Geoscience Reference
In-Depth Information
overall water quality. It is often more sensible to try
and assess water quality through indirect measure-
ment of something else that we know is influenced
by water quality. Two such proxy measures of water
quality are provided by biological indicators and
analysis of sediments in the river.
Another example of an indicator species used for
water-quality testing is
Escherichia coli
(
E. coli
). These
are used to indicate the presence or absence of faecal
contamination in water.
E. coli
is a bacteria present
in the intestines of all mammals and excreted in
large numbers in faeces. Although one particular
strain (
E. coli
157
) has toxic side effects the vast
majority of
E. coli
are harmless to humans. Their
presence in a water sample is indicative of faecal
pollution, which may be dangerous because of other
pathogens carried in the contaminated water. They
are used as an indicator species because they are easy
to detect, while viruses and other pathogens are
extremely difficult to measure. Coliform bacteria
(i.e. bacteria of the intestine) are detected by their
ability to ferment lactose, producing acid and gas
(Tebbutt, 1993). There are specific tests to grow
E.
coli
in a lactose medium, which allow the tester to
derive the most probable number per 100 ml
(MPN/100 ml).
Biological indicators
Aquatic fauna normally remain within a stretch of
water and have to try and tolerate whatever water
pollution may be present. Consequently the health
of aquatic fauna gives a very good indication of the
water quality through a reasonable period of time.
There are two different ways that this can be done:
catching fauna and assessing their health; or looking
for the presence and absence of key indicator species.
Fish surveys are a common method used for
assessing the overall water quality in a river. It is an
expensive field technique as it requires substan-
tial human resources: people to wade through the
water with electric stun guns and then weigh and
measure stunned fish. When this is done regularly
it gives very good background information on the
overall water quality of a river.
More common are biological surveys using indi-
cator species, particularly of macro-invertebrates.
Kick sampling uses this technique. A bottom-based
net is kicked into sediment to catch any bottom-
based macro-invertebrates, which are then counted
and identified. There are numerous methods that
can be used to collate this species information.
In Britain the BMWP (Biological Monitoring
Working Party) score is commonly used and pro-
vides good results. Species are given a score ranging
from 1 to 10, with 10 representing species that are
extremely intolerant to pollution. The presence
of any species is scored (it is purely presence/absence,
not the total number) and the total for the kick
sample calculated. The BMWP score has a maxi-
mum of 250. Other indicator species scores include
the Chandler index and the ASPT (Average Score
Per Taxon). Details of these can be found in a more
detailed water-quality assessment text such as
Chapman (1996).
Sediments
The water in a channel is not the only part of a river
that may be affected by water pollution. There are
many substances that can build up in the sediments
at the bottom of a river and provide a record of
pollution. There are two big advantages to this
method for investigating water quality: the sedi-
ments will reflect both instantaneous large pollution
events and long, slow contamination at low levels;
and if the river is particularly calm in a certain
location the sediment provides a record of pollution
with time (i.e. depth equals time). Not all water
pollutants will stay in sediments, but some are
particularly well suited to study in this manner
(e.g. heavy metals and phosphorus).
The interpretation of results is made difficult by
the mobility of some pollutants within sediments.
Some metals will bind very strongly to clay particles
in the sediments (e.g. lead and copper), and you can
be fairly certain that their position is indicative
of where they were deposited. Others will readily
disassociate from the particles and move around in
the interstitial water (e.g. zinc and cadmium)
