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….