What is it?
Electrical conductivity is a measure of a water’s ability to conduct electricity, and therefore a measure of the water’s ionic activity and content. The higher the concentration of ionic (dissolved) constituents, the higher the conductivity. Conductivity of the same water changes substantially as its temperature changes. This can have a confounding effect on attempts to compare this feature across different waters, or seasonal changes in this parameter for a particular body of water. The use of specific conductance [SC; units of microSiemens per centimeter (µS·cm-1) or miliSiemens per centimeter (mS·cm-1], the conductivity normalized to temperature of 25 ºC, eliminates this complication and allows valuable comparisons to be made.
How is it measured?
By definition, specific conductivity is the reciprocal of the specific resistance of a solution measured between two electrodes 1 cm2 in area and 1 cm apart. Conductivity is thus measured by placing two electrodes (with opposite electrical charge) in the water. For a known electrical current, the voltage drop across the electrodes reveals the water’s resistance. Since the resistance of aqueous solution changes with temperature (resistance drops with increasing temperature), the resistance is corrected to the resistance of the solution at 25 ºC
Why is it important?
SC is generally found to be a good measure of the concentration of total dissolved solids (TDS) and salinity. Elements whose ionic forms contribute the most to these measures include: calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), bicarbonate (HCO3-), sulfate (SO42-), and chloride (Cl-). Values of SC can differ greatly from system to system because the composition of inflowing tributaries reflects the geology of their watersheds. For example, the SC values of the Eastern Finger Lakes of New York, that are located in limestone-rich watersheds, are much higher than observed in the Catskill lakes and reservoirs of New York, where gneiss dominates. There are of course anthropogenic (human impact on nature) sources of these materials such as road salt, non-point source pollution (for example, agricultural run-off) and industrial inputs.
SC serves as a valuable tracer of water movement in aquatic systems. This is based on the nearly conservative (unreactive) behavior of the ionic constituents, and therefore SC, in most water bodies. Limnologists and environmental engineers often take advantage of this in conducting material budgets and in testing mathematical models (represents the behavior of a constituent according to equations) for lakes and reservoir. As a tracer, SC can also show the presence of density currents (see details) in a lake or reservoir.
What to look for in our systems?
Commonly, SC trends often track runoff conditions. Decreased runoff, commonly observed in summer, provides decreased dilution for ionic inputs in tributaries, usually manifested as increases in SC. Major runoff events may impart abrupt decreases in SC in Onondaga Creek and subsequently in the lake.