Geoscience Reference
In-Depth Information
conductivity at fifteen-minute intervals and are
monitored by the Environment Agency as they are
received in real time at the London office.
In addition to the oxygen-injecting boats, there
is tight water-quality management for the River
Thames through London. This is operated by the
Thames Estuary Partnership, a group of interested
bodies including the Environment Agency. Their
remit includes other factors such as protecting
London from flooding (using the Thames Barrier),
but also setting higher effluent standards for
sewage treatment works during the summer. The
emphasis is on flexibility in their management of
the Thames. There is no question that the River
Thames has improved from fifty years ago. In
many respects it is a river transformed, but it
still has major water-quality problems such as
you would expect to find where a small river is
the receptacle for the treated waste of over 10
million people. The water-quality management
of a river like the Thames needs consideration of
many facets of hydrology: understanding pollu-
tants, knowledge of stormflow peaks from large
rainfall events, and streamflow statistics.
will find that the majority of this chapter deals with
human-induced water-quality issues. This is an
inevitable response to the world we live in where we
place huge pressures on the river systems as
repositories of waste products. It is also important
to study these issues because they are something that
humans can have some control over, unlike many
natural water-quality issues.
Before looking at the water-quality issues of sub-
stances within a river system it is worth considering
how they reach a river system. In studying water
pollution it is traditional to differentiate between
point source
and
diffuse
pollutants. As the terminology
suggests, point sources are discrete places in space
(e.g. a sewage treatment works) where pollutants
originate. Diffuse sources are spread over a much
greater land area and the exact locations cannot be
specified. Examples of diffuse pollution are excess
fertilisers and pesticides from agricultural produc-
tion. The splitting of pollutants into diffuse
and point sources has some merit for designing
preventative strategies but like most categorisations
there are considerable overlaps. Although a sewage
treatment works can be thought of as a point source
when it discharges effluent into a stream, it has
actually gathered its sewage from a large diffuse
area. If there is a particular problem with a sewage
treatment works effluent, it may be a result of
accumulated diffuse source pollution rather than the
actual sewage treatment works itself.
A more useful categorisation of water pollutants
is to look at their impacts on the river system. In
this way we can differentiate between three major
types of pollutants.
•
Toxic compounds
, which cause damage to biological
activity in the aquatic environment.
•
Oxygen balance affecting compounds
, which either
consume oxygen or inhibit the transfer of oxygen
between air and water. This would also include
thermal pollution as warm water does not hold
as much dissolved oxygen as cold water (see
p. 134).
•
Suspended solids
- inert solid particles suspended
in the water.
Whether we approve or not, rivers are receptacles
for large amounts of waste produced by humans.
Frequently this is deliberate and is due to the ability
of rivers to cope with waste through degradation,
dilution and dispersion. Just how quickly these
three processes operate is dependent on the pollutant
load already present in the river, the temperature
and pH of the water, the amount of water flowing
down the river and the mixing potential of the river.
The last two of these are river flow characteristics
that will in turn be influenced by the time of
year, the nature of flow in the river (e.g. the shape
of the flow duration curve), and the velocity and
turbulence of flow. This demonstrates the strong
