Geoscience Reference
In-Depth Information
sediments means that they often stay in a river
system for a long period of time. Improvements
in water quality for a river can often be delayed
substantially by the steady release of phosphate from
sediments on the river bed.
/l)
Nitrate (mg/l)
20
15
10
Chlorine
5
Chlorine is not normally found in river water. It is
used as a disinfectant in the supply of drinking
water. It is used because it is toxic to bacteria and
relatively short lived. More common to find in river
water samples is the chloride ion. This may be an
indicator of sewage pollution as there is a high
chloride content in urine. Chloride ions give the
brackish taste of sea water, the threshold for taste
being around 300 mg/l. The European Commission
limit for drinking water is 200 mg/l.
0
Sept 1980
Sept 1981
Figure 7.5
Nitrate levels in the river Lea, England.
Three years of records are shown: from September 1979
until September 1982.
Source
: Data from the Environment Agency
Phosphates
Phosphorus can be found in three different forms:
orthophosphate, polyphosphate (both normally dis-
solved) and organic phosphate (bound to organic
particles). The ratio of different forms of phosphorus
in a water sample is highly pH dependent (Chapman,
1996). Like nitrogen, the availability of phosphorus
is a limiting factor in plant growth. The most com-
mon form of application for plants is as phosphate.
The major difference from nitrates is that phosphate
is not nearly as soluble. Consequently phosphate is
normally applied as a solid fertiliser, and less fre-
quently than nitrate. In river systems the main
source of dissolved phosphate is from detergents and
soaps that come through sewage treatment works.
Sewage treatment works remove very little of the
phosphate from detergents present in waste water,
except where specific phosphate-stripping units
are used. The largest amount of phosphate in river
systems is normally attached to particles of sediment.
Rodda
et al
. (1999) report maximum dissolved
reactive phosphorus levels of 0.2 mg/l but total
phosphorus levels of 1.6 mg/l. This is for intensive
dairy production, where the majority of phosphate
is from agricultural fertilisers.
Phosphates are a major contributor to eutrophi-
cation problems. The fact that they are bound to
Heavy metals
'Heavy metals' is the term applied to metals with
an atomic weight greater than 6. They are generally
only found in very low levels dissolved in fresh
water, but may be found in bed load sediments. In
acidic waters metals can be dissolved (i.e. found in
ionic form). They are often toxic in concentrations
above trace levels. The toxicity, in decreasing order,
is mercury, cadmium, copper, zinc, nickel, lead,
chromium, aluminium and cobalt (Gray, 1999). In
the aquatic environment copper and zinc are the
most frequent causes of toxicity. A major source of
zinc is derived from galvanised steel, particularly in
wire fencing and roofs (Alloway and Ayres, 1997).
Accumulation of lead in sediments has been a
problem for aquatic wildlife. Since the banning of
leaded petrol the major source has been through
the use of lead shot and fishing sinkers. Lead shot
has been banned in favour of steel shot in many
countries (e.g. USA, UK, New Zealand, Australia)
due to these problems (Dodds, 2002).
The sources of heavy metals in the aquatic
environment are almost always industrial or surface
runoff from roads. Sewage sludge (the product of
sedimentation at a sewage treatment works) is
