Sampling And Monitoring Water Quality

Water quality is monitored on a regular basis, including the quality of:

• Water received on the farm, whether it be water received from a river or underground water received through boreholes;
• Water contained in storage dams; and
• Water leaving the farm.

To monitor the quality of the water, samples are taken at the water source, tested and analysed.

Sampling is the process where the body of water to be sampled is reduced in volume so that it can be transported to a laboratory. Although this sounds strange, it is important to understand that a proper sample is not just a piece of the body of water, but a very small but true reflection or representation of the entire body. The volume of a water sample is usually 500ml, but this 500ml can represent millions of litres of water. Always keep this in mind when taking water samples.

Ensure that the 500ml taken is exactly the same as the millions of litres of water in the dam or river, in all aspects that are to be measured and tested.

Click here to view a video that explains water quality testing.

Monitor And Perform Basic Water Quality Tests And Analysis

Care must be taken when sampling to ensure that the sample is not contaminated while or after it is taken.

Containers: The most appropriate container for water samples is a plastic bottle with a screw cap. In practice, 500ml mineral water or cold drink bottles made from polycarbonate is very suitable for this purpose. Special bottles for water sampling are also available on the market.

Cleaning of containers: Wash all containers that are being reused properly with warm water, but do not use a detergent. Take care that the cap is also washed properly. Do not use caps with cardboard or absorbing seals. Before taking a sample in a properly washed reused or new container, rinse it three times with the water to be sampled. Also, rinse the caps.

Labelling: Never write directly on the bottle, because the lettering could be rubbed off during transportation. Stick a label onto the container or tie it to the neck of the bottle. Write all relevant information on the label with a waterproof pen. The minimum information on the label includes:

The name of the farm/farmer.

The identification number or name of the water source.

If only one water sample is submitted, also write the contact information of the farm or farmer on the label. Where more than one sample is submitted from the same farm or farmer, at least one label should contain all the relevant information and the rest only the farm or farmer name and the name or number of the water source.

Water Sampling Procedure

The standard procedure for sampling water is as follows:

• Rinse the container and cap three times with the water to be sampled.
• Fill the bottle to capacity and screw the cap on, leaving no air space at the top.
• Label the bottle.
• Put aside in the shade and dispatch as soon as possible to the laboratory.

Although this standard procedure remains the same on principle, special procedures are followed when sampling water from various water sources.

Canals – Sample the water in the middle of a running canal.

Boreholes – Let the pump run for at least one hour or for as long as it takes to fill all piping with fresh water. Never sample the water standing in the pipes.

Storage Dams – Take the sample away from the sides or wall of the dam and 50cm below the surface.

Rivers – Take the sample in the flow of the river. Standing water accumulates suspended material and has a higher concentration of salts.

Water from Irrigation Pipes – If the purpose of the sampling is to measure the quality of the irrigation water, remove a micro-sprayer or open the end of a lateral line. Let the water run to flush the pipes, and then fill the bottle. If the purpose is to measure the type and mass of the deposits, open the end of two or three laterals into a suitable container with a capacity of about 50 litres. Then take the sample from this container while stirring the water.

Performing Simple Chemical Water Quality Tests

Water quality tests that need to be done on a continuous basis are pH tests and electrical conductivity (EC) tests. These two tests are relatively simple and do not require a lot of expensive equipment.

pH Testing

The following equipment and instrumentation are required:

• A pH meter can be a handheld or desktop instrument.
• Alternatively, special pH-sensitive paper strips, but not Litmus paper.
• Glass or plastic beakers.
• The washing bottle contains demineralised water.
• Reagents.
• pH buffer solution for pH 7 and 4 or any other two whose range will include the pH of the water to be tested.

Demineralised Water: Demineralised water refers to water from which the dissolved salts (minerals) have been removed. Car battery water is for example demineralised. In chemical laboratories, all water that is used, even for washing the equipment, is demineralised.

