Why Measure Conductivity or Salinity?
The Main Points
- Conductivity measurements in freshwater lakes, rivers, and ponds are a rapid and convenient way to detect pollution events or other changes in the system. It is most useful as a long-term monitoring strategy.
- Conductivity does not measure the actual "salt" content of a body of water since it simply measures the total electrical current conducted by all cations and anions. Water hardness (CaCO3 concentrations) or NaCl concentrations have to be measured with more specific testing procedures.
As the name implies, conductivity is a measure of the ability of water to pass an electrical current. Because dissolved salts conduct electrical current, conductivity increases when there are more ions dissolved in the water.
We commonly think about salt as consisting of sodium chloride (NaCl) which is common or table salt. But chemists define "salts" as any compound that when dissolved in water releases a positively charged cation and a negatively charged anion. These ions are what can conduct the electricity. Thus a molecule of solid sodium chloride (NaCl) becomes one sodium ion (Na+) and one chloride anion (Cl-) when dissolved in water. Calcium carbonate (CaCO3) dissolves to form one positively charged Ca+2 cation and one negatively charged CO3-2 anion.
All natural water bodies contain some salts from the local rocks, minerals, and soils. Human activities that can increase the water's salt content include spreading road salt in the winter months, disturbing the soil, and directing stormwater runoff into the waterbody.
Conductivity is very useful as an overall measure of water quality. Every water body typically has a relatively constant range of conductivity. Conductivity measurements are made over time so that a baseline value can be established. Large changes in conductivity can then indicate when a discharge or some other source of pollution has entered the water. Water bodies with elevated conductivity often suggest impairments.
In freshwater we generally report conductivity measurements in units of micro- or milli- Siemens. (In case you are wondering, this unit of conductivity was named after Ernst Werner Siemens who also founded Siemens AG.) In salt or estuarine waters conductivity measurements are usually converted into salinity and reported as parts per thousand (ppt). For example, a liter of seawater may contain 30 grams of salt. Since a liter of seawater weighs about 1000 grams, the salinity is 30 grams per 1000 grams (30/1000 or 30 ppt).