lined with an impermeable EPDM rubber membrane to maintain a permanent water level below stone media. The wetland was filled with 2- to 4-inch stone and topped with pea stone for planting.
Infiltration chambers redistribute flow within the wetland’s inlet end, with water collected at the bottom of the outlet end to ensure all water entering the wetland passes through the stone media.
While the sizing criteria were based on Vermont’s Stormwater Management Manual, it served as a minimum requirement for the design. Hydraulic loading rates were investigated to ensure 100% of the water-quality volume design flow passes through the media.
Treatment is another consideration: 80% of the post-development total suspended solids (TSS) and 40% of phosphorous should be removed.
HydroCAD was used to base the water-quality design storm on watershed subcatchment area characteristics. Resulting influent flow intensities were modeled in PTC, a groundwater modeling software, to ascertain whether the storm surge would pass through the wetland or overflow the media.
Installing large, 2- to 4-inch stone media up to the surface of the wetland created an inlet “crash” zone. The stone had sufficient void space and a high enough hydraulic conductivity to allow the peak water-quality design storm flow to infiltrate the wetland. An extreme-storm-event bypass was installed to prevent larger flows from passing through the wetland and potentially damaging the system.
Photo: Fitzgerald Environmental Associates
Construction of a wetland stormwater treatment system
Another important design consideration for constructed wetlands is water-level control. Through such measures as an adjustable standpipe on the outlet and manufactured adjustable water-level control systems such as the DOS-IR, water can be drawn down to the desired design level beneath the top of the media after initial plantings receive elevated water levels required during the first growing season to establish root systems.
Another design consideration is plant selection, the key factor that makes these systems wetlands.
Factors affecting plant selection include shade, direction the wetlands are facing, and aesthetic impact. Native plant species are favored for their optimal survival rate. The design favors an even distribution of a variety of plants throughout the wetland to minimize die-off due to plant disease, nuisance pests, and other environmental parameters.
Over time, certain plants are expected to outperform others and begin to take over the wetland. Local sources for wetland plants ensure appropriateness for regional planting zones and microclimates.
Treating Airport Runoff
An increasing use of wetlands for water-quality treatment of runoff is taking place in North American airports.
Federal regulations are the driving force behind that, says Mark Liner, a senior engineer with Jacques Whitford North American Wetland Engineering (NAWE) in St. Paul, MN.
The USEPA is expected to release its Airport Deicing Effluent Guidelines proposed rule later this year. Although the Federal Aviation Administration requires deicing and anti-icing of aircraft and airfield pavement during icy conditions to ensure passenger and cargo operations safety, when the procedures are done without discharge controls, a negative environmental impact can result.
After reviewing airport stormwater discharge permits under the National Pollutant Discharge Elimination Program (NPDES) required to ensure the proper collection and treatment of deicing operations, the USEPA developed effluent guidelines. Airport wastewater samples were analyzed for a variety of pollutants with the data used to characterize pollutants in raw wastewater prior to treatment in an effort to document wastewater treatment and recycling system performance.
Final action on the proposed rule is expected by the end of 2009.
“Now, when a permit writer goes to an airport, he really has no centralized information for guidance,” says Liner. “The permit writer does what he thinks is right. This is going to be a guidance document that all permit writers can use for any airport that does deicing.”
Liner says that many states, such as New York, already have stringent deicing requirements whereas some airports have none, creating a process dependent upon the permit writer’s judgment call.
Airports are typically located near water bodies.
“Airports already have very large stormwater concerns in the summer, because it’s very important to keep the water off the runways and also to keep bird strikes down,” says Liner. “They are well drained. The question is what happens in the wintertime when, instead of trying to get the water off as soon as possible, you’re actually trying to slow it down, hold onto it, and treat it. Most airport drainage systems aren’t set up that way; they’re set up to get the water off the airfield as quickly as possible.”
Liner explains that after the deicing fluid hits the pavement, the question is how much can be collected.
“There are recovery or collection efficiencies and you can vacuum it up, but it still doesn’t get everything,” he says. “Where do the leftovers go? In some cases, it gets funneled to a sewage plant; in other cases, it won’t have any treatment—it will just go off the airfield. That’s been a concern.”
