Geoscience Reference
In-Depth Information
(Alloway and Ayres, 1997). In this case you cannot
be sure that a particularly high reading at one depth
is from deposition at any particular time.
to eutrophic ('good nutrition') and ends with
hypertrophic ('excess nutrients'). In limnology this
classification is viewed as part of a natural progres-
sion for bodies of water as they fill up with sediment
and plant matter. Eutrophication is a natural process
(as part of the nitrogen and phosphorus cycles), but
it is the addition of extra nutrients from anthro-
pogenic activity that attracts the main concern in
hydrology. In order to distinguish between natural
and human-induced processes the term 'cultural
eutrophication' is sometimes used to identify the
latter.
The major nutrients that restrict the extent of a
plant's growth are potassium (K), nitrogen (N) and
phosphorus (P). If you buy common fertiliser for
a garden you will normally see the K:N:P ratio
expressed to indicate the strength of the fertiliser.
For both aquatic and terrestrial plants nitrogen is
required for the production of chlorophyll and green
leaves, while potassium and phosphorus are needed
for root and stem growth. In the presence of abun-
dant nitrogen and phosphorus (common water
pollutants, see pp. 136-138), aquatic plant growth,
including algae, will increase dramatically. This can
be seen as positive as it is one way of removing the
nitrate and phosphate from the water, but overall it
has a negative impact on the river system. The main
negative effect is a depletion of dissolved oxygen
caused by bacteria decomposing dead vegetative
matter in the river. In temperate regions this is a
particular problem in the autumn when the aquatic
vegetation naturally dies back. In tropical regions
it is a continual problem. A second negative effect
is from algal blooms. In 1989 there was an explosion
in cyano-bacteria numbers in Rutland Water, a
reservoir supplying drinking water in central
England (Howard, 1994). (NB These are also called
blue-green algae, despite being a species of cyano-
bacteria.) The cyano-bacteria produce toxins as
waste products of respiration that can severely affect
water quality. In the 1989 outbreak several dogs
and sheep that drank water from Rutland Water
were poisoned, although no humans were affected
(Howard, 1994). In an effort to eliminate future
problems the nutrient-rich source water for Rutland
MODELLING WATER QUALITY
The numerical modelling of water quality is fre-
quently required, particularly to investigate the
effects of particular water-quality scenarios. The
type of problems investigated by modelling are: the
impact of certain levels of waste discharge on a river
(particularly under low flow levels); recovery of
a water body after a pollution event; the role of
backwaters for concentration of pollutants in a river;
and many more. The simplest water-quality models
look at the concentration of a certain pollutant in a
river given knowledge about flow conditions and
decay rates of the pollutant. The degradation of a
pollutant with time can be simulated as a simple
exponential decay rate equation. A simple mass
balance approach can then be used to calculate the
amount of pollutant left in the river after a given
period of time (James, 1993). More complex models
build on this approach and incorporate ideas of
diffusion, critical loads of pollutants and chemical
reaction between pollutants in a river system. If
the problem being researched is to track pollutants
down a river then it is necessary to incorporate
two- or three-dimensional representation of flow
hydraulics. There are numerous water-quality
models available in the research literature, as well
as those used by consultants and water managers.
EUTROPHICATION
'Eutrophication' is the term used to describe the
addition of nutrients to an aquatic ecosystem that
leads to an increase in net primary productivity. The
term comes from limnology (the study of freshwater
bodies, e.g. lakes and ponds) and is part of an overall
classification system for the nutrition, or trophic,
level of a freshwater body. The general classification
moves from oligotrophic (literally 'few nutrients'),
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