Water Sampling Basics
The sample collected should be small in volume, enough to accurately represent the whole water body
The water sample tends to modify itself to the new environment. It is necessary to ensure that no significant changes occur in the sample and preserve its integrity till analysed (by retaining the same concentration of all the components as in the water body)
The essential objectives of water quality assessment are to:
- define the status and trends in water quality of a given water body
- analyse the causes for the observed conditions and trends
- identify the area specific problems of water quality and provide assessments in the form of management to evaluate alternatives that help in decision-making
Sampling sites for the waterbody/lake are selected to represent the water quality at different points and depths. Generally three sampling sites are selected for monitoring
Inlet: the point where the principal feeder opens into the lake
Center: the point that gives the general water quality of the lake
Outlet: the place where the overflow occurs
Types of water sampling
The quality of water varies with time in a water body due to various natural and human induced factors. The monitoring has to be done in a way that records all the changes in the quality. The sampling frequencies generally adopted in monitoring are:
- Weekly sampling for one year
- Consecutive day sampling during study period
- Hourly sampling for 24 hours
- Monthly sampling at predefined time
Variations in water quality are mainly due to changes in the concentrations of the components of the water flowing into the water body. These variations can be man-made or natural and can either be cyclic or random
Random variations: due to spasmodic, often unpredictable events such as accidental oil spills, sewage leaks, overflows, etc.
Cyclic variations: may be a result of regular seasonal changes triggering certain natural processes such as rainfall, snowmelts and seasonal temperature changes, altering the ecosystem. Seasonal growth and decay of vegetation will also rise due to cyclic changes in the composition of water
In lakes, the mass of water and good lateral mixing provide inertia against any rapid modifications due to inputs and outputs.
The sampling container should not react with the sample, be of adequate capacity to store the sample and be free from contamination
Grab sampling is done at the inlet, center and outlet in most of the water bodies studied to assess their physical and chemical qualities at monthly intervals. The samples are collected in thoroughly cleaned 2.5-litre inert plastic containers, which were rinsed with distilled and lake/tank water before collection
- Avoiding floating materials in sample.
- The stoppers of the sample containers to be closed properly to prevent outside contamination.
- The container should be labelled describing the name of the water body, date, time, sampling-point, and conditions under which it was sampled
Preservation of the sample
Between the time a sample is collected and analysed in the laboratory, physical, chemical and biochemical reactions may take place in the sample container leading to changes in the intrinsic quality of the sample, making it necessary to prevent or minimize these changes with suitable preservatives such as alcohol and mercuric chloride. Highly unstable parameters such as pH, temperature, transparency, free carbon-di-oxide, dissolved oxygen, etc. are measured at the sampling site.
The preservation procedure includes keeping the samples in the dark, adding chemical preservative, lowering the temperature to retard reactions, or combinations of these.