UPDATE: Fondriest Environmental is offering their expertise in conductivity through their new online knowledge base. This resource provides an updated and comprehensive look at conductivity and why it is important to water quality. To learn more, check out: Conductivity, Salinity and TDS.
Salinity and conductivity measure the water’s ability to conduct electricity, which provides a measure of what is dissolved in water. In the SWMP data, a higher conductivity value indicates that there are more chemicals dissolved in the water.
Conductivity measures the water’s ability to conduct electricity. It is the opposite of resistance. Pure, distilled water is a poor conductor of electricity. When salts and other inorganic chemicals dissolve in water, they break into tiny, electrically charged particles called ions. Ions increase the water’s ability to conduct electricity. Common ions in water that conduct electrical current include sodium, chloride, calcium, and magnesium. Because dissolved salts and other inorganic chemicals conduct electrical current, conductivity increases as salinity increases. Organic compounds, such as sugars, oils, and alcohols, do not form ions that conduct electricity.
Aquatic animals and plants are adapted for a certain range of salinity. Outside of this range, they will be negatively affected and may die. Some animals can handle high salinity, but not low salinity, while others can handle low salinity, but not high salinity.
In addition to its direct effects on aquatic life, salinity also has many other important effects on water chemistry and water density.
Salinity is most commonly reported as parts per thousand, or the equivalent term, grams per liter. For instance, seawater has an average salinity of 35 ppt, which is equivalent to adding 35 grams of salt to 1 Liter of water
Conductivity is reported in a unit called a Siemen, or its smaller versions, the milliSiemen, which is one-one thousandth of a Siemen, and the microSiemen, which is one-one millionth of a Siemen. Most commonly, a special type of conductivity, called specific conductivity, is used.
Conductivity and salinity are both measured by an electric probe on the data logger. This probe measures how much electrical current moves through the water. Salinity is then calculated from this value.
Conductivity is determined by measuring how easily an electrical current flows between two metal plates. These metal plates are called electrodes, and are spaced a specific distance apart. Dissolved salts in solution will be attracted to the plate with the opposite charge. In many probes, a four-electrode cell is used. Two of the electrodes measure the current of the solution, while the other two electrodes maintain a constant current between them and are used as a reference.
The best method to determine salinity is to perform a chemical analysis of the concentrations of different ions in water, such as calcium, sodium, chloride, and carbonate. However, because this method is time-consuming, tedious, and expensive, salinity is estimated from conductivity. Because salts in water conduct current, conductivity will be proportional to the salt concentration. The data logger uses a complex mathematical equation to estimate salinity from conductivity. This equation accounts for the temperature dependence of conductivity.