Constructed Wetlands

Growers face increasing pressure to remediate and treat runoff, both to reduce deterioration of surface and ground water quality and also to facilitate conservation through recycling and reuse of this vital resource. Many nurseries and greenhouses are implementing water and nutrient management plans to manage inputs and runoff, to save money and to comply with regulations. These plans may include treatments based on chemical, thermal, or radiation methods (e.g., chlorine, pasteurization and UV, respectively) to remove contaminants and pathogens from runoff.

Growers with an eye toward future sustainability need alternatives to chemical treatment to manage and recycle runoff. Constructed wetlands (CWs) are an effective, research-based ecological alternative for removing various contaminants from runoff.

Surface flow wetland designed for treating nutrient rich runoff from agricultural production areas.

Nutrient, pesticide contaminants
Residential, urban and agricultural land uses contribute to nonpoint source runoff. Excess nitrogen and phosphorus in runoff can lead to increased rates of eutrophication.

Local, state and federal environmental agencies are under pressure to limit pollutant discharges from identifiable nonpoint source contributors to further protect and improve water quality. Agricultural nonpoint sources can include greenhouse and nursery operations that do not capture and recycle runoff. States including California, Florida, Maryland, Oregon and Texas have adopted regulations mandating runoff capture or control by irrigated agricultural operations. Similar regulation in other states is likely as efforts continue to protect and maintain the quality of surface and ground water resources. In January 2009, the EPA in concert with the Florida Department of Environmental Protection proposed an expedited schedule for establishing numeric nutrient criteria limiting nitrogen and phosphorus pollution in Florida lakes, rivers, streams, springs and canals. This precedent could set the stage for regulations in other states in the next decade. California has implemented the Irrigated Lands Regulatory Program which controls runoff from all irrigated agricultural operations, including nurseries and greenhouses, in the state.

Pathogens, biological contaminants
Irrigation and runoff water can be infested with a variety of pathogens, and the risk of disease increases when water is recycled. All major pathogen types— fungi, water molds, bacteria, viruses, and nematodes— have been shown to be transmitted via recycled water. Of major concern are waterborne phytopathogens (Phytophthora, Pythium, etc.), which are perennial problems across all facets of agriculture and are responsible for billions of dollars of crop losses. Moreover, recycled water may contain high levels of bacteria. A University of Florida survey of 24 greenhouse and nursery growers throughout the United States found that bacterial load in 76 percent of all recycled irrigation waters sampled exceeded the levels needed to avoid clogging of water-conserving drip irrigation systems. A number of chemically based treatment methods have proved effective in controlling or reducing disease incidence and clogging of irrigation lines due to biofilms. Drawbacks to chemically based treatment systems include high initial investment costs, continuous operational expenses, worker safety issues and a potential for environmental harm if not properly managed.

Horizontal, subsurface flow wetland schematic with design considerations for treatment of nutrient-rich agricultural runoff.

Alternative treatment options
Constructed wetlands enable both small and large green industry operations to maintain the water quality levels necessary for successful crop growth while potentially facilitating the future use of alternative water sources.

Wetlands are considered the “kidneys” of the landscape because of their capacity for cleansing polluted waters. Success in cleansing municipal and industrial point-source discharges led to the widespread use of CWs.

There are various types of CWs: free water surface (surface flow), subsurface flow (horizontal and vertical flow), mobile, and floating vegetation wetlands.

Surface flow constructed wetlands
A surface flow CW resembles a shallow (0.5 to 2.5 feet) freshwater marsh and generally requires a large land area for wastewater treatment. A five-year study funded as part of the USDA-ARS Floriculture and Nursery Research Initiative and conducted by researchers at Clemson University examined the nutrient-removal capacity of a 9.31-acre surface flow CW receiving runoff from 120 acres of container production at a large nursery in Cairo, Ga. The CW was highly efficient at removing nitrogen (nitrate, nitrite and ammonia) from nursery runoff from mid-spring through late fall in the southeastern United States, although it failed to consistently lower phosphorus levels. These CWs may also include vegetation on floatation devices. This vegetation facilitates biological processes that breakdown pollutants in the water.

Surface flow CWs work best for high to moderate runoff volumes, and should be designed to retain water for 3 to 3.5 days. The recommended surface area can be reduced if depth is increased (typical depth is 2 to 3 feet, maximum depth is 4 feet), which promotes anaerobic (low oxygen) conditions that facilitate nitrogen removal.

Subsurface flow constructed wetlands
Subsurface flow CWs utilize a smaller “footprint” than surface flow CWs and can remediate both nitrogen and phosphorus if properly designed. A subsurface flow CW consists of a lined or impermeable basin filled with a 2-foot-deep layer of coarse medium pea gravel with a high hydraulic conductivity and wetland plants. Wastewater flows horizontally or vertically below the surface of the media to prevent exposure to humans or wildlife; remediation is aided by plants and associated microbial populations.

This portable subsurface flow wetland was established with bulrush. Water moves through the wetland via a solar-powered pump.

Subsurface flow CWs are better for winter treatment than surface flow CWs and emit less total ammoniacal nitrogen (NH3-N and NH4+-N) to the atmosphere. The gravel substrate of subsurface flow CWs is costly, and treatment longevity is finite because substrate clogging may occur after several years of operation.

Mixed system constructed wetlands
When phosphorus treatment is needed, simply passing water through a surface flow CW is not adequate. A mixed system, using a surface flow CW for nitrogen removal and subsurface flow CW for phosphorus, may be the most effective treatment option. When targeting phosphorus, instead of pea gravel, the subsurface flow CW substrate should be a pre-screened fired-clay nugget, such as Oil-Dri Agsorb. Lab verification of phosphorus-binding capacity is necessary to insure adequate treatment capacity. The clay nuggets used should be large enough to prevent clogging and to allow water infiltration and movement. Phosphorus-removal efficiency declines as binding sites fill, so monitoring is necessary to determine when to replace the clay nuggets. These secondary treatments can be greater than 80 percent efficient in reducing phosphorus concentrations in discharge.

Small-scale system
Nursery operations limited in production space and land expense may find the small-scale treatment systems, including mobile/portable wetlands, to be more effective.

A portable subsurface flow CW system was developed by Mobile Environmental Solutions Inc., Tustin, Calif. The portable wetland uses bulrush (Schoenoplectus spp.) planted in a lightweight medium of 3/8-inch pumice.

Inflow and outflow pipes manage the movement of water in this self-contained system, which can be transported by a midsized pickup truck. CWs can be used to recycle water or to assure compliance with increasingly stringent environmental regulations regarding the discharge of nonpoint-source pollutants.

Lorence Oki is a University of California Cooperative Extension Landscape Horticulture Specialist, Department of Plant Sciences, and Sarah White is a Nursery Extension Specialist, Environmental Horticulture Department, Clemson University.