The following procedure is followed when a pH meter is used:

• Switch the pH meter on and let it warm up according to the instructions of the manufacturer.
• Rinse the electrode with demineralised water.
• Mark two beakers pH 4 and pH 7 and pour the appropriate pH buffer into each beaker.
• Insert the electrode into buffer solution pH 4. Set the pH meter to read pH 4.
• Remove the electrode from the buffer and rinse with demineralised water in a separate beaker.
• Insert the electrode into buffer solution pH 7. Set the pH meter to read pH 7.
• Remove the electrode from the buffer and rinse with demineralised water in a separate beaker.
• Repeat the process with buffer solutions until the instrument reads 4 and 7 when inserted into the buffer solutions.
• Rinse the electrode well with demineralised water.
• Insert the electrode into the water sample. Let the reading stabilise before it is noted on the report (reading A).
• Rinse the electrode and take a reading of one of the pH buffer solutions. Note the reading (reading B).

pH-sensitive paper strips change colour when coming into contact with a substance that is not pH neutral. The following procedure is followed when pH-sensitive paper strips are used:

• Pour either the pH buffer solution of pH 4 or pH 7 into a marked beaker.
• Insert a strip into the beaker and keep it there for the required period, according to the product instructions.
• Verify the colour of the paper strip with the colour chart. This compares to reading B in the previous procedure.
• Insert a strip into the water sample and keep it there for the required period.
• Compare the colour of the paper strip with the pH colour chart. Note the pH according to colour. This compares to reading A in the previous procedure.

Click here to view a video that explains how to test pH in water.

Electrical Conductivity (EC) Testing

The electrical conductivity of water refers to its ability to conduct an electrical current. The more salts dissolved in the water, the higher the ability to conduct current, hence an increase in the EC value.

The following equipment and instrumentation are required:

• An appropriate EC meter, which can be a handheld or desktop version.
• Washing bottle with demineralised water.
• Glass or plastic beakers.
• Reagents.
• Standard solution of for instance 1,000g potassium chloride per litre water.

The following procedure is followed:

• Standardise the instrument according to the instructions of the manufacturer using the standard potassium chloride solution.
• Rinse the electrode of the EC meter in demineralised water.
• Insert it into the water sample.
• Note the reading (reading C).
• Rinse the electrode of the EC meter in demineralised water.
• Insert it into the standard solution.
• Note the reading (reading D).

Click here to view a video that explains EC-testing.

Reporting On Water Quality Tests

Reporting on water quality involves three steps, namely:

• Ensuring that the value of the reference standard is acceptable.
• Ensuring that the units are correct.
• Comparing the results with reported results from previous tests at the same source.

If this sequence is followed, the accuracy and quality of the test are guaranteed.

Reference Standard

In the methods described above, reference standards are used for pH and EC to calibrate the instruments. The same solutions are used to verify the results by using it as an unknown sample together with the water sample. A water sample cannot be used as a reference, since the composition will change over time. Use one of the calibration standards, which are chemically stable as a reference.

Example: pH Testing - Let us assume that buffer pH 7 was used as an unknown and was read at the end of the procedure. The reading is recorded as “Reading B”, with the value of 7.03. Compared to what it should read, namely 7.00, one can conclude that the deviation is acceptable and that the reading on the water can also be accepted as correct. Therefore “Reading A” is accepted and reported. Deviations of less than 5% are acceptable. A deviation of 0.03 from the expected 7.00 represents a deviation of 0.03 / 7.00 x 100 = 0.43% which is acceptable.

Example: EC Testing - The same procedure is followed for verifying the EC reading. At 25°C, 1,000mg potassium chloride per litre of water has an EC of 185mSm-1. Let us assume that the reading of the potassium chloride solution at the end of the procedure was 150mSm-1 (Reading D). This deviation is not acceptable, and the value noted for the water cannot be accepted. Therefore, the procedure needs to be repeated, ensuring that the EC meter is correctly calibrated. A deviation of 35 from the expected 185mSm-1 represents a deviation of 35 / 185 x 100 = 18.92%, which is not acceptable.

Units

The pH value has no unit. It is only a value referring to the pH scale which runs from 0 (acid) to 14 (alkaline).

The international standard for EC is milli-Siemens per meter (mSm-1). There are however other units that are used, which may create confusion. In the table below, the relationship between the internationally accepted mSm-1 and other measurements are shown for reference purposes.

Information: Relationship between EC Units

One Siemen = One mho

mScm-1 = mmhocm-1

1 mScm-1 x 100 = 1 mSm-1

1 mSm-1 = 1 mmho m-1 = 0.01 mmho cm-1 = 10 micro mho cm-1

“mho” is a unit of electrical conductance, with the symbol Ω-1. This unit has been renamed the siemens. Conductance in mho being the reciprocal of resistance in ohms, mho is ohm spelled backwards.

Comparisons

Once the reference standards and units are verified, the results can be compared with the previous results from the same source. Please keep in mind that these comparisons need to be done on a seasonal basis. Do not compare the values of samples taken in spring with those of samples taken during summer or autumn. Compare results from water samples taken in spring with those of samples previously taken in spring.