(Ours et al., 1997). Hydraulic conductivity in peat tends to
range widely from the orders of 10
À2 to 10
À7 cm s
À1
(Chason and Siegel, 1986). The high organic matter content that can exceed 98 % (Hobbs, 1986) results in large
specific surface area (up to 200 m
2 g
À1
) and high
cation exchange capacity (CEC) ranging from 24 to
180 meq 100 g
À1 (Bunt, 1988) and dominated by major
metal cations (Ca
2+ , Mg
2+ , K
+ , Na
+ ).
The composition of peat is complex (due to the presence of organic matter in various stages of decomposition)
with lignin and cellulose (major components of the cell
walls of plants) as major constituents. Although some controversy still exists, lignin is thought to be the precursor of
humic acids in peat. Humic substances (including humic
acids and fulvic acids) typically reach 22 % content in peat
soil (McDonnell et al., 2001). These components, and
especially lignin, contain polar groups such as alcohols,
aldehydes, carboxylic acids, phenolic hydroxides, and
ethers that can be involved in chemical bonding (Bailey
et al., 1999). Peat can attenuate the movement of contaminants from groundwater by adsorption (or the process by
which molecules are attracted and retained on the surface
of peat molecules). Due to the polar character of peat,
adsorption is very effective on dissolved solids such as
metals, organic contaminants, and divalent cations
(Brown et al., 2000). These processes, inducing sorption
in peat, are also a subject of controversy. Theories
to explain how metal ions are sorbed onto peat include
ion exchange, surface adsorption, chemisorption
(or adsorption by chemical forces instead of physical),
and complexation (or formation of complexes, structures
conformed by a metal atom surrounded by a number of
negatively charged ions, Brown et al., 2000). It is commonly accepted that ion exchange and complexation are
the most prevalent of these mechanisms, and they are
dependent on conditions such as pH, ionic strength, peat
type, and concentration (Bailey et al., 1999). Solute retardation has been largely used to explain sorption processes
in peat, while the properties of the peat matrix and its contribution to retardation have been commonly ignored
(Hoag and Price, 1997).
Types of peat and peat accumulating systems
Definitions of peat are generally not very strict in terms of
organic content requirements, and although it is generally
accepted that most peats contain less than 20 % inorganic
matter (i.e., unburnable), accumulations of poorly
decomposed vegetable matter are still considered peat
with values up to 35 % inorganic matter or even 55 % inorganic matter (in certain commercial operations).
According to the USDA (United States Department of
Agriculture), organic matter can be classified as (1) peat
(or fibric), if virtually all remains are intact enough to
allow identification of plant forms; (2) muck (or sapric),
if decomposition of remains is enough to limit recognition; and (3) mucky peat (or hemic), if remains can be
partially recognized. Furthermore, peat can be classified
according to its origin as (1) sedimentary peat formed
from floating aquatic plants such as algae; (2) moss peat
formed from mosses and including Sphagnum; (3) herbaceous peat formed from herbaceous plants such as sedges,
reeds, and cattails; and (4) woody peat formed from
woody plants such as trees and shrubs.
Peat soil accumulations result in the formation of
peatlands, wetland terrains characterized by a naturally
accumulated layer of peat and dominated by peat-forming
plants. Classification and definition of wetlands have been
traditionally difficult and controversial mainly due to the
lack of international consensus, with few attempts to
establish a global system only lead by the Ramsar Convention on Wetlands of International Importance (Scott
and Jones, 1995). According to the US Environmental
Protection Agency (EPA), wetlands are defined as “areas
where water covers the soil, or is present either at or near
the surface of the soil all year or for varying periods of
time during the year, including during the growing season,” and can be classified as (1) marshes or continually
water-inundated wetlands characterized by emergent
soft-stemmed vegetation adapted to saturated soil conditions; (2) swamps or wetlands dominated by woody
plants; (3) bogs or peat-forming wetlands (peatlands)
characterized by spongy peat deposits, acidic waters, and
a floor covered by a thick carpet of Sphagnum moss that
usually receive all or most of their water from precipitation
rather than from other sources (e.g., runoff, groundwater,
or streams), therefore resulting in low presence of nutrients for plant growth (ombrotrophic); and (4) fens or
peatlands that receive nutrients from sources other than
precipitation (minerotrophic) that tend to be less acidic
and present higher nutrient levels as compared to bogs.
Bogs are very low productive ecosystems when compared to other peatlands such as fens. Peat is
a heterogeneous material controlled by highly complex
spatial and temporal gradients. The two main factors
responsible for these gradients (water budget and water
quality) are strongly interrelated to the controls on productivity and vegetation type in peatlands: pH, available
nutrients, mineral concentration, CEC, and water-table
position. Rainwater nourishing bogs are usually very
acidic and low in nutrients (e.g., N, P, K, Ca, Mg). Mineral
content of bog plants is lower than in plants from fens or
swamps (Archibold, 1995). Bogs are characterized by
low water pH and high organic content of peat (high
capacity to absorb ions). The rate of water flow controls
the supply of nutrients to the plants and affects their rate
of growth (Clymo, 1973). Water level variations also
induce vegetation gradients, by shifting from wet- to
dry-tolerant plant species. Most of the nutrients present
in peat soils are not available for plant growth due to their
previous incorporation into organic compounds. Soluble
elements (such as Ca) are washed out and may accumulate
in the underlying peat. Other elements (such as K) are lost
through litter (uppermost layer of organic debris)
PEAT
477
range widely from the orders of 10
À2 to 10
À7 cm s
À1
(Chason and Siegel, 1986). The high organic matter content that can exceed 98 % (Hobbs, 1986) results in large
specific surface area (up to 200 m
2 g
À1
) and high
cation exchange capacity (CEC) ranging from 24 to
180 meq 100 g
À1 (Bunt, 1988) and dominated by major
metal cations (Ca
2+ , Mg
2+ , K
+ , Na
+ ).
