Large hydrophobic organics and metals are removed by ion exchange, adsorption, absorption, and precipitation through geochemical and microbial oxidation and reduction. Sorption occurs as metals in the water contact humic or other organic substances in the soil medium. Oxidation and reduction reactions that occur in the aerobic and anaerobic zones, respectively, transform or degrade organics and precipitate metals as hydroxides and sulfides. Precipitated and adsorbed metals settle in quiescent ponds, or are filtered out as the water percolates through the soil or substrate.

Status: Innovative

Constructed wetlands are used primarily to treat halogenated and nonhalogenated volatile organic compounds (VOCs), nonhalogenated semivolatile organic compounds (SVOCs), and inorganics. Constructed wetlands have most commonly been used in wastewater treatment for controlling organic matter; nutrients, such as nitrogen and phosphorous; and suspended sediments (e.g., agricultural runoff. The wetlands-based treatment process is also suitable for controlling trace metals and other toxic materials (e.g., acid mine drainage). Constructed wetlands have been used to treat acid mine drainage that has extreme acid conditions and high concentrations of iron, sulfate, and other trace metals. Constructed wetlands not only show a reduction in iron and sulfate concentrations, but also show a recovery in pH levels. In the area of stormwater treatment, wetlands have shown the ability to remove fecal coliform bacteria, total petroleum hydrocarbons, and metals including lead, chromium, and zinc.

Wetland treatment has been applied with some success to wastewater in the eastern United States. The process may have to be adjusted to account for differences in geology, terrain, trace metal composition, and climate in the metal mining regions of the western United States.

The following factors may limit the applicability and effectiveness of the process:

• The long-term effectiveness of constructed wetland treatment is not well known. Wetland aging may be a problem that may contribute to a decrease in contaminant removal rates over time.
• The cost of building an artificial wetland varies considerably depending on such site conditions as topography, soil permeability, and climate.

Wetland treatment is a long-term technology intended to operate continuously for 10 to 30 years or longer. Factors that affect the duration of wetland operation and maintenance include the following:

• Cleanup goals
• The amount and concentration of the contaminants
• Sediment erosion patterns
• Surrounding land use
• Wetland water balance
• Slope
• Local climate (i.e., aridity, rainfall, temperature)
• Organism type and density.

$0.15 to $1.00 per 1,000 gallons

The major cost items included in the cost estimate range for constructed wetland treatment are designated in the following table :