Deicing is a Catch-22 operation, says Liner.
“It goes to the issue of safety—you cannot save any money on safety,” he says. “It’s up to the pilots how much deicing fluid they are going to put on the planes, and they spare no expense.
“However, with the glycol treatment system, there’s this question of how much you can afford to protect the local environment. If you have a major storm event that happens once in 10 years, it’s going to be very difficult to manage the environmental effects. Maybe you can handle 85% of occurrences, but to have a handle on 100% of occurrences is a very tough proposition.”
Airport managers consider liability, Liner points out.
“You have these tenants like Northwest and United, and usually they are in charge of safety of their planes, as they should be. And they deice, deice, deice. But after they are done deicing, it’s the airports who own the problem afterward, so it’s a difficult situation.”
Among the many airport wetlands projects Jacques Whitford NAWE is working on is the use of aerated wetlands at the Buffalo Niagara International Airport (BNIA).
The use of aeration is critical for the below-grade gravel beds designed to treat spent glycol found in stormwater, says Liner.
The system is designed to supply oxygen to bacteria attached to the gravel and can be controlled relative to the level of glycol being treated. The system is currently designed for 10,000 pounds of oxygen demand per day and is about the size of four football fields.
At full build-out, the wetland will consist of eight wetland cells excavated from an existing open area near the airport’s main runway, Liner says. Only a field of grasses will be observable growing from a “dry” mulch surface at ground level, he adds.
Photo: Western States Reclamation
The Richmond Hills wetlands mitigation project
Liner says the gravel size used and the porosity of the bed are important to the design. Gravel measuring 0.5 to 0.75 inch in diameter permits bacterial biofilm accumulation, a slime that grows during the deicing season and degrades in the summer.
A detailed analysis of biomass growth, storage, and decay was undertaken to ensure gravel beds would not clog, Liner says. A vertical flow configuration was selected in which stormwater is distributed uniformly over the beds and flows downward to an underdrain system, he adds. The large application area, coupled with large gravel voids, minimizes clogging potential.
Stormwater flow and concentration will be closely monitored and controlled, as necessary, to optimize wetland performance. Air and nutrients will be supplied to the system to match the pounds of glycol measured, Liner says, adding that the system is engineered to maintain an active biomass within the wetland throughout the winter.
The system is constructed below ground with an insulating mulch layer on top to maximize water temperature.
During warmer summer months, accumulated biomass will degrade and be consumed by larger “bugs” that graze on the slime-covered gravel, says Liner. The natural digestion of biosolids is a seasonal means for managing sludge generated in glycol treatment.
The project is in compliance with the state of New York stormwater permit, which limits the concentration of biochemical oxygen demand in stormwater to 30 milligrams per liter. An additional regulatory restriction on the project is the volume of stormwater discharge to Scajaquada Creek, south of the airport.
The discharge from the southern half of the airfield is limited to a maximum flow rate of 181 cubic feet per second, which effectively translates to 154 cubic feet per second for the project area, says Liner.
To meet this restriction, the airport previously constructed a stormwater system with a 3-million-gallon vault. Pumps from the vault discharged to a culvert feeding Scajaquada Creek, he adds.
Liner says that there are often competing interests between stormwater management and wastewater treatment. “What the stormwater guy wants is not what the wastewater guy wants,” he adds. The BNIA project is a marriage of the different goals.
Whereas the stormwater engineer is focused on storage volume and flood prevention, a wastewater engineer looks at tank volume for equalizing flow and concentrations with both gaming during the design process to maximize tank volume dedicated to meet their respective goals.
In the end, stormwater management volume wins out because it affects the core of airport operations and airfield safety, says Liner. In the BNIA project, there is also volume for equalization, he adds.
The airport has more than 1 million gallons of tanks dedicated to glycol storage, which will be used to smooth out peaks and pace the discharge of glycol-contaminated stormwater to the wetland system, Liner says.
At the close of deicing season in the spring, the wetland treatment system will be used as a tool in the stormwater volume management. The water level in the gravel beds is fully adjustable, allowing the operator to utilize the beds to buffer the flow from summer storm events, Liner says, adding that because the system is already piped for managing peak flows, no additional infrastructure modifications are necessary. The beds provide treatment in the winter and storage in the summer.
