ABSTRACT
Old and new landfills produce leachates, typically formed from infiltrating waters and the products of solid waste decomposition. Those contaminated leachate waters are a potential threat to surface and subsurface receiving waters. A very wide spectrum of pollutants is possible, reflecting the character of the materials contained in the stack. The principal categories of undesirable substances are (1) volatile organic compounds (VOCs), (2) nutrients, notably nitrogen, (3) heavy metals, and (4) priority toxic organic compounds. Wetland treatment of the leachate is one option for water quality improvement. The volumetric flow of leachate is often fairly small, compared with other wastewaters such as domestic, 18animal, and industrial or urban or agricultural runoff. Thus, long detention times are possible for small wetland/landfill area ratios.
Wetlands offer a wide spectrum of natural processes that may serve to reduce leachate contaminants. VOCs are air-stripped from the surface of the wetland waters and biodegraded by consortia of wetland microbes Ammonium nitrogen may also volatilize and undergo nitrification and denitrification. The wetland carbon cycle provides the energy source for nitrate reduction. Nutrients are seasonally utilized by wetland biota, and residuals accrete as new wetland sediments and soils. Metals are sequestered in tissues of growing plants, ion-exchanged onto wetland sediments, and precipitated as sulfides and oxyhydroxide coprecipitates. Many toxic organics are biodegraded, either in the oxic surface zones, or in the anoxic or anaerobic sediments. The anoxic sediments are the site of sulfate reduction, which provides sulfides for metal removal.
Treatment wetlands may be designed to operate in a passive mode, and only minimal and infrequent maintenance activities are necessary. These features are exceptionally attractive for treatment in perpetuity, because of low present worth compared with other technologies. However, it is difficult, if not impossible, to design for removal down to detection limits, as may be required by regulation. The stochastic component of wetland performance is larger than for other technologies. Design data are lacking for many of the leachate constituents. Some leachate concentrations may be toxic to wetland biota, either the vegetation that supports wetland functions or the animals that may be attracted to the system. In the former case, pretreatment may be necessary. In the latter case, the more costly subsurface flow variants of the technology may be required.