The composition of peat is complex (due to the presence of organic matter in various stages of decomposition)
with lignin and cellulose (major components of the cell
walls of plants) as major constituents. Although some controversy still exists, lignin is thought to be the precursor of
humic acids in peat. Humic substances (including humic
acids and fulvic acids) typically reach 22 % content in peat
soil (McDonnell et al., 2001). These components, and
especially lignin, contain polar groups such as alcohols,
aldehydes, carboxylic acids, phenolic hydroxides, and
ethers that can be involved in chemical bonding (Bailey
et al., 1999). Peat can attenuate the movement of contaminants from groundwater by adsorption (or the process by
which molecules are attracted and retained on the surface
of peat molecules). Due to the polar character of peat,
adsorption is very effective on dissolved solids such as
metals, organic contaminants, and divalent cations
(Brown et al., 2000). These processes, inducing sorption
in peat, are also a subject of controversy. Theories
to explain how metal ions are sorbed onto peat include
ion exchange, surface adsorption, chemisorption
(or adsorption by chemical forces instead of physical),
and complexation (or formation of complexes, structures
conformed by a metal atom surrounded by a number of
negatively charged ions, Brown et al., 2000). It is commonly accepted that ion exchange and complexation are
the most prevalent of these mechanisms, and they are
dependent on conditions such as pH, ionic strength, peat
type, and concentration (Bailey et al., 1999). Solute retardation has been largely used to explain sorption processes
in peat, while the properties of the peat matrix and its contribution to retardation have been commonly ignored
(Hoag and Price, 1997).
Types of peat and peat accumulating systems
Definitions of peat are generally not very strict in terms of
organic content requirements, and although it is generally
accepted that most peats contain less than 20 % inorganic
matter (i.e., unburnable), accumulations of poorly
decomposed vegetable matter are still considered peat
with values up to 35 % inorganic matter or even 55 % inorganic matter (in certain commercial operations).
According to the USDA (United States Department of
Agriculture), organic matter can be classified as (1) peat
(or fibric), if virtually all remains are intact enough to
allow identification of plant forms; (2) muck (or sapric),
if decomposition of remains is enough to limit recognition; and (3) mucky peat (or hemic), if remains can be
partially recognized. Furthermore, peat can be classified
according to its origin as (1) sedimentary peat formed
from floating aquatic plants such as algae; (2) moss peat
formed from mosses and including Sphagnum; (3) herbaceous peat formed from herbaceous plants such as sedges,
reeds, and cattails; and (4) woody peat formed from
woody plants such as trees and shrubs.
Peat soil accumulations result in the formation of
peatlands, wetland terrains characterized by a naturally
accumulated layer of peat and dominated by peat-forming
plants. Classification and definition of wetlands have been
traditionally difficult and controversial mainly due to the
lack of international consensus, with few attempts to
establish a global system only lead by the Ramsar Convention on Wetlands of International Importance (Scott
and Jones, 1995). According to the US Environmental
Protection Agency (EPA), wetlands are defined as “areas
where water covers the soil, or is present either at or near
the surface of the soil all year or for varying periods of
time during the year, including during the growing season,” and can be classified as (1) marshes or continually
water-inundated wetlands characterized by emergent
soft-stemmed vegetation adapted to saturated soil conditions; (2) swamps or wetlands dominated by woody
plants; (3) bogs or peat-forming wetlands (peatlands)
characterized by spongy peat deposits, acidic waters, and
a floor covered by a thick carpet of Sphagnum moss that
usually receive all or most of their water from precipitation
rather than from other sources (e.g., runoff, groundwater,
or streams), therefore resulting in low presence of nutrients for plant growth (ombrotrophic); and (4) fens or
peatlands that receive nutrients from sources other than
precipitation (minerotrophic) that tend to be less acidic
and present higher nutrient levels as compared to bogs.
Bogs are very low productive ecosystems when compared to other peatlands such as fens. Peat is
a heterogeneous material controlled by highly complex
spatial and temporal gradients. The two main factors
responsible for these gradients (water budget and water
quality) are strongly interrelated to the controls on productivity and vegetation type in peatlands: pH, available
nutrients, mineral concentration, CEC, and water-table
position. Rainwater nourishing bogs are usually very
acidic and low in nutrients (e.g., N, P, K, Ca, Mg). Mineral
content of bog plants is lower than in plants from fens or
swamps (Archibold, 1995). Bogs are characterized by
low water pH and high organic content of peat (high
capacity to absorb ions). The rate of water flow controls
the supply of nutrients to the plants and affects their rate
of growth (Clymo, 1973). Water level variations also
induce vegetation gradients, by shifting from wet- to
dry-tolerant plant species. Most of the nutrients present
in peat soils are not available for plant growth due to their
previous incorporation into organic compounds. Soluble
elements (such as Ca) are washed out and may accumulate
in the underlying peat. Other elements (such as K) are lost
through litter (uppermost layer of organic debris)
PEAT
477