Geoscience Reference
In-Depth Information
from the land into the aquatic environment is
intrinsically linked with the hydrological path-
way (i.e. the route by which the water moves
from precipitation into a stream), and hence the
hydrological processes occurring. A good example
of this is in Heppell
et al
. (1999) where the
mechanisms of herbicide transport from field to
stream are linked to runoff pathways in a clay
catchment.
10
Erosion
1
0.1
Transportation
Deposition
0.01
3
Employment of hydrologists
. It is rare for someone
employed in water resource management to be
entirely concerned with water quantity, with
no regard for quality issues. The maintenance
of water quality is not just for drinking water
(traditionally an engineer's role) but at a wider
scale can be for maintaining the
amenity value
of rivers and streams.
0.001
0.001
0.01
0.1
Particle size (mm)
1
10
100
Figure 7.1 The Hjulstrom curve relating stream
velocity to the erosion/deposition characteristics for
different sized particles (x-axis). In general the
slower the water moves the finer the particles that
are deposited and the faster the water moves the
larger the particles being transported.
It is easy to think of water quality purely in terms
of pollution; i.e. waste substances entering a river
system as a result of human activity. This is an
important issue in water-quality analysis but is by
no means the only one. One of the largest water
quality issues is the amount of suspended sediment
in a river, which is frequently a completely natural
process. Suspended sediment has severe implications
for the drinking-water quality of a river, but also for
other hydrological concerns such as reservoir design
and aquatic flora and fauna. As soon as a river is
dammed the water velocity will slow down. Simple
knowledge of the
Hjulstrom curve
(see Figure 7.1)
tells us that this will result in the deposition of
suspended sediment. That deposition will eventu-
ally reduce the capacity of the reservoir held behind
the dam. In high-energy river systems, for even
a very large reservoir, a dramatic reduction in
capacity can take place within two to four decades.
It is critically important for a hydrologist involved
in reservoir design to have some feeling for the
quantities of suspended sediment so that the
lifespan of a reservoir can be calculated. In South
Korea, reservoir management includes under-
standing the sediment plume entering a reservoir
during the rainy season and using a multiple level
abstraction to release this sediment laden water
during the wet season, i.e. avoiding sedimentation
in the dam (Kim
et al
., 2007).
Spatial variations in water quality may be
influenced by many different environmental factors
(e.g. climate, geology, weathering processes, vege-
tation cover and anthropogenic). Often it is a
combination of these factors that makes a particular
water-quality issue salient for a particular area. An
example of this is acid rain (also discussed in
Chapter 2) as a particular problem for north-eastern
North America and Scandinavia. The sources of the
acid rain are fossil-fuel-burning power stations and
industry. It becomes a particular problem in these
areas for a number of reasons: it is close to the
sources of acid rain; high rainfall contributes a lot
of acid to the soil; the soils are thin after extensive
glaciation and derived from very old rocks; and the
soils are heavily leached (have had a lot of water
passing through them over a long time period) and
have a low buffering capacity (see p. 34). This com-
bination of influences means that the water in the
rivers has a low
pH
, and - of particular concern to
gill-bearing aquatic fauna - has a high dissolved
aluminium content (from the soils).
Having argued for the role of natural water-
quality issues to be considered seriously, the reader
