Measuring the salinity of water
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AgNote LC0064 – This AgNote replaces LC0064 (1999).
Version: September 2018
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This AgNote explains how to collect water samples, test salinity levels and record the results.
Salinity refers to the concentration of soluble salts in soil or water. All natural waters contain some dissolved salts such as sodium, magnesium and calcium. Sodium chloride (table salt) is the most common of all the salts; it is the main constituent of seawater. Salts based on calcium and magnesium also affect the hardness and alkalinity of water.
The level of salt in water affects its suitability for irrigation, stock and domestic use. Collecting samples of farm water and measuring salinity levels will enable landholders to make better use of limited water resources.
Table 1 shows the salinity tolerance levels for livestock water.
Table 2 shows the effects of water salinity on plants.
Table 1: Salinity tolerance levels for livestock water
Production decline begins EC (µS/cm)
|Lactating ewes, weaners||6,000||10,000|
|Sheep, dry feed||9,300||21,800|
Source: Agriculture Victoria (2016) Managing farm water supplies.
Water for farm use in Victoria originates from a variety of sources including farm dams, rivers, streams and groundwater. Most groundwater supplies and many rivers and streams contain high levels of dissolved salts.
The level of salt in water affects its suitability for irrigation, stock and domestic use. Excessive levels of salt can result in lost production or death of the plant or animal.
There are two main methods of determining the salt content of water; Electrical Conductivity (EC), and Total Dissolved Solids (TDS). EC is a measure of how much electrical current can flow through a sample of water while TDS is a measure of the relative weight of dissolved materials in a sample of water.
Table 2: Effects of water salinity on plants
Effect and suitability
Low salinity water – can be used for most crops on most soils with all methods of water application. Little likelihood that a salinity problem will develop.
Medium salinity water – can be used if a moderate amount of leaching occurs. Plants with a medium salt tolerance can be grown, usually without special practices for salinity control.
High salinity water – cannot be used on soils with restricted drainage. Even with adequate drainage, special management for salinity control may be required. The salt tolerance of the plants to be irrigated must be considered.
Very high salinity water – not suitable for irrigation under ordinary conditions. For use soils must be permeable with adequate drainage. Water must be applied in excess to provide considerable leaching. Salt tolerant crops should be selected.
Extremely high salinity water – occasional emergency use for salt tolerant crops on permeable well drained soils under good management. High rates of leaching are required.
Source: Adapted from Victorian Irrigation Research and Advisory Committee, 1980, Quality aspects of farm water supplies.
EC is measured by passing an electric current between two metal plates (electrodes) in the water sample and measuring how readily current flows (conductance) between the plates. The more dissolved salt in the water, the stronger the current flow and the higher the EC.
TDS is measured by evaporating a known volume of water to dryness, then weighing the solid residue remaining. Measurement of TDS is tedious and cannot be carried out in the field. EC measurement is much quicker and simpler, is very useful for field measurement and can be used to give an estimate of TDS. EC can be expressed using a variety of units as outlined below.
EC meters are relatively inexpensive and available at various water equipment dealers.
An EC meter measures conductance through a solution in a container of specific dimensions. It is necessary to clearly define the units of both conductance and length when talking ECs. To say a water sample is 2000 EC is like saying a table is 2000 long, without specifying millimetres, centimetres or metres. The standard EC unit used by the Victorian Salinity Program and the Murray Darling Basin Authority is microSiemens per centimetre (μS/cm) at 25oC.
You will however see other units and need to be aware of the relationships between them. These other units are listed in Table 3 and Table 4. The relationship between the various salinity units is shown in Table 5.
TDS is recorded in milligrams of dissolved solids in one litre of water (mg/L) or parts per million (ppm).
Table 3: Salinity units (EC)
microSiemens per centimetre
milliSiemens per centimetre
deciSiemens per metre
Table 4: Salinity units (TDS)
Total Dissolved Solids
parts per million
milligrams per litre
Table 5: Relationship between salinity units
1 mS/cm = 1 dS/m
1 mS/cm = 1000 μS/cm
*μS/cm x 0.6 = TDS (mg/L)
1 mg/L = 1 ppm
Collecting water samples
To ensure reliable, accurate results water needs to be collected using the procedure outlined below.
- Make sure the collecting container is very clean, as previous contents could affect the result. Use a container with an opening large enough to take the EC meter. A container with an opening of 50 mm should be adequate for most meters. Do not use jars which smell (eg. vegemite, pickle jars) if samples are to be kept for a while.
- Choose a sample which is representative of the body of water being considered. Try not to take the sample too close to the surface, bottom or sides of the waterbody.
- Flowing water – For rivers and creeks, try to take the sample in a place where the water is flowing. Sample well below any stream junction (a rule-of-thumb is the equivalent of 10 stream widths downstream) to allow good mixing.