Another airport project is taking place in Edmonton, AB, in Canada.
“Canada has a different environmental situation,” says Liner. “We have an existing wetland system at Edmonton International Airport designed for treatment of snowmelt during the winter. A lot of deicing fluid that hits the snow or that melts gets channeled down to a water body outside of the airport and, during winter, they will run it through the wetlands for treatment.”
Liner points out that Edmonton is undergoing a large expansion in support of the oil sands area (dirt saturated with oil) to the north.
“It’s a real boomtown there now, so they are expanding, trying to meet the incredible growth in the Edmonton area,” he says. “The question is, how they can expand the airport? Our project is trying to remove bottlenecks in the existing system.”
One approach is to add aeration to the existing system to remove the glycol.
“The project is to increase the capacity and we know by our initial study that we were probably going to be able to increase the capacity by tenfold,” says Liner.
The present system is not aerated. It is a large passive wetland with 12 cells measuring 50 meters by 50 meters; the aeration would be added to those cells.
The biggest challenge is in managing the upstream flow of the treatment unit, says Liner.
“I call this a ‘bathtub study’—it’s a question of how big the bathtub’s got to be to store all of the water,” he says. “The treatment system is the hole in the bathtub. You don’t want the bathtub to overflow, so now one of the big challenges is trying to figure out how big to make that hole in the bathtub—how big of a treatment system do we really need to handle the spring melt.”
In terms of technologies being considered, Liner says that the current system is a horizontal flow bed, with flow going from left to right; his company is looking at converting it into a vertical-flow system so that the water is flooded on top and then flows straight downward.
“That allows a larger window for the water to flow through,” says Liner. “If it’s a horizontal flow, it’s a small rectangle, so all of that flow has to go through that small rectangle. When we do vertical flow, it’s a much larger footprint, so that means we’re not hydraulically impeded at all. It’s about the same volume of gravel.”
Jacques Whitford NAWE uses vertical flow technology on many of its domestic wastewater systems.
“The idea of taking water and distributing it over the top is different for wetlands,” says Liner. “With most wetlands, you think of horizontal flow.”
Ultimately, every airport operation has a different deicing activity associated with its regional weather.
“There’s not a one-size-fits-all approach for them,” he says.
Rehabbing Natural Wetlands
Meanwhile, Wilco Industrial Services in New Roads, LA, performs rehabilitation of natural wetlands using a patented jet spray technology dredging system. The spoils from the dredging operations are spread thinly over existing wetlands adjacent to the project.
“This process mimics the natural flood process that used to occur in the past when the Mississippi River would flood the wetlands every year, and, as a result, deposit a new layer of sediment over the wetlands,” says Mike Johnson, executive vice president of dredging. “Now, with levees, that natural process has been significantly impacted. Because of that, the Louisiana Department of Natural Resources mandates that all dredge spoils from operations are utilized in a beneficial manner. As a result, they advocate the use of jet spray technology to revitalize marsh in those areas.
“For projects that occur now within the oil fields in Louisiana, the state mandates that spoils from dredging operations are utilized in a beneficial manner, so they would either have to be used to create additional new wetlands or to revitalize existing wetlands,” says Johnson.
Johnson says his company employs two techniques. One involves basic hydraulic dredging of sediment. The company deposits the sediment adjacent to or within existing marshes to create new wetland areas in addition to the existing ones there.
The other technique involves a jet spray. “We spray the spoils over that existing marsh in a thin layer; it revitalizes the existing marsh by basically adding a new layer of sediment and nourishing the existing marsh,” Johnson says.
The specially designed equipment acts like a large lawn sprinkler that is mounted directly on the back of the dredge equipment. In some cases, the company pipes the spoils from the dredge to a remote barge or piece of equipment where the spray unit is mounted, and spraying is conducted from there.
One of the challenges involved centers on monitoring the thin-layer application technique, Johnson says.
“We have a rigid quality-control system in place, because normally we are controlling lifts of that sediment anywhere from 2 inches to generally not more than 6 inches,” he says. “That way, it has the most beneficial use it can. Generally, when you exceed that type of application, it has a much more significant impact upon the existing vegetation.
