Conductivity
What is conductivity?
Conductivity, or electrical conductivity (EC), is a measurement of the concentration of total
dissolved ions in solution.
What affects conductivity?
The higher the concentration of total dissolved ions and salts in solution, the greater the
conductivity. Dissolved ions and salts come from substances which dissociate into ions in
solution, including electrolytes (“salts”) and inorganic compounds. The concentration of free-
moving ions is what makes a solution a stronger or weaker conductor. While electrolytes and
inorganic compounds are good conductors, organic compounds (oils, phenols, sugars, and
alcohols) are not. Distilled water is purified of ions, so it is a poor conductor. Temperature
affects solution conductivity, as well. The higher temperature, the better the conductor.
Why measure conductivity?
We measure conductivity levels in order to determine the solution’s capacity for electrical
conduction by measuring the concentration (in microsiemens per centimeter us/cm) of total
dissolved ions in solution. In ecosystem studies, we measure the conductivity levels of solutions
within ecosystems in order to determine their health. First, we determine the normal
conductivity level of a solution. For example, a scientist picks a body of water such as a stream,
and monitors its conductivity level, throughout a year. As long as it has been a typical year (no
chance events which cause irregular chemical activity in the water), the scientist is able to
accumulate “baseline” water conductivity data. In future years, if scientists notice an irregular
fluctuation in water conductivity, they may wonder what environmental factor has caused the
change.
Which environmental factors affect conductivity?
Conductivity levels change due to: the geology and size of the waterway, amount of entering
wastewater, and bacterial metabolism. There are a few other factors, but these are the main
sources of dissolved ions in wa ter.
Geology: As water flows, it passes over bedrock and soils. Rocks and soils of different chemical
composition provide naturally occurring ions such as chloride, iron and aluminum. So for
example, if water flows over a bedrock of limestone, it tends to have a higher conductivity level
because of the dissolved carbonate minerals leaching in from the bedrock.
Size: The smaller the body of water, the larger the effect of conductivity-influencing factors.
For example, take a small stream and a large stream, side by side. On a sunny day, the sun exerts
the same amount of heat and causes the same amount of evaporation from both streams. Yet, as
streams heat up, the smaller stream’s relative concentration of ions, and water which evaporates,
becomes greater than the larger stream.
