Measuring the Salinity of Water
How to collect water samples and water samples and record their salinity levels.
There are two main methods of determining the salt content of water: Total Dissolved Salts (or Solids) and Electrical Conductivity.
Total Dissolved Salts (TDS) is measured by evaporating a known volume of water to dryness, then weighing the solid residue remaining. Electrical conductivity (EC) is measured by passing an electric current between two metal plates (electrodes) in the water sample and measuring how readily current flows (ie conducted) between the plates. The more dissolved salt in the water, the stronger the current flow and the higher the EC. Measurements of EC can be used to give an estimate of TDS.
Measurement of TDS is tedious and cannot be carried out in the field. EC measurement is much quicker and simpler and is very useful for field measurement. There are however a few simple precautions to note in doing so and these are outlined here.
TDS is recorded in milligrams of dissolved solid in one litre of water (mg/L). Parts per million (ppm) is equivalent to mg/L but it is not a favoured unit.
EC measures the charge carrying ability (ie conductance) of liquid in a measuring cell 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 Commission is microSiemens per centimetre (μS/cm) at 25oC.
You will however see other units and need to be aware of the relationships between them. μS/cm relates to other units as follows:
1000 μS/cm = 1 deciSiemen/metre (dS/m)
1000 μS/cm = 1 milliSiemen/centimetre (mS/cm)
10 μS/cm = 1 milliSiemen/metre (mS/m)
Relationship of total dissolved salts to EC
EC can be effectively converted to TDS for natural Victorian waters by the following relationship:TDS (mg/L) = EC (μS/cm at 25oC) x 0.6
Collecting your water sample
- Make sure that your collecting container is very clean.Previous contents could affect your result. Use a container with an opening large enough to take the EC meter. 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. It needs to be a sample which is like most of the water you want to get information about. If you don't collect a representative sample you're wasting your time. Try not to take your sample too close to the surface, bottom or sides of the waterbody
- Flowing Water - For rivers and creeks try to take your sample in a place where the water if 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 - eg. Dams, swamps and lakes. Saline water is denser than fresh water. This means, that in a still water body, the saline water will settle to the bottom. If you have an offtake pipe from the base of the dam, sample water from here.
- Groundwater - Stock bores can be tested at the trough. However, the water should be freshly pumped. The salinity of water sitting in an unused trough may be higher than the actual groundwater salinity level due to concentration of the salts through evaporation. Investigation bores may be tested using a bailer to collect a water sample. Make sure you ask the permission of the individual or department responsible for the bore.
- Rinse the container two or three times with some of the water to be sampled.
- Collect the sample.
Taking your salinity reading
- Ensure your 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 to remove bubbles from around the electrodes.
- This will ensure good contact is achieved between water and electrodes (do not swirl it around as this may actually drive water out of the probe).
- 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. Some meters automatically correct the reading taken at water temperature to a reading at 25oC.
- If the meter has automatic temperature compensation, wait about 30 seconds before taking your reading if the water and probe are about the same temperature. 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 take the temperature of the sample and use a correction table to get the right value.
- Read the display, and record the result as mentioned below.
- Rinse the probe with tank water and drain off any excess water, between each sample and at the end of sampling for the day. This will prevent false readings due to salt residues on the meter from the last sample.
Recording your results
The results of any sampling, should be recorded in a notebook for future reference. The information should include:
- Name of collector
- Date of sampling Salinity levels fluctuate throughout the year. The date of sampling becomes important then when comparing readings.
- Sampling location. Make a note of where the sample was taken from. Further samples may then be taken from the same site in the future.
- Water source Make a note of the water source. eg. River, Creek, Lake, Dam, Swamp, Drain, Groundwater Bore, etc.
- EC reading. Readings should all be recorded as microSiemens per centimetre (μS/cm). See Salinty units (page 1) to convert readings in other units to μS/cm.
- Temperature reading. If the meter has no automatic compensation, record temperature and adjust resulting EC value from a calibration table.
EC meters should be calibrated regularly to ensure they are reading accurately.
The best idea is to calibrate your meter 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 measuring bottle with calibration solution. (Discard the solution). Pour 100 ml of the Calibration solution into the measuring bottle.
- 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 calibration solution. DO NOT pour the remaining solution back into the original bottle, as this will contaminate your 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 your meter
Rinse the probes with tank water after you have finished testing to prevent salt build up.
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.
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.
Water quality guidelines
|EC range (μS/cm)||Usefulness of water|
|0 - 800|
|800 - 2,500|
|2,500 - 10,000|