Conductivity measures the ability of water to pass an electrical current. Just like copper wire transfers an electrical current from a power source to our TV and oven, water can conduct an electrical current. The conductivity in freshwater bodies varies greatly from 150-500 microsiemens per centimeter (abbreviated μS/cm, it is the unit label for electric flow). The conductivity of each waterbody varies depending on the surrounding geology (bedrock and soils) and presence of naturally occurring electrolytes. For example, where the Hudson River meets the Long Island Sound the conductivity will be high, because of the addition of salt from incoming seawater. It is important to note that conductivity levels stay consistent over time for each waterbody, so for example the Lower Hudson River should always have a high conductivity reading. Drastic changes in conductivity readings over time indicate a problem with pollution.
Water naturally conducts electrical current, but when inorganic dissolved solids (salts, metals, etc) are in the water, current is transferred more efficiently. The transfer of electrical current is possible because inorganic dissolved solids possess either a positive or negative charge. The presence of naturally occurring electrolytes (such as salts) and the geology (bedrock and soil) of the area determine how well water conducts an electrical current. These inorganic dissolved solids are essential for aquatic life, however drastic changes in levels will make the aquatic system unbalanced. Often an overload of inorganic dissolved solids leads to low dissolved oxygen and poor tasting water
Presence of the following substances will increase the conductivity (water will have a higher reading):
- Chlorides
- Nitrates
- Sulfates
- Phosphates
- Sodium
- Magnesium
- Calcium
- Iron
- Aluminum
Presence of the following substances will decrease the conductivity (do not pass electricity very well, so will lower reading):
- Oil
- Alcohol
- Sugar
Testing conductivity levels is important, because it establishes a baseline value of conductivity for the waterbody. Data samples collected over a period of time should maintain conductivity values that are similar. If there are significant increases or decreases in conductivity levels at any time, it is very probable there has been some unnatural impact to the water body.
Possible pollution sources and their effect on conductivity values:
- Failing septic systems (up)
- Heavy metals (up)
- Road salts (up)
- Agricultural runoff (up)
- Fertilizer runoff (up)
- Industrial discharge (depending on discharge content the conductivity could be up or down)
- Oil spill (down)