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natural systems to improve water quality of wastewater discharge or sewage treat-
ment, storm water runoff, land reclamation after mining or refineries (USEPA 2004).
The uptake and accumulation of different elements in the wetlands mostly
depends upon the diverse factors like metal concentrations in soils, organic matter
content, pH, cation exchange capacity, and diversity of macrophytes present in the
region. However, it is well proven that the concentration of metals in soil is the
predominant factor; additionally, soil pH also governs the uptake of metal by plants
(Jung 2008). Further, active uptake of elements through their “nutrient pumps”
(Odum 1988), promote immobilization of metals in high concentration in plant
tissues endorsing the use of such wetland plants in phytoremediation for both natural
and constructed wetlands for wastewater treatment (Kadlec and Knight 1996).
Among the varied phytoremediation technologies, in wetlands, plants may be
used either for immobilization and storage of metals (phytostabilization) below
ground in roots and/or soil, or “phytoextraction” in which hyperaccumulators may
be used to remove metals from the soil and concentrate them in aboveground tissues
(McGrath and Zhao 2003). The process of phytoextraction mostly needs mainte-
nance as accumulator plants must be, in turn, harvested and disposed of to prevent
recycling of accumulated metals when the plants decompose. Again, the mechani-
cal aspects of harvesting plants would be disparaging to wetlands comprised of
rooted plants. However, for application, the patterns and processes of metal uptake,
distribution, and removal by different species of wetland plants needs to be taken
care for. This data is very important to monitor the effects on the residence time of
metals in plants and in wetlands, and the potential release of metals into the system
(may be dead plant tissues); otherwise, wetlands themselves in due course would
turn out to be the source of metal contamination to the vicinity. Thus, highly metal
enriched deceased plant material is a concern as the elements may be released again
into the surrounding, polluting through leaching and mineralization by litter adsorp-
tion or microbial immobilization. Accordingly, application of wetland plants for
wastewater treatment should be done after proper scientific study as limitation of
the plants to sequester the contamination and the assimilative capacity should also
be taken care for (Verkleij and Schat 1990).

7.7 Wetland Sediment and Contaminant Uptake by Plants

Generally, sediments are the sites of sink for metals. However, quality of soils also
differ in terrestrial and wetland systems. In terrestrial systems, soils are mostly
oxidized, but in wetlands, due to saturation of water, sediments become anoxic in
nature. Thus the bioavailability of the metals is low in wetland areas. Further, metal
bioavailability also depends upon metals association with different fractions. Most
available form is water-soluble fraction of metals. Metals associated with inorganic
compound or humus materials or adsorbed to hydrous oxides are less available than
dissolved in aqueous solution. However, metals are essentially unavailable when