Geoscience Reference
In-Depth Information
Some of the trace organic compounds accumulate
through the food chain so that humans and other
species that eat large aquatic fauna may be at risk.
Of particular concern are endocrine disrupting
chemicals (EDCs), which have been detected in
many rivers. These chemicals, mostly a by-product
of industrial processes, attack the endocrine system
of humans and other mammals, affecting hormone
levels. Some chemicals (e.g. DDT) have the ability
to mimic the natural hormone oestrogen. Because
oestrogen is part of the reproductive process these
chemicals have the potential to affect reproductive
organs and even DNA. Studies have shown high
levels of oestrogen-mimicking compounds in sew-
age effluent (Montagnani
et al
., 1996) and that male
fish held in cages at sewage effluent discharge sites
can develop female sexual organs (Jobling and
Sumpter, 1993).
Trace organics can be detected using gas chro-
matography, although this is made difficult by the
sheer number of compounds to be detected. They
are removed from drinking water supplies using
activated carbon filters, or sometimes oxidation by
ozone.
pp. 143-145) - the breakdown of organic nitro-
genous compounds into a stable and relatively
harmless nitrate. There are two problems with this
process occurring in the natural river environment.
First, there is the oxygen demand created by the
nitrification process. Second, the intermediate
ammonia stage is highly toxic, even in very low
concentrations. Under extremely low dissolved
oxygen concentrations (less than 1 mg/l) the nitri-
fication process can be reversed, at least in the first
stage. In this case nitrates will turn into nitrite
and oxygen will be released. Unfortunately, this is
not a ready means for re-oxygenating a river as by
the time the dissolved oxygen level has dropped to
1 mg/l the fish population will have died or moved
elsewhere.
The levels of nitrate in a water sample can be
expressed in two different ways: absolute nitrate
concentration, or the amount of nitrogen held as
nitrate (normally denoted as NO
3
-N). The two are
related by a constant value of approximately 4.4.
As an example the World Health Organisation
recommended that the drinking water standard for
nitrate in drinking water be 45 mg/l. This can also
be expressed as 10 mg/l NO
3
-N.
As indicated above, one source of nitrate is from
treated sewage. A second source is from agricul-
tural fertilisers. Farmers apply nitrate fertilisers
to enhance plant growth, particularly during the
spring. Plants require nitrogen to produce green
leaves, and nitrates are the easiest form to apply as
a fertiliser. This is because nitrates are extremely
soluble and can easily be taken up by the plant
through its root system. Unfortunately this high
solubility makes them liable to be flushed through
the soil water system and into rivers. To make
matters worse a popular fertiliser is ammonium
nitrate - (NH
4
)
2
NO
3
. This has the added advantage
for the farmer of three nitrogen atoms per molecule.
It has the disadvantage for the freshwater environ-
ment of extremely high solubility and providing
ammonium ions in addition to nitrate. The appli-
cation of nitrate fertilisers is most common in areas
of intensive agricultural production such as arable
and intensive livestock farming.
Nitrogen compounds
Nitrogen exists in the freshwater environment in
four main forms:
• organic nitrogen - proteins, amino acids and urea
• ammonia - either as free ammonia (NH
3
) or the
ammonium ion (NH
4
+
)
• nitrite (NO
2
-
)
• nitrate (NO
3
2-
).
If organic nitrogen compounds enter a river
(e.g. in untreated sewage) then an oxidation process
called nitrification takes places. An approximation
of the process is outlined below:
Organic N + O
2
→
NH
3
/NH
4
+
+ O
2
→
NO
2
-
+ O
2
→
NO
3
2-
For this to occur there must be nitrifying bacteria
and oxygen present. This is one of the main pro-
cesses operating in a sewage treatment works (see
