Concerns arise when excessive irrigation of the container crops grown in soilless media leads to leaching and loss of nutrients and chemicals in runoff. The resulting runoff can escape from production areas and have a negative impact on surface and ground water.

The presence of nutrients in runoff and concerns of their impact on surface and groundwater quality has undergone increasing interest and scrutiny from the public, environmental groups, governmental agencies, and elected officials.

Since its enactment, the U.S. Environmental Protection Agency (EPA) has enforced provisions of the Clean Water Act related to point-source pollution. In 1999, the EPA began enforcing nonpoint source pollution controls, mandating that all states implement a Total Maximum Daily Load (TMDL) program for all watersheds and bodies of water.

Constructed wetlands (CWs) have been promoted as inexpensive, low-technology approaches to treating agricultural, industrial, and municipal wastewater to comply with increasingly stringent environmental regulations. CWs, or marshes built to treat contaminated water, incorporate soil and drainage materials, water, plants, and microorganisms.

"Surface-flow" constructed wetlands resemble shallow freshwater marshes and generally require a large land area for wastewater treatment. More effective for greenhouse and nursery operations with limited production space and expensive land are a type of constructed wetland called "subsurface flow".

Subsurface flow wetlands consist of a lined or impermeable basin filled with a coarse medium, typically gravel, and wetland plants. Wastewater flows horizontally or vertically below the surface of the media to prevent exposure to humans or wildlife.

Robert Polomski and his colleagues at Clemson University published a study in the June 2008 issue of HortScience that investigated the nitrogen and phosphorus removal potential by a vegetated, laboratory-scale subsurface flow system.

"In this study, we investigated a cost-effective approach of reducing water treatment costs. Instead of traditional wetland plants, we found that commercially available aquatic garden plants can be used in a production/remediation system."

Over an eight-week period, five commercially available aquatic garden plants received a range of nitrogen and phosphorus that spanned the rates detected in nursery runoff.

According to Polomski, "the results support the use of aquatic garden plants as aesthetic and economically viable alternatives to traditional wetland plants in constructed wetlands. Although more research is necessary to address other variables, the study concluded that the use of commercially produced plants in constructed wetlands has the potential to generate revenue for producers."

Related Links
American Society for Horticultural Science
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