82 ENVIRONMENTALENGINEERINGThe grab sample, as the name implies, measures water quality at only one sampling
point. Grab samples accurately represent the water quality at the moment of sampling,
but say nothing about the quality before or after the sampling. A composite sample
is obtained by taking a series of grab samples and mixing them together. The flow-
weighted composite is obtained by taking each sample so that the volume of the sample
is proportional to the flow at that time. The last method is especially useful when daily
loadings to wastewater treatment plants are calculated. Whatever the technique or
method, however, the analysis can only be as accurate as the sample, and often the
sampling methods are far more sloppy than the analytical determination.
DISSOLVED OXYGENOne of the most important measures of water quality is dissolved oxygen. Oxygen,
although poorly soluble in water, is fundamental to aquatic life. Without free dissolved
oxygen, streams and lakes become uninhabitable to aerobic organisms, including fish
and most invertebrates. Dissolved oxygen is inversely proportional to temperature, and
the maximum amount of oxygen that can be dissolved in water at 0°C is 14.6 ma.
The saturation value decreases rapidly with increasing water temperature, as shown in
Table 5-1. The balance between saturation and depletion is therefore tenuous.
The amount of oxygen dissolved in water is usually measured either with an
oxygen probe or by iodometric titration. The latter method, known as the Winkler test,
Table 5-1. Solubility of Oxygen in WaterWater temperature Saturation concentration
(“C) of oxygen in water (ma)
0 14.6
2 13.8
4 13.1
6 12.5
8 11.9
10 11.3
12 10.8
14 10.4
16 10.0
18 9.5
20 9.2
22 8.8
24 8.5
26 8.2
28 8.0
30 7.6