Overview
Conductivity and salinity measure water's ability to conduct electricity, and also help to describe a total dissolved solids (T.D.S.) measurement. Pure, distilled water is a very 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.
Influencing Factors
Conductivity and salinity are dependent on many factors including geology, precipitation, surface runoff, and evaporation. Conductivity is also temperature dependent. In a marine estuary, conductivity and salinity are influenced by tides, and can change dramatically. In a freshwater estuary, conductivity and salinity change only slightly due to seiches and rainfall.
In saltwater estuaries, rising tides increase salinity and conductivity. Other things that cause high conductivity and salinity include: increased temperature, fertilizers from agriculture, sewage, road runoff containing automobile fluids and deicing salts, and a local geology with rocks that dissolve easily, such as limestone. Conductivity and salinity also increase when water evaporates.
Often, small daily fluctuations in conductivity and salinity are seen as a result of evaporation during the day, or condensation and groundwater recharge at night. Storms and rainwater lower conductivity and salinity values because they dilute surface water. Rainwater has a lower conductivity than river water. In freshwater estuaries, seiches can push lakewater up into the mouth of the estuary, which slightly lowers the conductivity and salinity. In saltwater estuaries, increased flow from freshwater streams, such as after a storm, decreases the conductivity and salinity in the estuary.
Importance
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 decreases the ability of water to hold oxygen. For instance, ocean water holds about 20% less oxygen than freshwater. Salinity also increases the density of water, and ocean water is heavier than freshwater. This is why freshwater floats on seawater.
Different waters have characteristic ranges of salinity and conductivity. Spikes outside of this range may indicate a pollution event, such as an overdose of fertilizers or the illegal dumping of chemicals. In the north, the use of road salt in the winter often cause spikes in conductivity. Conductivity and salinity are also very useful in determining how freshwater and seawater mix in saltwater estuaries, or how streamwater and lakewater mix in freshwater estuaries.
Conductivity Measurement
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 so that the conductivity of water can be compared at different temperatures. This is because conductivity increases as water temperature increases. Specific conductivity adjusts the conductivity reading to what it would be if the water were 25°C.
Historically, conductivity was reported in the unit mho. A mho is the reverse of an Ohm, which is the standard unit of resistance. In fact, mho is the word Ohm spelled backwards. This makes it easy to remember that Ohms means resistance and mhos means conductance. The term mho has mostly been replaced by the equivalent term Siemen. 1 Siemen equals 1 mho.
Conductivity is measured by an electric probe. Values of conductance are 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.
A four-electrode cell is commonly used in field applications. 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.
Salinity Measurement
Salinity is most commonly reported as parts per thousand (ppt), or the equivalent term, grams per liter. For instance, seawater has an average salinity of 35 parts per thousand, which is equivalent to adding 35 grams of salt to 1 Liter of water.
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. A complex mathematical equation is used to estimate salinity from conductivity. This equation accounts for the temperature dependence of conductivity.