Geography Reference
In-Depth Information
longer tolerate its chemistry and either die or quickly leave that
section of the water body. Often, water quality and pollution are
measured from a human perspective in terms of how safe the water
is to drink or how easy or costly it is to treat the water before it
can be consumed. Water that has a good taste and is not dangerous
to health still contains dissolved substances. The highest quality
waters with the best taste tend to come from reservoirs and lakes
that have collected the majority of runoff from undisturbed land-
scapes with mainly surface water runoff. However, many ground-
waters can taste good too. These tend to be where rocks only
weather slowly. If groundwater has picked up lots of dissolved sub-
stances from the surface, soils and rocks it is more likely to have an
unpleasant taste (see Box 4.3).
The runoff routes for water across and through the landscape are
important for controlling the concentrations of different chemicals
within rivers, lakes and groundwater. As precipitation inputs nor-
mally have low concentrations of dissolved chemicals where a site
is dominated by rapid water movement to rivers (macropore flow
or iniltration-excess overland flow) there is little time for chemical
interactions with soil or rock and so river water chemistry will be
similar to rainwater chemistry (e.g. in blanket peatlands). However,
if there is significant overland flow which brings lots of sediment
with it, perhaps with other added fertilisers or industrial chemicals,
then that can cause pollution problems. Throughflow in upper soil
layers and water coming back out of the soil as part of saturation-
excess overland flow will tend to have quite different solute con-
centrations to that of precipitation since it will have had more time
to interact with the soil and for weathering reactions to take place.
Solute concentrations in groundwater are often greater than else-
where due to the longer contact time of water with soil and rock.
The groundwater composition is affected by the geochemistry of
the surrounding geology since different minerals weather at differ-
ent rates and produce different solutes.
Water quality varies with time because the water flow processes
across the landscape also vary with time. As groundwater and
throughflow deep within the soil are the major source of base
cations to river water from mineral weathering, base cation con-
centrations in river water tend to be greater during dry conditions
when groundwater is the main source of river flow. In wetter