- Still water – Dams, swamps and lakes deeper than 1m are likely to ‘stratify’, forming a layer of saline water at depth. Saline water is denser than fresh water and may underlie the fresher water in bodies of water that are not mixed by wind activity. Turbidity and temperature variations between layers facilitate this stratification process. For this reason, it is preferred that a sample is obtained from depth or from an outlet pipe at the base of a dam. It is also important to note that the salinity levels in a dam may rise significantly over summer due to evaporation.
- Groundwater – Bores, wells or springs can be tested in-situ or at any outlet point. Pumping or bailing may be needed to ensure a representative sample. The salinity of water sitting in a trough may be higher than the groundwater due to concentration of the salts through evaporation. Investigation bores may be tested using a bailer to collect a water sample. Approval may be required to sample a bore owned or managed by a different individual or organisation.
- Rinse the container two or three times with some of the water to be sampled, then collect the sample.
Taking your salinity reading
It is important that the following steps are followed carefully to ensure reliable, accurate results. If in doubt take your samples to your nearest Agriculture Victoria office for testing.
- Ensure the EC meter has been calibrated (see notes below).
- Remove the protective cap, switch the meter on and insert the probe into the water sample up to the immersion level.
- Move the probe up and down or tap it to remove air bubbles from around the electrodes. This will ensure good contact is achieved between water and electrodes.
- Allow the probe to reach the temperature of the water before taking a reading.
- Temperature has a significant impact on the salinity reading. EC units are standardised to a temperature of 25oC. Most meters automatically correct the reading taken at water temperature to a reading at 25oC.
- If the meter has a temperature compensation mechanism and the water and probe are about the same temperature, wait about 30 seconds before taking the reading. If the water is much colder than the probe, allow a longer period, say two minutes before taking a reading.
- If the meter has no temperature compensation mechanism, take the temperature of the sample and use a correction table to get the right value.
- Read the display, and record the result as described below.
- Rinse the probe with fresh water and drain off any excess between each sample and at the end of sampling for the day. This will prevent false readings due to a build up of salt residues.
- The results of sampling should be recorded in a notebook for future reference. The information should include:
- Name of collector.
- Date of sampling. As salinity levels fluctuate throughout the year the date of sampling becomes important when comparing readings.
- Sampling location. Make a note of where the sample was taken from. Further samples may need to be taken from the same site in the future.
- Water source. Make a note of the water source, e.g. River, Creek, Lake, Dam, Swamp, Drain, Groundwater Bore, etc.
- EC reading. Readings should all be recorded as microSiemens per centimetre (μS/cm). Readings taken using different units can be converted to μS/cm using the tables on page 1.
EC meters should be calibrated regularly to ensure they are reading accurately.Ideally your meter should be calibrated before each sampling session.
You will need:
- Bottle of Calibration Solution.
- Bottle of Check Solution.
Clean measuring bottle with a lid and opening large enough to take the EC meter probe.
- Select a calibration solution about midway within the range of readings you are likely to record.
- Rinse the measuring bottle and probe with calibration solution. (Discard the solution). Pour enough of the calibration solution into the measuring bottle to completely cover the probe.
- Put the EC meter into the solution, allowing time for it to adjust for temperature.
- Using a small screw driver or the calibration tool provided turn the screw until the display reads the same as the known salinity of the calibration solution.
- Discard the calibration solution. Do not pour the remaining solution back into the bottle, as this will contaminate your original calibration solution.
- Use the check solution to test the accuracy of the calibration.
- An unopened bottle of calibration solution has a shelf life of about two years.
Care of the EC meter
Keep the EC meter probe clean and rinse it regularly with distilled or deionised water to prevent a build up of salt.
To improve performance, clean the stainless steel electrodes periodically by rinsing in pure alcohol (eg. methylated spirits) for 10 to 15 minutes.
Variations in readings or a faint display can indicate battery failure. Replace the batteries regularly.
A note on accuracy
Pocket salinity meters such as the TDScan 4 and the DiST 4, when properly calibrated, have a resolution of 100 μS/cm (i.e. they read to the closest 100 μS/cm). However, if samples are not collected properly or the meter has not been calibrated, large errors can occur.
Groups undertaking a salinity monitoring program comparing readings from site to site and from year to year, should use a meter with a resolution of at least 10 μS/cm.
Boulton, B. and Brock, M (1999). Australian freshwater ecology: Processes and management. Gleneagles Publishing, Glen Osmond.
Chaffey, B., ed., (1992). Principles of Sustainable Agriculture – Dryland Salinity. Department of Agriculture, Victoria.
Victorian Irrigation Research and Advisory Services Committee, (1980). Quality Aspects of Farm Water Supplies, 2nd ed. Government Printers, Melbourne.