“When we are able to maintain that quality control and keep it thin like that, the vegetation basically very quickly comes right through that layer and revegetates the area very rapidly.”
The other massive challenge that Wilco Industrial Services has in the operation is the proximity of the actual dredging operation to the wetlands over which the material will be sprayed.
“We address that by running several thousand feet of pipe; we connect the dredge to a remote spray location that is mounted either on a barge that can be moved to different areas or on a small marsh buggy that can travel and advance the spray head as we revitalize the marsh,” Johnson says.
Looking back on some of the projects his company has done over the years, Johnson says that they are performing “great.” One year after a marsh renourishment project at the Rockefeller State Wildlife Refuge in Cameron and Vermilion parishes, “there was a very visible difference between those areas and the adjacent areas,” he notes. “Vegetation seemed to be more vibrant, maybe even higher than the adjacent vegetation.”
Dredge spoils are also the center of an approach being used by Dredge America of Kansas City, MO, as the company works on a wetlands project in a 19-acre saltwater marsh at the edge of the Gulf of Mexico in Pascagoula, MS, in conjunction with the US Army Corps of Engineers.
The project creates wetlands through the beneficial use of dredge spoils, says Dan McDougal, the company’s president. The project is adjacent to refineries and shipbuilding operations.
“There’s a rock dike encircling the outside of it. Basically, we’re pumping from an old dredge spoil area and filling this area in to create a saltwater marsh area. When we get finished and get all the material pumped for the grade, the grasses that will grow will be planted,” says McDougal.
Access is one of the biggest challenges in the project, he says.
“We’re working in and around swamp or low land areas just to do this work, because it is very difficult. We have a marsh buggy; that’s the only way you can access the area every time it rains just to be able to get close to the area to work. It’s a difficult environment to work in.”
Turbidity from pumping is one of the water quality issues involved in the area, notes McDougal.
“We have a small dredge, and we’re pumping this material and making it an even grade and making sure the turbidity inside of the rock area doesn’t affect anything outside of that area,” he says.
That involves monitoring the pumping rate and where the pipe is moved, says McDougal.
“We have computer equipment on the dredge that shows us where we are and how far we’ve advanced. The marsh buggy is at the end of the pipe, and we make sure we have a consistent grade. We have less than a foot of tolerance, because if we put it in too high the grass won’t grow, and if we put it in too low it won’t grow; it’s a very narrow range that the wetland grasses will grow in.”
The computer software Dredge America utilizes in its mission is HyPack’s Dredgepack, a hydrographic survey software.
“It uses GPS satellite, and as you’re dredging, it shows a picture of the dredge moving on your computer screen, so it actually tracks where you’ve been—your depths and exact location,” McDougal says.
He is noting a more widespread use of created wetlands for stormwater treatment in his work.
“It is definitely going to be a thing of the future—you’re going to be seeing more of it all the time,” he says. “In this case, we’re making space available in the dredge spoil for the navigation channel that will eventually need to be dredged again, so we’re making a beneficial use to create this wetland. It’s a win-win, which is what everyone looks for.”
Construction and Maintenance
At Western States Reclamation where David Chenoweth and Liz Chenoweth serve as president and vice president respectively, the company has become more engaged in wetlands construction in the past several years.
The company has done the earthwork portion of wetlands through wet excavation, using machinery such as low-ground-pressure dozers and trackhoes, says David Chenoweth.
The company also performs the additional tasks of setting riprap, planting wetland species and upland shrub communities, seeding, using geotextiles and hard-armor products, and doing the maintenance and reporting commonly required by the US Army Corps of Engineers.
“We are the select contractor for a couple of different engineering firms we team up with when there is a design/build effort,” says Chenoweth.
A large percentage of work done for the Colorado Department of Transportation involves wetlands work using a seed mix with a considerable number of plant species, he says.
“We are either seeding or planting sedges, rushes, and those kinds of species, and we’ve set ourselves up with different pieces of equipment to do that type of seeding if it’s a large enough area,” he explains. “We have all-terrain vehicles and broadcast spreaders mounted with harrows and other soil-preparation equipment so we can get good seed-to-soil contact and cover the seed properly.
