Geology Reference
In-Depth Information
inorganic solutes, and particularly those which determine its hard-
ness and alkalinity. The second component is suspended sediment,
also referred to as suspended solids. These are particles, which are
sufficiently small to remain suspended with the water column for
significant periods of time where they provide a surface onto which
dissolved substances may deposit or from which material may
dissolve. The third major component of the system is the bottom
sediment. This is an accumulation of particles and associated pore
water, which has deposited out of the water column onto the bed
of the stream, river, or lake. The size of the sediment grains is
determined by the speed and turbulence of the water above. A fast-
flowing river will retain small particles in suspension and only large
particles (sand or gravel) will remain on the bottom. In relatively
stagnant lake water, however, very small particles can sediment
out and join the bottom sediment. In waters of this kind, sediment
accumulates over time and therefore the surface sediments in
contact with the water column contain recently deposited material
while the sediment at greater depths contains material deposited
tens or hundreds of years previously. In the absence of significant
mixing by burrowing organisms, the depth profile of some chem-
icals within a lake bottom sediment can provide a very valuable
historical record of inputs of that substance to the lake. Ingenious
ways have been devised for determining the age of specific bands of
sediment. While the waters at the surface of a lake are normally in
contact with the atmosphere and therefore well aerated, water at
depth and the pore water within the bottom sediment may have a
very poor oxygen supply and therefore become oxygen-depleted
and are then referred to as anoxic or anaerobic. This can affect the
behaviour of redox-active chemicals such as transition elements,
and therefore the redox properties of freshwaters and their sedim-
ents are an important consideration.
(ii) Salt waters. The waters of seas and oceans differ substantially
from freshwaters by virtue of their very high content of dissolved
inorganic material and their very great depth at some points on
the globe. These facets confer properties, which although over-
lapping with those of freshwaters, can be quite distinct. Some
inorganic components will behave quite differently in a very high
salinity environment than in a low ionic strength freshwater.
Historically, therefore, the properties of seawater have tradition-
ally been studied separately from those of freshwaters and are
presented separately, although the important overlaps such as in
the area of carbonate equilibria are highlighted.
