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POW!! Lightning from a passing summer storm strikes the water with a bright flash. Within milliseconds the electricity from the bolt spreads out over the surface of the water in all directions, and its charge is dispersed and weakens almost as quickly as it struck. Luckily, minutes earlier, the lifeguard blew her whistle and evacuated the water of all the swimmers.
Water, regardless of whether it is fresh or saline, serves as one of the best electrical conductors on the planet. Conductivity is the measure of how well a material transports an electric charge. The components of water, known as constituents, and their amount, will determine how good of a conductor water will be. Electrolytes, such as calcium, sulfates, and bicarbonates will all change the speed at which electricity can move through the water. Salt water, which contains high concentrations of sodium (Na+) and chloride (Cl-), has a substantially greater ability to conduct electricity than freshwater, and thus, a much higher conductivity. Conductivity is also correlated with water temperature. As the water temperature increases by 1șC, conductivity increases by approximately 1.9%.
Conductivity is an important abiotic parameter, in addition to water temperature, turbidity or clarity, and current speed and direction, to measure in both fresh and salt water environments. In freshwater systems, low conductivity values are characteristic of high-quality, oligotrophic (low nutrients) waters, whereas elevated values are characteristic of eutrophic (high nutrients) waters. These high values are often indicative of non-point source pollution, including fertilizer run-off, industrial discharges, road salt, and faulty septic systems. In saltwater systems, we can expect to see higher conductivity due to the increased amounts of the charged ions: sodium (a cation, meaning a positively charged ion) and chloride (an anion, a negatively charged ion). In this type of environment, conductivity is one way to determine the salinity of the water, or the amount of salt it contains. Salt content is a very important determinant of the flora and fauna that live in a habitat, as these organisms must be physiologically adapted to handle the salty environment.
Measured in the SI units of siemens per meter (S/m), conductivity is also often reported in microsiemens (”S) or millisiemens (mS) per centimeter (”S OR mS/cm). The SI unit, the siemen (S), is the inverse of the ohm, or the measure of electrical resistance. Because the siemen is the inverse of the ohm, it is also often reported in mhos, or ohms backwards (1 ”mho = 1 ”S = 1/ohm). At standard temperature of 25șC, distilled water has conductivity from 0.5 to 2 ”mhos/cm, drinking water is generally between 50 to 1500 ”mhos/cm, and full strength seawater (35 ppt) has a conductivity of 53,000 ”mhos/cm. Comparatively, air has a conductivity of approximately 120 ”mhos/cm.
Conductivity is measured via a conductivity meter, typically handheld, which can be purchased or made. Available in various shapes, sizes, and accuracy, the meter's probe typically contains two electrodes. By passing a current between to the two electrodes and measuring the time it takes to travel between them, the conductivity is determined. Once this measure is taken, it can be used, along with water temperature and pressure, to determine the water's salinity. To calculate salinity by hand, or Excel (recommended), you can view the algorithm in html code by selecting "Source" from the View menu at the top of the page (see below).
Ocean Observing Systems
A recent hot-topic among scientists, educators, policy-makers, and the general public, including fishermen, surfers, and beach-goers, especially since the horrific tsunami last December, is the need for a global network of ocean observing systems (OOS). These systems, which link land-based meteorological stations, sensor-packed buoys, research vessels, undersea monitors, and satellites all measure and report important biotic (living) and abiotic (nonliving) data to computers and the internet, where it is typically available in real-time or archives.
With literally millions of individual systems all over the planet, efforts are currently underway to unite them and form one integrated ocean observing system (IOOS) in the United States, and then also a global ocean observing system, GOOS. The goal of these integrated systems is to create a network capable of describing the current state of the oceans, bays, rivers, and streams, including their living resources, forecasting future conditions and hazards, and evaluating and forecasting global warming.
Most parameters that can be measured are observed by buoys world wide. Common parameters measured include:
Other measured parameters include:
Data Activity & Discussion
Using five coastal observing stations around the United States, let's explore conductivity and investigate what affects it.
First, locate the approximate location of the five observing stations on a map of the United States or on a globe:
Next, click on each of the following observing sites and, using the table below, record the data for the first 4 columns -- data and time of latest observation, conductivity, and water temperature (click here for a printable worksheet). For some stations, this information may be tricky to find (small print), but it is available! *BE SURE ALL UNITS MATCH!
|Hudson River, NY|
|Bodega Bay, CA|
|CORIE Elliott Point|
|York River, VA|
1 S = 1,000 mS (milliSiemens) = 1,000,000 µS (microSiemens)
1 m = 100 cm
Once you have recorded the data, convert your water temperature and conductivity data to salinity using this calculator. Pressure at all stations is 10 decibars, or 100 kPa. The calculator calls for the pressure to be entered in 10kPa, so you will enter 10. Enter the calculated salinity values in the last column.
Now, using the Salinity Calculator, re-enter the data from the CORIE Red26 buoy, except this time, lower the Temperature measurement (make the water colder).
Now, increase the Pressure measurement until there is a change in salinity.
For related information and activities, check out the Bridge's Chemistry resources.
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