Water sampling methods

1. Water Sampling
Processing

2. 2
Representative sample
A representative sample is taken from a large body of water to make up a sample
(sub-samples of 500 mL) which represents the whole body of water. To
minimize the effect of natural variation in the data collected, the sample should
be collected:
 From exactly the same location
 At approximately the same time each day
 Analyze them in the same way.
Water samples can be taken directly from the water body (stream or dam), or if a
pump is used, from the first outlet along the supply line. Below, some
instructions for taking a representative of water samples:
 Stream: Sample taken from main stream flow.
 Stock drinking water: Sample taken from near the edge.

3. 3
 Dam: If water is pumped from a dam, and the sample is not taken from the outlet
side of the pump, take the sample from near where suction side of the pump
draws water. If wishing to assess changes in quality throughout the dam, where
the water may settle into well-defined layers, it may be necessary to sample the
chosen layers separately. The practicality of sampling from specific depths needs
to be considered. The sample is taken from upper layers first.
 Underground source: When water is drawn from an underground source, i.e. a
bore or well, this is often the only way to sample. Allow the pump to run for
sufficient time to flush out water which has been in the pipe, then take samples at
time intervals of 5-10 minutes, from the first off take point, e.g. tap, trough, or
sprinkler head.
 Reticulated source: If the water is drawn from a reticulated source, e.g.,
irrigation scheme, city or town water supply, check with the local supplying
authority, as they may have analytical data on the chemical and other
characteristics of the water.
 Water irrigation: Where the water is used for irrigation or piped for livestock or
domestic purposes, the simplest approach is normally to let the pump do the
sampling.

4. 4
Type of water sampling
The two types of samples most commonly obtained are grab samples and
composite samples:
Grab samples are single samples collected at a specific spot at a site over a short
period of time not exceeding 15 minutes. Thus, they represent only the
composition of its source at the time and place of collection. A grab sample is
required for test parameters which must be measured immediately after collection,
and for tests which require the entire contents of a single sample container for
analysis or for tests where the parameter will change over time. Examples of test
parameters which require grab samples are temperature, pH, residual chlorine, and
dissolved oxygen.
Composite samples are obtained by taking an appropriate number of grab
samples collected at equal intervals or proportional to flow. Flow proportional
composite samples are collected when the flow and waste characteristics are
continually changing. This single sample reflects the average conditions of a
point-source during an interval. The nature of the composite sample requires the
tested parameters to be stable in the container for the duration of sampling, often
24 hours (this is usually means that the samples are refrigerated to 4 °C).

5. 5
Protecting and preserving samples
The physical and chemical characteristics of a water sample can change during
transport and storage. Therefore, protecting and preservation methods only slow
chemical and biological changes that inevitably occur after collecting sample. Some
parameters change more quickly than others, and so need to be analyzed on site.
Protecting and preservation of samples is the main thing to obtain good quality
result.
1. The most common preservative approach is cooling to 4 °C. This can be
achieved by making slurry of ice and water and placing the collected sample in
the slurry. This serves to slow biological activity in the sample and keep
dissolved gases in solution. However, it can hasten certain physical processes
such as precipitation of metals.
2. Other common preservatives (prevent known chemical reactions and maintain a
target analyte in solution) include:
- Use of sodium thiosulfate (final concentration is 0.008 %) to remove chlorine.
- Use of copper sulfate or mercuric chloride to halt biological activity.
- Use of zinc acetate to trap sulfides.

6. 6
- Appropriate pH adjustment to either, over 12 SU (Standard Units) with sodium
hydroxide (NaOH) or less than 2 SU with hydrochloric acid (HCl), sulfuric acid
(H2
SO4
) or nitric acid (HNO3
) (depending on the intended analyze) can
inactivate biological processes.
3. The analyst must be aware that acidification of nitrate-nitrite samples with
HNO3
or preservation of SO4
samples with H2
SO4
will invalidate the analyses,
so the sampler must make adequate notation.
4. Samples which are not preserved must be analyzed immediately.
Handling and storage of samples for analysis
 As soon as the water samples are received at the water analysis laboratory,
they should be checked with the accompanying information list (including
sample number and date of sampling should be written on the container.
Information regarding samples should be entered in a register and each sample
be given a laboratory number.

7. 7
 The sample(s) should be kept refrigerated or on ice in a cooler (but not allowed
to freeze) as soon as possible. Changes caused by growth of microorganisms
are best slowed by cold and dark.
 In general, the shorter the time between collection and analysis, the more
reliable the results will be.
Health and safety
 Laboratory personnel who are routinely exposed to water samples, that may
contain live microorganisms, are encouraged to protect themselves from water-
borne illnesses by wearing clean disposable gloves and washing their hands
frequently.
 Care must be taken when handling hazardous reagents by using protective
clothing and safety goggles.