Water sampling is the act of collecting a portion of material for analytical purposes that accurately represents the material being sampled with respect to stated objectives, (APHA and others, 2001).
We can also say that water sampling is the process of taking a portion of water e.g. from borehole, river, spring, lake, dam ormunicipal council for purpose of analysis to verify that the water is fit for domestic use or intended purpose.
There are two types of water samples i.e. grab samples and composite samples. Grab samples are usually taken where information specific to a particular sampling location, time or distinct areas within a sampling location are required. On the other hand composite samples are usually taken when an average representation of a sampling location or time is needed.

Water samples may be collected from a tap outlet, standing water body (surface water) or groundwater.The objectives of any water sampling arrangement should be clearly formulated beforewater samples are collected and finally analyzed. For domestic water supplies and specifically those that have undergone some mode of treatment, the sampling objectivesare determined by the information that is required such as;

  • The efficiency of the water treatment e.g. Reverse Osmosis, UV, filtration or chlorination
  • The level of treatment required to ensure that a water supply is suitable for domestic use.
  • Possible contamination of the water in the distribution system; such as from bacteria or from the pipes.
  • The water quality at the point of use.

The objectives of the sampling programme in turn determine issues such as;

  • How are samples to be taken?
  • Which substances in the water are of interest e.g. physiochemical, bacteriological, pesticides, or heavy metals.
  • Which analytical methods are to be used?
  • Where and when are samples to be taken?
  • How results are to be reported.
  • What is to be done with the reported results.

1. SAMPLING FROM A TAP:If there are several taps in the area of test, choose a tap which is most frequently used. Remove any external fittings such as filters and remove any contaminants (e.g. grease, slime, sediment build-up etc.) around the spout with a clean cloth. Tap cleanliness is particularly important with microbiological testing. Tap outlets which are suspected to be contaminated must be disinfected first before taking the sample. Disinfect by swabbing the outside of the tap and as much of the inside as possible with methylated spirit or a 0.1% sodium hypochlorite solution. Prepare the 0.1% solution by diluting commercially available sodium hypochlorite solution (approx. 10%) by a factor of 100. Allow to stand for a few minutes (to allow full disinfection) before proceeding to the next step shown below.
CAUTION: Sodium hypochlorite is a strong oxidizing agent and is highly corrosive. Handle with great care and wear appropriate PPE (gloves, safety glasses). If contact with skin or clothing occurs, wash immediately with copious quantities of water.

  • Check for the correct sample bottle and label;
  • Turn on tap water at maximum flow and start timing;
  • Let water flow for 2 to 5 minutes depending on how often the tap is used (If the internal plumbing system has not been used for a long period of time, flush the system thoroughly before sampling).
  • After flushing, open cap of the sample bottle;
  • Keep holding the sample bottle cap in one hand while sample is being collected to ensure it does not come into contact with anything to avoid contamination;
  • Rinse the bottle with the same sample up to three times.
  • Fill the sample bottle carefully to prevent overfill;
  • Carefully put the cap back on the sample bottle;

The following should be noted during sampling:
i. The sampling bottle shall be so held that the water does not come in contact with the hand before entering into the bottle;
ii. Make sure that all samples are correctly labeled (sampling point, date and sampler).
iii. Store water samples in ice-boxes with freezer packs and deliver to laboratory on the same day.
Iv. When samples for chemical and microbiological analyses are collected from the same tap thesample for microbiological examination should be collected first. Thisprecautionary measure is to avoidthe danger of contamination of the water at the sampling point.

2. SURFACE WATERS: This includes shallow and deep lakes, rivers, creeks, streams etc. The sample should be representative of the source of supply. In this respect, it is important to consider location and depth of the water. Taking samples very close to the bank may not be representative of the source of supply. Proper safety precautions must be observed when collecting surface water samples.

  • For a surface water sample, simply hold the bottle firmly and plunge the neck downwards to a depth of about 0.5m.
  • Turn the bottle until the neck points upward and mouth is directed towards the current (if present). If a sample is taken from a boat, always collect the sample from the upstream side of the boat. 
  • Alternatively, a clean bucket (of about 10 L capacities) or other suitable vessel such as a large beaker can be used to collect the surface sample.
  • Dip the bucket or beaker into the stream, withdraw and then transfer to the laboratory sample container. Rinse the sample bottle with same sample three times. If a composite sample is to be submitted for analysis, pour equal portions of freshly collected samples into a suitable container.
  • Be careful not to disturb bottom sediment.

Procedural Precautions
The following precautions should be considered when collecting surface watersamples.

  • Special care must be taken not to contaminate samples. This includes storing samples in a secure location to preclude conditions which could alter the properties of the sample. Samples shall be custody sealed during long-term storage or shipment.
  • Collected samples are in the custody of the sampler or sample custodian until the samples are relinquished to another party.
  • If samples are transported by the sampler, they will remain under his/her custody or be secured until they are relinquished.
  • Surface water samples will typically be collected either by directly filling the container from the surface water body being sampled or by decanting the water from a collection device such as a stainless steel scoop or other device.
  • During sample collection, if transferring the sample from a collection device, make sure that the device does not come in contact with the sample containers.
  • Place the sample into appropriate, labeled containers. Samples collected for VOC analysis must not have any headspace. All other sample containers must be filled with an allowance for ullage.
  • All samples requiring preservation must be preserved as soon as practically possible, ideally immediately at the time of sample collection.

Quality Control
If possible, a control sample should be collected from a location not affected by thepossible contaminants of concern and submitted with the other samples. In streams orother bodies of moving water, the control sample should be collected upstream of thesampled area
3. GROUNDWATER: Groundwater is the water present beneath Earth’s surface in soil pore spaces and in the fractures of rock formations. This includes boreholes and shallow wells. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table.
Sampling of groundwater without pump.

  • Secure a weight under the sample bottle and lower a plastic sample bottle in borehole/well until the bottle is submerged in the water.
  • Fill bottle with groundwater.
  • When raising the bottle to the surface ensure that the sample bottle does not get contaminated as a result of touching the inner walls of the borehole/well.
  • Keep sample container closed and in a clean condition up to the point where it has to be filled with the water to be tested.
  • At sampling point remove cap but do not contaminate inner surface of cap and neck of sample bottle with hands.
  • Do not rinse the bottle.
  • Fill sample with groundwater and replace the cap immediately.
  • Leave ample air space in the bottle (at least 2.5 cm) to facilitate mixing by shaking, before examination.
  • Place sample bottle in a cooled container (e.g. cool box) directly after collection. Try and keep cooled container dust-free and out of any direct sunlight.

Sampling of borehole/well with pump

  • Find the nearest tap on the line where you must collect the water sample.
  • Open the tap and let water run to waste for at least 3 minutes. At the sampling point remove cap but do not contaminate inner surface of cap and neck of sample bottle with hands.
  • Remove cap of sample bottle, but do not contaminate inner surface of cap and neck of sample bottle with hands.
  • Fill sample bottle after rinsing with sample water for three times.
  • Replace the cap immediately.
  • When the sample is collected leave ample air space in the bottle (at least 2.5 cm) to facilitate mixing by shaking before examination.
  • Place sample bottle in a cooled container (e.g. cool box) directly after collection. Try and keep cooled container dust-free and out of any direct sunlight.

Note:

  • Operate the pump to flush out stagnant water from the pipe.
  • Do not sample a newly drilled borehole/well or a rarely used one unless the facility has been pumped for more than 48 hours.

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