11. Monitoring Methodology
It is not the intention to go into great detail here on the different methodologies used to sample water bodies. However, there is a basic need to incorporate quality control and assurance procedures described in the next section into the taking of samples or into making measurements. This applies equally to quality and quantity measurements, and also to chemical, physical and biological determinants. Mistakes or inconsistencies at this stage can invalidate data as much as poor quality control in the laboratory can.
For chemical monitoring three basic methodologies of obtaining samples can be identified.
- Discrete manual sampling, in which samples are taken from a water body manually and generally transported to another location for analysis. Such samples represent snapshots in time and space.
- Discrete or cumulative automatic sampling. Instead of taking samples by hand, machines are programmed for the collection of different sized samples for discrete or cumulative samples collected over a pre-set time span. Again these samples will be snapshots in space but can be more representative of quality/quantity over a time period.
- Continuous on-line monitoring where a suitable measuring device is placed directly into the water to obtain a measurement, or a sample of the water is pumped to a bankside measuring device. The process is automated, and results and information can be telemetered to a central control location. Continuous monitoring can give real time information and, potentially, also over long time periods. They are widely used as early warning or control monitors, for example, for the detection of step changes in quality perhaps in relation to pollution spills. The use of continuous on-line monitors is often a balance between the value of (near) continuous data, and the cost of constructing and maintaining the instruments at a level where the data are known to have the required accuracy and precision. Continuous monitors are discussed in more detail in Appendix B.
With all of these methodologies quality control must be maintained over aspects such as appropriate sampling vessels (e.g. material, size), sample preservation, sample return (to the analytical laboratory) times, sampling location and times (day, year etc.).
Sampling aquatic biology should also have the same basic quality control considerations as for chemical sampling. Biological sampling is often achieved through the use of nets, pumps or grabs rather than bottles. However, sample treatment and preservation are of equal importance as for chemical determinants.