Background

Background

Background

Phytoremeditation

Phytoremediation is new and rapidly developing form of bioremediation which uses green plants and their associated microorganisms to destroy, remove, contain, or otherwise detoxify environmental contaminants (Cunningham et al, 1996). Phytoremediation is still in its early stages of research and development, yet it has the potential for the decontamination of a variety of pollutants including petroleum hydrocarbons, pesticides, heavy metals, and nutrients. There are four destruction and removal processes that can occur in phytoremediation which include (USEPA, June 1999):

	Phytovolatization - The enhancement of the volatization process from the soil or through the plant's roots or shoots via plant transpiration of volatile contaminants or transformation of contaminants to more volatile forms.
	Phytodegradation - The degradation of contaminants either by uptake by the plant or by excression of root exudates that subsequently metabolize the contaminate via plant enzymes to form benign products.
	Phytoextraction - The uptake of contaminants by the plant and absorption of the contaminant into the plant tissue for subsequent harvesting or disposal.
	Phytostimulation - The stimulation of rhizosphere microorganisms that degrade the contaminant to benign products by producing an environment favorable to the microorganisms within the root zone.

As well, phytoremediation can sequester contaminants by the process of phytostabilization which makes the contaminant less bioavailable to humans and other receptors. The process of phytostabilzation includes three forms (Rittmann and McCarty, 2001):

	Humification - Incorporation of the contaminants into the soil organic matter, or humus, through binding reactions of plant and microbial enzymes.
	Lignification - Incorporation of the contaminant into plant cell-wall constituents.
	Aging - The slow binding of the contaminant into the soil mineral fraction.

Another aspect of phytoremediation is hydraulic control in which during the growing season, plants transpire large volumes of water to the atmosphere. This transpiration can be used to limit or stop downward flow of precipitation from the vadose zone to the saturated zone; as well, as "pump" water from the saturated zone to the atmosphere. These mechanisms can be exploited to slow or stop migration of contaminants within both the vadose and saturated zones.

Urban Stormwater Constructed Wetland Systems

Historically, the most common application of constructed wetland systems has been the treatment of municipal sewage effluent to remove pollutants such as biochemical oxygen demand, total suspended solids, nitrogen and phosphorous. The first modern use of constructed wetlands started in Europe about 50 year ago and the idea was later introduced to the United State in the mid-1970's. Today, constructed wetland systems have been developed to treat agricultural and industrial effluents, mine drainage, landfill leachate, hazardous wastes, and stormwater flows. The use of constructed wetlands to control urban stormwater flow and quality is a recent application of the technology and the use of this technology is rapidly increasing. (Davis, 1995)

The advantages of wetland systems for urban stormwater include it's passive operation, relative ease to construct, simple designs, low cost, minimal operational attention, high degree of flow control, and increased aesthetic and habitat value. The disadvantages include odor generation, attraction to pests, potential public and wildlife exposure, and a somewhat restricted list of useable species. (Tsao, 2003)