Conductivity (Total Dissolved Solids)
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for a more in-depth look at conductivity, click here (from our April 2004 newsletter)

Water is one of the most efficient solvents in the natural world and has the ability to dissolve a great many solids. Many of these solids when put into solution carry an electrical charge. For example, chloride, nitrate and sulfate carry negative charges, while sodium, magnesium and calcium have a positive charge. These dissolved substances increase water's conductivity - its ability to conduct electricity. Therefore, measuring the conductivity of water indirectly indicates the amount total dissolved solids (TDS). It's not a perfect measure because some substances, particularly organic compounds like oil, alcohol or sugar do not conduct electricity well and have low conductivity, but conductivity is a rough approximation of TDS. Each stream tends to have a relatively constant range of conductivity that, once established, can be used as a baseline for comparison with regular conductivity measurements. Significant changes in conductivity could then be an indicator that a discharge or some other source of pollution has entered a stream. Conductivity tends to decrease in the winter when heavy rainfall and runoff increase the amount of fresh water flow. With more water, mineral concentrations are more dilute. On the other hand, in late summer and fall, especially during periods of drought, the dissolved solids are more concentrated, raising conductivity. Conductivity is also affected by temperature: the warmer the water, the higher the conductivity. For this reason, conductivity is reported as conductivity at 25 degrees Celsius (25 °C). The basic unit of measurement is the siemen. Conductivity is measured in micro-siemens per centimeter (µs/cm) or milli-siemens per centimeter (ms/cm). Distilled water has a conductivity in the range of 0.5 to 3 µs/cm. The conductivity of rivers in the United States generally ranges from 50 to 1500 µs/cm. Drinking water usually has to meet a standard of 500 mg/L TDS - a conductivity of roughly 1000 µs/cm. Conductivity in Santa Barbara and Ventura streams is usually above 1000 µs/cm because of high mineral content in the easily eroded marine sediments that form our coastal mountains.

 

Observations made on Stream Team about conductivity: