Environmental Engineering Reference
In-Depth Information
sediments and the potential deleterious effects per-
sistent pollutants have on benthic ecosystems
(Anderson
et al.
1996; Martin-Diaz
et al.
2004).
This chapter discusses various approaches to sedi-
ment ecotoxicology, the advantages and limitations,
and its role in environmental monitoring and impact
assessment.
nitrate, ammoniac) and dissolved gasses (O
2
, CO
2
),
are greatly infl uenced by geochemical and biological
processes in the surrounding catchments. CO
2
has a
high solubility in water. Chemical reactions with
water molecules enable 100-200 times more CO
2
to
dissolve in water than would be possible through
physical interactions alone. CO
2
dissolves in water
to form carbonic acid (H
2
CO
3
), which in turn dis-
sociates to hydrogen carbonate (
HCO
−
) and carbon-
ate ions (
CO
−
), depending on pH. Whereas the pH
of freshwater does vary, depending on the underlying
geology and organic input, the pH of normal seawa-
ter is around 8, caused by the surplus of acid cations.
Under these slightly alkaline conditions, all three
forms of carbonate ions can be found. Initially, addi-
tion of protons (acid) or hydroxyl ions (base) causes
the carbonate species to shift in the relative amounts,
but does not release free hydrogen ion. Therefore,
seawater is generally very well buffered, whereas
the buffering capacity of freshwater depends on the
geological setting (Ott 1988; Brown
et al.
1992;
Libes 1992).
The salinity in estuaries can vary greatly, owing to
tidal action, and the magnitude of river discharge,
which facilitates fl uctuations in pH and shifts in the
sorption behavior and bioavailability of many chem-
ical compounds. Granulometric characteristics are
largely governed by hydrodynamic conditions. In
fast-fl owing water, sediments tend to consist of
coarse sand, gravel, and pebbles, whereas slow-mov-
ing water allows for the deposition of fi ne-grained
sand, silt, and clay (Riedl & Ott 1982). In fact, the
grain size-frequency distribution is fundamental to
the biogeochemistry of sediments and is used in their
classifi cation. Like sedimentologists, benthic ecolo-
gists use arbitrarily graded scales, either logarithmic
or geometric, to classify sediment grain size spectra
for habitat characterization (Buchanan 1971; Ott
1988; Libes 1992).
Grain size, shape, and packing density determine
the porosity and in turn the volume of pore or inter-
stitial water of sediments (Ott 1988). Coarse-grained,
loosely packed sediments are characterized by a
small surface area and high porosity, whereas fi ne-
grained, densely packed sediments exhibit a very
large surface area with low porosity. Low porosity
increases the pore water residence time, reduces
oxygen supply and favors the establishment of
distinct biogeochemical gradients, giving rise to
6.1.1 Sediments
Aquatic sediments represent an open, dynamic, and
heterogeneous biogeochemical system (Sundby
1991), formed by an accumulation of particulate
matter introduced to the aquatic environment from
a variety of sources, such as continental run-off,
coastal erosion or atmospheric fall-out, which is then
deposited on the bottom of a water body. Typically,
sediments are a structured accumulation of particu-
late mineral matter, inorganic matter of biogenic
origin, organic matter in various stages of decompo-
sition or synthesis, and water (Knezovich
et al.
1987;
ASTM 1994b). Sediments usually consist of an inor-
ganic matrix (silica, alumina, and carbonates) which
is coated with organic matter, manganese, and iron
oxides, but can be anything from pure inorganic to
pure organic in composition (Rand
et al.
1997),
giving rise to a wide variety of physical, chemical,
and biological characteristics. For experimental pur-
poses, sediments can be formulated from particulate
matter of known origin and characteristics to create
specifi c controlled conditions (ASTM 1994b; Suedel
& Rodgers 1994; Hartl
et al.
2000; Quevauviller &
Ariese 2001).
6.1.2 Assessing the characteristics of
aquatic sediments
Sediments are largely site specifi c, depending on a
multitude of physico-chemical parameters, most
notably, salinity, grain size, sedimentation rates, and
the organic carbon fraction.
Water, as the universal solvent, dissolves more
substances than any other liquid. Five anions (Cl
,
SO
2−
,
HCO
−
, Br
−
, H
3
BO
3
) and fi ve cations (Na
+
,
Mg
2+
, Ca
2+
, K
+
, Sr
2+
) constitute 99.9% of ions dis-
solved in seawater. These “conservative” constitu-
ents always maintain their relative proportions,
regardless of salinity. The proportions of “non-con-
servative” constituents, such as nutrients (phosphor,
−
