Waterwatch Australia national technical manual

Module 4 - physical and chemical parameters
Waterwatch Australia Steering Committee
Environment Australia, July 2002
ISBN 0 6425 4856 0

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Methods (continued)

Electrical conductivity

What is it and why does it matter?
What factors affect electrical conductivity?
Suggested methods, equipment and reporting
Data confidence
Interpreting your results

What is it and why does it matter?

Electrical conductivity: the property of a substance which enables it to serve as a channel or medium for electricity

Salty water conducts electricity more readily than purer water. Therefore, electrical conductivity is routinely used to measure salinity. The types of salts (ions) causing the salinity usually are chlorides, sulphates, carbonates, sodium, magnesium, calcium and potassium.

While an appropriate concentration of salts is vital for aquatic plants and animals, salinity that is beyond the normal range for any species of organism will cause stress or even death to that organism. Salinity also affects the availability of nutrients to plant roots.

Depending on the type of salts present, salinity can increase water clarity. At very high concentrations, salts make water denser, causing salinity gradations within an unmixed water column and slightly increasing the depth necessary to reach the water table in groundwater bores.

What factors affect electrical conductivity?

Electrical conductivity in waterways is affected by:

geology and soils
land use, such as agriculture (irrigation), urban development (removal of vegetation, sewage and effluent discharges), industrial development (industrial discharges)
flow (electrical conductivity is generally lowest during high flows and increases as flows decrease, with extreme levels occurring during droughts)
run-off
groundwater inflows
temperature
evaporation and dilution.

Contamination discharges can change the water's electrical conductivity in various ways. For example, a failing sewage system raises the conductivity because of its chloride, phosphate, and nitrate content, but an oil spill would lower the conductivity. The discharge of heavy metals into a waterbody can raise the conductivity as metallic ions are introduced into the waterway.

Suggested methods, equipment and reporting

The basic unit of measurement of electrical conductivity is microSiemens per centimetre (µS/cm) or deciSiemens per meter (dS/m). MicroSiemens per centimetre are sometimes called EC units. The total dissolved solids (or TDS) content of a water sample, in milligrams per litre (mg/L), is also a measure of salinity. The sample's electrical conductivity can be converted to TDS.

The electrical conductivity of water samples should be measured on the spot at the waterbody. Measurement can be delayed by up to 1 month if the sample is refrigerated (but NOT frozen) immediately on being taken, and if the sample bottle is filled completely, with no air gap at the top.

Equipment

The equipment you will need for this method includes:

electrical conductivity meter
calibration solution
deionised water

An electrical conductivity meter uses two electrodes, one of which detects an electrical current sent by the other. The meter should also measure temperature and automatically compensate for temperature in the conductivity reading, but non-compensating meters do exist.

If you have a non-compensating meter, you must measure the water temperature at the same time as the electrical conductivity.

When comparing salinities of different samples, it is important to standardise the reading to 25°C. Do this by increasing the electrical conductivity reading by 2% per degree for samples with temperatures below 25°C, and decreasing it by 2% per degree for samples above 25°C.

Different meters are available for fresh water (0-1990 µS/cm), and brackish (slightly salty) water, (0-19 900 µS/cm), and sea water (~50 000 µS/cm). Use a meter that matches the expected conductivity range of your waterway.

Procedure

Before going to the site, calibrate your meter.
On site, rinse the electrode in deionised water.
Collect a water sample using the normal sampling technique.
Dip the electrode into the sample and, if appropriate, select the appropriate conductivity range.
Do not immerse the probe too far (some probes/meters are not waterproof above a certain point).
Move the electrode slowly in a circle for one minute until the digital read-out stabilises or continually jumps between two numbers.
Repeat the test on a replicate sample.
Record both results on a water quality results sheet.
Dispose of the sample downstream of your test site.
Rinse the electrode with deionised water before testing the next sample.

If the conductivity of the sample exceeds the range of the meter, you may dilute the sample. Be sure to perform the dilution according to the manufacturer's instructions because the dilution may not have a simple linear relationship to the conductivity.

Maintenance

Rinse the electrodes with deionised water from a squeeze bottle. Dry the electrodes with a paper towel, otherwise the electrodes will corrode. Replace the cap and place the meter back in your kit. The stainless steel electrodes need to be kept clean and dry.

