Geoscience Reference
In-Depth Information
frequently heavy metal-rich, derived from industry
discharging waste into the sewerage system. When
untreated sewage is discharged into a river heavy
metals can be found in the sediments. Where there
is a combined sewage and storm-water drainage
system for an urban area, untreated sewage can be
discharged during a storm event when the sewage
treatment works cannot cope with the extra storm
water. Runoff from roads (through a stormwater
system) frequently shows high levels of copper from
vehicle brake pads. When washed into a river
system, particularly in summer storms, the copper
levels can be extremely high and cause toxicity
problems to aquatic fauna.
when used for the reduction in a hydrometry (water
quantity) network, is in Pearson (1998). The main
concern is that there are enough measurements to
capture the temporal variability present and that the
sample site is adequately representative of your river
stretch.
One important consideration that needs to be
understood is that the sample of water taken at a
particular site is representative of all the catchment
above it, not just the land use immediately adjacent.
Adjacent land use may have some influence on the
water quality of a sample, but this will be in addi-
tion to any affect from land uses further upstream
which may be more significant.
Gravimetric methods
WATER-QUALITY MEASUREMENT
Gravimetric analysis depends on the weighing of
solids obtained from a sample by evaporation,
filtration or precipitation (or a combination of these
three). This requires an extremely accurate weighing
balance and a drying oven, hence it is a laboratory
technique rather than a field one. An example of
gravimetric analysis is the standard method for
measuring total dissolved solids (TDS). This is to
filter a known volume of water through 0.45
m
(1 micron = one-millionth of a metre or one-thous-
andth of a millimetre) filter paper. The sample of
water is then dried at 105°C and the weight of
residue left is the TDS.
Other examples of gravimetric analysis are
total suspended solids and sulphates (causing a
precipitate and then weighing it).
The techniques used for water-quality analysis
vary considerably depending on equipment avail-
able and the accuracy of measurement required.
For the highest accuracy of measurement water
samples should be taken back to a laboratory, but
this is not always feasible. There are methods that
can be carried out in the field to gain a rapid assess-
ment of water quality. Both field and laboratory
techniques are discussed on the following pages.
Before discussing the measurement techniques it is
important to consider how to sample for water
quality.
Sampling methodology
It is difficult to be specific on how frequently a water
sample should be taken, or how many samples
represent a given stretch of water. The best way of
finding this out is to take as many measurements
as possible in a trial run. Then statistical analyses
can be carried out to see how much difference it
would have made to have had fewer measurements.
By working backwards from a large data set it is
possible to deduce how few measurements can be
taken while still maintaining some accuracy of over-
all assessment. An example of this type of approach,
Volumetric methods
Volumetric analysis is using titration techniques
to find concentrations of designated substances. It
is dependent on measuring the volume of a liquid
reagent (of known concentration) that causes a
visible chemical reaction. This is another laboratory
technique as it requires accurate measurements
of volume using pipettes and burettes. Examples of
this technique are chloride and dissolved oxygen
(using the Winkler method).
