Environmental Engineering Reference
In-Depth Information
8
WETLANDS
8.1 
INTRODUCTION
and coral reefs, and they perform a variety of essential
ecological functions. The combination of shallow water,
high levels of nutrients, and primary productivity is ideal
for the development of organisms that form the base of
the food web and feed many species of fish, amphibians,
shellfish, and insects. Wetlands are points of groundwater
recharge, and they absorb water and airborne pollutants,
attenuate floodwater, control erosion, cycle minerals,
such as nitrogen, produce organic matter through carbon
fixation, and provide feeding refuges and reproductive
habitats for a wide variety of fish and wildlife. More than
one-third of the threatened and endangered species in
the United States live only in wetlands, and nearly half
use wetlands at some point in their lives.
Wetlands are found on every continent in the world
except Antarctica, and constitute 5-8% of the land
surface on Earth (Mitsch and Gosselink, 2007). Wet-
lands are found at the interface of terrestrial ecosys-
tems, such as upland forests and grasslands, and aquatic
ecosystems in rivers, lakes, and oceans. Wetlands are
also found in seemingly isolated areas, where the nearby
aquatic ecosystem is often a groundwater aquifer.
Approximately half of the state of Alaska is classified
as wetlands (70-80 million hectares), with Florida (4.5
million hectares), Louisiana (3.6 million hectares), Min-
nesota (3.5 million hectares), and Texas (3.1 million
hectares) having the next largest areas of wetlands.
In the past, wetlands were considered as sources of
disease (malaria) and an obstacle to human use of land
resources for growth, agriculture, and economic devel-
opment. In reality, in addition to attenuating surface
runoff (flood control) and ecological function, wetlands
Wetlands are areas where water covers the surface of
the ground or is present at or near the ground surface
for varying periods of time during the year. The pro-
longed presence of above-ground water creates condi-
tions that favor the growth of specially adapted plants
(hydrophytes) and promote the development of charac-
teristic wetland (hydric) soils. Although wetlands are
often wet, a wetland might not be wet year-round. In
fact, some of the most important wetlands are only sea-
sonally wet.
Wetlands can be broadly classified as either natural
wetlands or constructed wetlands. Natural wetlands are
formed by nature with limited human influence, whereas
constructed wetlands are engineered systems that are
designed to function like natural wetlands. Constructed
wetlands are commonly used to treat municipal and
industrial wastewater, agricultural runoff, and urban
runoff.
8.2  NATURAL WETLANDS
Natural wetlands occur at the interface between land
and water and are transition zones where the flow of
water, the cycling of nutrients, and the energy of the sun
meet to produce a unique ecosystem. The wetlands eco-
system is characterized by unique hydrology, soils, and
vegetation that make these areas important features of
a watershed. Wetlands are among the most productive
ecosystems in the world, comparable with rain forests
 
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