To ensure accurate readings, you should periodically clean the meter with methylated spirits. Put the electrodes into a beaker with enough methylated spirits to just cover them, and leave them to stand for 15-20 minutes. Remove the electrodes and wipe them with a soft tissue soaked in methylated spirits. Finally rinse them thoroughly with distilled water.

Calibration

Use a conductivity calibration solution - usually potassium chloride (KCl) - to calibrate the meter to the range you will be measuring. For example, a 0.01 molar KCl solution will have a conductivity of 1413 µS/cm, and a 0.001 molar KCl solution will have a conductivity of 147 µS/cm.

To prepare a 0.01 molar conductivity solution, dissolve 0.7456 g of KCl (that has been dried overnight at 105°C) in freshly boiled deionised water and dilute to 1 L (can be stored for 6 months). To prepare a 0.001 molar solution, use only 0.0746 g of KCl (can be stored for 3 months). Store solutions in a dry, dark and cool room.

Significant errors can result from not calibrating your meter at 25°C, e.g. if the meter is calibrated using a solution at 15°C, it will give erroneous water sample readings that are 20% too high.

Tip a small volume of calibration solution into a small clean container for use when calibrating the meter. Do not immerse the meter or probe in the stock solution because this will contaminate it, making it unusable. Rinse the electrodes with deionised water.

Data confidence

For quality control, calibrate the meter with a standard before each sampling run. The standards must be at 25°C and of similar concentrations to test samples. Also, check that the meter has held its calibration at the end of sampling run. Test a Waterwatch mystery sample (available from your coordinator) every six months.

Interpreting your results

Each waterway or waterbody tends to have a relatively consistent range of electrical conductivity values that, once known, can be used as a baseline against which to compare regular measurements of conductivity. Significant changes in conductivity may then indicate that a discharge or some other source of contamination has entered the waterway.

The electrical conductivity of Australian rivers varies widely across the country. Trigger values, given by the revised national water quality guidelines for the protection of freshwater ecosystems (ANZECC/ARMCANZ 2000), range from 20 µS/cm (in upland rivers in tropical Australia and lakes in south-east Australia) to 5000 µS/cm (in lowland rivers in south-central Australia). If samples have salinities equal to or greater than those trigger values, management action is needed to reduce the salinity in that water. Values measured by Waterwatch groups in waterbodies should ideally be smaller than the local trigger values. Check with your Waterwatch coordinator for the range of salinities to be expected in your area, and to discuss the relevant trigger values. Table 4.3 compares typical salinity readings in various types of water.

Estuaries have a higher conductivity than freshwater since, as salinity increases, conductivity also increases. The electrical conductivity of bore water varies and can be several times saltier than sea water.

**Table 4.3: Some electrical conductivity ranges**
Water type	Electrical conductivity (µS/cm)
Deionised water	0.5-3
Pure rainwater	<15
Freshwater rivers	0-800
Marginal river water	800-1600
Brackish water	1600-4800
Saline water	>4800
Seawater	51 500
Industrial waters	100-10 000

Source: Suttar S., Ribbons of Blue Handbook. Scitech, Victoria, 1990.

Safety considerations when measuring electrical conductivity

Let someone know where you are going and when you will return. Don't work alone.
Students must be fully supervised by their teacher in accordance with Education Department guidelines.
Ensure safe and easy access. Beware of slippery rocks and ground.
If sampling near a road or from a bridge, be wary of passing traffic.
Be able to swim if you fall in.
Avoid contact with contaminated water. Use gloves while measuring, but take them off as soon as you've finished. Don't touch your skin with wet gloves.
Keep a first aid kit available.
Feet should be covered; remember sunblock, hat, t-shirt.
After each piece of used equipment has been rinsed with distilled water, pour the rinse water into the liquid waste bottle.
Do not put solid waste into the liquid waste bottle.
Place all used gloves, used paper towels, empty reagent packaging and any other rubbish from testing into a plastic garbage bag with no tears in it, and take it with you when you leave. Try to leave the site cleaner and tidier than you found it-pick up and transport out any rubbish.
Consult your Waterwatch coordinator about disposing of liquid waste.

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