What is electrical conductivity?
Electrical conductivity is a measure of how well water conducts electricity. Pure water is a poor conductor of electricity, but as more salts are dissolved in it, the conductivity increases. Measuring the electrical conductivity of the water therefore allows us to estimate the amount of salt or solid material dissolved in it.
Most groundwater originates as rain and is dilute, so its electrical conductivity is low. As the water percolates downward into the ground, it dissolves material from the soil and rock it comes into contact with, and its electrical conductivity increases. The substances dissolved in groundwater can be natural, such as calcium carbonate dissolved from limestone, or they can be generated by human activities, such as nitrate fertiliser leached from agricultural soil.
Seawater is very saline and has high electrical conductivity. The difference in conductivity between seawater and fresh groundwater is very marked, so electrical conductivity can be a useful indicator of seawater contamination in coastal wells. Even without seawater contamination, however, groundwater near the coast tends to be slightly more saline than farther inland because of salt deposited on the ground by sea spray.
How is it measured?
Electrical conductivity is measured in the field when groundwater samples are collected. Field staff use hand-held meters that are maintained and calibrated following standard protocols. Samples can also be collected and sent to a laboratory for analysis. In some cases, chemical processes in the sample bottle may change the conductivity slightly between the field and the laboratory, but there is debate about which measurement is more reliable. In any case, the differences tend to be small and can usually be disregarded.
Several units are commonly used for reporting electrical conductivity. LAWA reports values in units of microSiemens per centimetre (µS/cm). Other units commonly in use include milliSiemens per metre (mS/m) and milliSiemens per centimetre (mS/cm). Conversion between the different units is straightforward.
Conductivity varies with temperature, and the values measured by regional councils and reported on LAWA are standardised to 25 °C.
What do the values mean?
For reference, high quality deionized water (nearly pure) has a conductivity of about 0.05 μS/cm, whereas seawater has a conductivity of approximate 50,000 μS/cm. The electrical conductivity of fresh groundwater is typically less than 100 μS/cm. Electrical conductance is defined as the conductance of a cubic centimetre of water at a temperature of 25 degrees Celsius.
There are no health or aesthetic guideline values for electrical conductivity in water.