Specific conductance

A measure of the ability of a water to conduct an electrical current. Specific conductance is related to the type and concentration of ions in solution and can be used for approximating the dissolved solids concentration in water. In general, for the san antonio river basin, conductivity *.6 approximates tds. People monitoring water quality can measure electrical conductivity quickly in the field and estimate tds without doing any lab tests at all. See tds.

Source: Corrosion-Doctors.org

Wikipedia:  https://en.wikipedia.org/wiki/Specific_conductance