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Conductivity (Total Dissolved Solids)
an excerpt from our April 2004 Newsletter
What is Conductivity?
Conductivity is the ability to conduct electricity. Water conducts electricity because it contains dissolved solids that carry electrical charges. For example, chloride, nitrate, and sulfate carry negative charges, while sodium, magnesium, and calcium carry positive charges. These dissolved solids affect the water’s ability to conduct electricity. Therefore, measuring the conductivity of the water indirectly indicates the amount of Total Dissolved Solids (TDS) in the water.
Why is it important?
Most streams have a fairly constant range of conductivity under normal circumstances. Therefore, significant changes in conductivity can be an indicator that a discharge or some other source of pollution has entered the water. The composition of the water can be critical for aquatic organisms as well, as many critters have very specific ranges that they can tolerate.
How do we measure Conductivity?
Stream Team uses a conductivity meter that applies voltage between two electrodes. The drop in voltage between the two electrodes is used to measure the resistance in the water, which is converted into conductivity.What factors affect Conductivity?
Conductivity can be affected by many factors. Examples include:
•The addition of fresh water (rain) lowers conductivity because rainwater has low conductivity and the increase in water levels dilutes mineral concentrations. Conversely, during low flow conditions (summer and fall) the dissolved solids are more concentrated and therefore conductivity levels are higher.
•Conductivity is affected by temperature: the warmer the water, the higher the conductivity.
•Soil and rocks release dissolved solids into the waters that flow through or over them. Therefore, the geology of a certain area will determine the conductivity.
•In coastal streams or estuaries, salt water often mixes with fresh water. The addition of salt water greatly increases conductivity.
What are expected Conductivity levels?
According to the California State Water Resources Control Board, typical conductivity ranges are:Distilled Water: 0.5 to 3.0 mS Melted Snow: 2 to 42 mS
Drinking Water: 30 to 1500 mS Freshwater Streams: 100 to 2000 mSStream Team Conductivity results
Here in the Santa Barbara/Ventura area, our coastal mountains are formed from easily eroded marine sediments. Therefore, conductivity in our local streams is usually above 1000 mS, and can be as high as 3000 or even 4000 mS. The highest result we have ever seen was 8280 mS, this was at San Jose Creek in Goleta (site SJ1) in January 2004.
One very interesting thing to note is that heavy rain seems to have very different effects in Ventura and Goleta. In Goleta, heavy rain greatly lowers conductivity, whereas in Ventura, heavy rain seems to increase conductivity levels. This is very interesting, and will require more investigation!