From aspwe.org, Wetland Engineering and Restoration
Wetlands for Treatment
We are not aware of wetlands being used intentionally for primary treatment.
A demonstration wetlands project in Sacramento California was specifically designed for removal of metals using treatment wetlands, they found mass removal rates for six metals (silver,cadmium,copper,mercury,lead and zinc) consistently exceeded 60 percent. Removal rates for arsenic, chromium and nickel steadily declined over the course of the study.
There is a wealth of information on attempts to remove phosphorous from waters entering into the Everglades in South Florida. Over time we hope to accumulate links to some of the key resources from that effort. For now I will point you to a single abstract that captures the essence of the work that is underway: Stormwater Treatment Areas: Constructed Wetlands for Phosphorus Removal in South Florida Surface Waters by Wossenu Abtew, Gary Goforth, Guy Germain and Tim Bechtel. See also: Constructed Wetlands for Water Quality Improvement in Surface Water Discharges to the Everglades: Stormwater Treatment Areas