Specific conductance is the measure of water's ability to conduct electricity. This conductivity is based on the amount of solid compounds which are dissolved into the water sample. The greater the dissolved solids, the greater the conductivity. Specific conductance can be used to measure water quality and indirectly test the level of pollution present in water.
Conductance happens because more ions are mobile and present in a solution than in a pure substance. Pure water contains molecules of H2O, which are neutral. Salt water, however, contains molecules of H2O and ions, positively or negatively charged atoms, of chlorine (Cl-) and sodium (Na+) from the dissolved salt. Since they have a charge, they can conduct electricity. The more ions present, the greater the conductivity.
This conductance is measured using a sensor that actually measures how well a substance does not conduct electricity, or resists electric current. The unit used to measure specific conductance is in siemens, which indicates a measure of electrical conductance. Measurements are usually given in microsiemens per centimeter (μs/cm).
Distilled water, which is theoretically pure water, usually has a measure of 1 μs/cm. Absolutely pure water should contain no dissolved compounds and therefore should have a measurement of zero. It is nearly impossible to create water that is devoid of even trace compounds, however, which is why the actual reading differs from the theoretical reading. Conversely, sea water, which contains high concentrations of dissolved salt, has a reading of about 50,000 μs/cm.
As in the case of sea water, specific conductance may be high for natural reasons. Certain rocks may release ions when water flows over them, adding to the specific conductance of the water they come in contact with. Also, rain water often has a higher conductance than pure water because it picks up dust and gas particles as it falls from the sky.
Many causes of high specific conductance are due to human pollution, however. Runoff from farmland may contain pesticides or fertilizer used on the crops, just as runoff from roads can contain fluids from cars and road salt used to de-ice roadways in the winter. Abandoned mines also create high conductance because they may have acidic drainage which can contaminate nearby water supplies.
Although there are not regulations for acceptable levels, there are regulations for acceptable levels of compounds present in water. These levels vary depending on whether the water is used for industrial, agricultural, or domestic uses. Purifying polluted water is usually done by reverse osmosis and can be costly.