“We also have used our dozer with a broadcast spreader mounted on the front so we can creep out in the areas and apply the seed as well as track it in with cleat marks, which has been very successful,” says Chenoweth.
He says that the company gets many calls for this type of work because it has a significant labor force, including 70 workers from Mexico who work through the US Department of Labor H2B visa program and who have been trained for many years.
“We are on the second generation of workers, and our efficiency is there, especially under the supervision of a foreman or superintendent who’s got more of the academic knowledge of the plant species and the planting patterns,” says Chenoweth.
Cost-effectiveness also is realized through equipment efficiencies, he adds. “We’re seeing that there has been a lot of experimentation with different installation practices, so contractors and engineers like us have been able to zero in on the most cost-effective practice based on successes.”
It’s also cost-effective to be cognizant of which species will best reestablish from seed rather than from nursery stock, he notes.
“We’re finding that with the three-year maintenance period something we are able to do more cost-effectively as compared to previous years is using a lot of polymers. That’s reduced our watering significantly, and we are able to pass those savings on to the client,” he says.
“It goes back to research and development and looking at a number of projects where there had been three-year maintenance and being able to see where your failures and successes are coming from.”
Chenoweth says that his company has learned many lessons over the years with respect to wetland areas. “Maintenance is absolutely necessary during that three years to have diverse, functioning wetlands. Even though that’s hard on the contractor at times from a scheduling standpoint, a company really has to devote specific crews and specific scheduling to those projects to make sure they’re sustainable.
“Polymers are a help, but you’ve got a host of things that have to be done to get those products released. As a contractor, you don’t get the final retainer or bond released until you provide the clients sustainability as defined by the Corps of Engineers.”
That involves weed control, touch-up seeding, and plant material replacements that need to be done in a timely manner, says Chenoweth.
“It’s not a stand-alone project,” he adds. “[Wetlands] have to be maintained and examined on a monthly basis, especially during the growing season.”
One of the biggest lessons that the company learned quickly as it got more involved with soil excavation—even with specialized low-ground-pressure dozers—is that allowances must be made for equipment getting stuck.
“You have to have resources close by to pull that equipment out and train your labor force properly on proper planting techniques, especially making sure the small 10-cubic-inch materials are not too high or too low,” says Chenoweth.
“You also learn that before any planting is done, that you or the contractor needs see if the hydrology on the project is right,” he adds. “We’ve seen a number of projects where on paper the engineering drawings looked right, but the hydrology was not, and we had to send a request for information to the client or engineering firm stating our concerns over the excavation depth and [the hydrology] not properly functioning before planting was started.”
Chenoweth points out that after the earthwork is done and before the first plant goes in, one must evaluate the effectiveness of the plan.
“Is it really going to work? Are you really going to have a saturated base for planting? We’ve learned the hard way on a few projects where we just did what we were told as a contractor, and that’s a lousy attitude. You really need to be proactive with the client if you don’t feel it’s going to function.”
Among the wetlands projects in which Western States Reclamation has been involved:
- A revegetation and wetlands mitigation project for the Federal Highway Administration’s Hahn’s Peak Road improvements. The company planted more than 13,000 10-cubic-inch wetlands plantings, including water sedge, slender beaked sedge, small winged sedge, Nebraska sedge, field horsetail, Torrey’s rush, and American brookline. It also planted 500 1-gallon wetlands shrubs, installed 9,000 square yards of erosion control blanket, and performed 4 acres of wetlands seeding and 40 acres of upland seeding.
- Richmond Hills wetlands mitigation project, in which the company performed channel shaping, placed approximately 1,700 cubic yards of topsoil, placed riprap, installed turf reinforcement mats and erosion control blankets, placed willow stakes, performed wetland and upland seeding, and planted 10,500 wetlands plugs and cottonwood trees.
- Numerous Colorado Department of Transportation projects—an average of four state highway revegetation projects per year since 1983. Most of these projects have included wetlands seeding and mulching, wetlands plantings, brush layer cuttings (willow cuttings), willow wattles, 1-gallon and 5-gallon shrub plantings, 1.5-inch to 3-inch cottonwood plantings, and wetland plug transplants.
Carol Brzozowski specializes in topics related to stormwater and technology.