Geoscience Reference
In-Depth Information
Warm water may enter a river as thermal pollu-
tion from power stations and other industrial
processes. In many power stations (gas, coal and
nuclear) water is used as a coolant in addition to the
generation of steam to drive turbines. Because of
this, power stations are frequently located near a
river or lake to provide the water source. It is normal
for the power stations to have procedures in place
so that hot water is not discharged directly into a
river; however, despite the cooling processes used,
the water is frequently 1-2°C degrees warmer on
discharge. The impact that this has on a river system
will be dependent on the river size (i.e. degree of
dilution and rate of dispersion).
Conductivity
TDS
Conductivity
(7.1)
K
=
or TDS
=
K
This relationship gives a very good surrogate
measure for TDS. The
K
term is a constant (usually
between 0.55 and 0.75) that can be estimated by
taking several measurements of conductivity
with differing TDS levels. Conductivity is a simple
measurement to take as there are many robust
field instruments that will give an instant reading.
This can then be related to the TDS level at a later
stage. Electrical conductivity is measured in
Siemens per metre, although the usual expression is
microsiemens per centimetre (
S/cm). Rivers
normally have a conductivity between 10 and 1,000
S/cm.
Dissolved solids
Suspended solids
In the first chapter, the remarkable ability of water
to act as a solvent was described. As water passes
through a soil column or over a soil surface it will
dissolve many substances attached to the soil
particles. Equally water will dissolve particles from
the air as it passes through the atmosphere as rain.
The amount of dissolved substances in a water
sample is referred to as the
total dissolved solids
(TDS)
. The higher the level of TDS the more
contaminated a water body may be, whether that be
from natural or anthropogenic sources. Meybeck
(1981) estimates that the global average TDS
load in rivers is around 100 mg/l, but it may rise
considerably higher (e.g. the Colorado River has an
average TDS of 703 mg/l).
The amount of suspended solids has been high-
lighted at the start of this chapter as a key measure
of water quality. The carrying of suspended sedi-
ment in a river is part of the natural erosion and
sediment transport process. The sediment will be
deposited at any stage when the river velocity drops
and conversely it will be picked up again with
higher river velocities (see Figure 7.1). In this
manner the
total suspended solids (TSS)
load will
vary in space and time. The amount of TSS in a river
will affect the aquatic fauna, because it is difficult
for egg-laying fish and invertebrates to breed in an
environment of high sediment. Suspended sediment
is frequently inert, as in the case of most clay and
silt particles, but it can be organic in content and
therefore have an oxygen demand.
TSS is expressed in mg/l for a water sample
but frequently uses other units when describing
sediment load. Table 7.2 shows some values of
sediment discharge (annual totals) and calculates an
average TSS from the data. It is remarkable to see
the data in this form, enabling contrast to be drawn
between the different rivers. Although the Amazon
delivers a huge amount of sediment to the oceans
it has a relatively low average TSS, a reflection of
the extremely high discharge. In contrast to this the
Electrical conductivity
A similar measurement to TDS is provided by the
electrical conductivity. The ability of a water sample
to transmit electrical current (its conductivity)
is directly proportional to the concentration of
dissolved ions. Pure, distilled water will still con-
duct electricity but the more dissolved ions in water
the higher its electrical conductivity. This is a
straight-line relationship, so equation 7.1 can be
derived.
