is it important?
Electrical conductivity (EC)
estimates the amount of total dissolved salts (TDS),
or the total amount of dissolved ions in the water. EC is controlled
- geology (rock
types) - The rock composition determines the chemistry of the
watershed soil and ultimately the lake. For example, limestone
leads to higher EC because of the dissolution of carbonate minerals
in the basin.
- The size
of the watershed (lake basin) relative to the area of
the lake (Aw : Ao ratio) - A bigger watershed
to lake surface area means relatively more water draining into
the lake because of a bigger catchment area, and more contact
with soil before reaching the lake.
- "other" sources
of ions to lakes - There are a number of sources of pollutants
which may be signaled by increased EC:
sewage treatment plants (point source pollutants;
septic systems and drainfield on-site wastewater treatment
and disposal systems (nonpoint
source pollutants; see: links )
runoff from roads (especially road salt; see: links).
This source has a particularly episodic nature with pulsed
inputs when it rains or during more prolonged snowmelt periods.
It may "shock" organisms with intermittent extreme
concentrations of pollutants which seem low when averaged
over a week or month (see: Measures of Variability Lesson and
runoff of water draining agricultural fields typically
has extremely high levels of dissolved salts (another major nonpoint
source of pollutants; see: links). Although a minor
fraction of the total
dissolved solids, nutrients (ammonium-nitrogen, nitrate-nitrogen
and phosphate from fertilizers) and pesticides (insecticides
and herbicides mostly) typically have significant negative
impacts on streams and lakes receiving agricultural drainage
water. If soils are also washed into receiving waters, the organic matter
in the soil is decomposed by natural aquatic bacteria which
can severely deplete dissolved
oxygen concentrations (see above).
inputs of ions are typically relatively minor except
in ocean coastal zones where ocean water increases the salt
load ( "salinity" ) of dry aerosols and wet (precipitation)
deposition. This oceanic effect can extend inland about 50-100
kilometers and be predicted with reasonable accuracy.
in the world are microSiemens per centimeter (µS/cm)?
- evaporation of
water from the surface of a lake concentrates the dissolved solids
in the remaining water - and so it has a higher EC. This is a
very noticeable effect in reservoirs in the southwestern US (the
major type of lake in arid climates), and is, of course, the
reason why the Great Salt Lake in Utah and Mono Lake, California
and Pyramid Lake, Nevada are so salty.
metabolism in the hypolimnion when lakes are thermally
stratified for long periods of time (in Minnesota this might
be May - November depending on the basin shape, lake depth
and weather). During this period, there is a steady "rain" of detritus (dead
stuff, mostly algae and
washed in particulate material from the watershed) down to
the bottom. This material is decomposed by bacteria in the water
column and after it reaches the sediments. Their metabolism releases
the potential energy stored in the chemical bonds of the organic
carbon compounds, consumes oxygen in
oxidizing these compounds, and releases carbon
dioxide (CO2) after the energy has been liberated
(burned). This CO2 rapidly dissolves in water to
form carbonic acid (H2CO3), bicarbonate ions
(HCO3- ) and carbonate
ions (CO3-) the relative amounts
depending on the pH of the water. This newly created acid gradually
decreases the pH of the water and the "new" ions
increase the TDS, and therefore the EC, of the hypolimnion.
Essentially, they are "eating" organic matter much
as we do and releasing CO2. We oxidize organic carbon
using O2 that we breathe out of the air as an oxidant.
We use the energy to drive our metabolism and exhale the oxidized
carbon as CO2. The oxygen is simultaneously chemically
reduced and exhaled as water vapor (H2O; the oxidation
state of gaseous molecular oxygen is reduced from 0 to -2 in
the process). Other higher aquatic organisms
that have aerobic metabolisms, such as zooplankton,
insects and fish also consume oxygen dissolved in the water
while releasing carbon
dioxide as they metabolize organic carbon (food items).
are the units for electrical conductivity (EC). The sensor simply
consists of two metal electrodes that are exactly 1.0 cm apart
and protrude into the water. A constant voltage (V) is applied
across the electrodes. An electrical current (I) flows through
the water due to this voltage and is proportional to the concentration
of dissolved ions in the water - the more ions, the more conductive
the water resulting in a higher electrical current which is measured
electronically. Distilled or deionized water has very few dissolved
ions and so there is almost no current flow across the gap (low
EC). As an aside, fisheries biologists who electroshock know that
if the water is too soft (low EC) it is difficult to electroshock
to stun fish for monitoring their abundance and distribution.
until about the late 1970's the units of EC were micromhos per
(µmhos/cm) after which they were changed to microSiemens/cm (1 µS/cm
= 1 µmho/cm). You will find both sets of units in the published
scientific literature although their numerical values are identical.
Interestingly, the unit "mhos" derives from the standard
name for electrical resistance reflecting the inverse relationship
between resistance and conductivity - the higher the resistance
of the water, the lower its conductivity. This also follows from Ohms
Law, V = I x R where R is the resistance of the centimeter
of water. Since the electrical current flow (I) increases with
increasing temperature, the EC values are automatically corrected
to a standard value of 25°C and the values are then technically
referred to as specific electrical conductivity.
conductivity data are temperature compensated to 25°C (usually
called specific EC). We do this because the ability of the water
to conduct a current is very temperature dependent. We reference
all EC readings to 25°C to eliminate temperature differences
associated with seasons and depth. Therefore EC 25°C data
reflect the dissolved ion content
of the water (also routinely called the TDS or total dissolved
much salt is there in lakewater?
below was developed to give you an idea of how much salt (dissolved
solids and ions) is present in some of the WOW lakes and to compare
them to a range of other aquatic systems. TDS, in milligrams per
liter (mg/L) stands for total dissolved salts or solids and is
the weight of material left behind were you to filter a liter of
water to remove all the suspended particulates and then evaporate
the water from the container (usually done in a drying oven in
the lab unless you work on Lake Mead in southern Nevada where you
can just set it outside for a few minutes in the summer). Each
of the piles represents the amount of salt present in a liter of
water. We used sodium bicarbonate (baking soda) for the lakes and
sodium chloride (table salt) for the ocean.
AND TOTAL DISSOLVED SALT VALUES
is a softwater, acid
rain sensitive lake in northeastern Minnesota; Superior
and Tahoe are ultra-oligotrophic lakes;
Ice and Independence are WOW lakes; Mead is an unproductive reservoir
(the largest in the U.S.) but has a high TDS due to the salt content
of the Colorado River which provides >98% of its water; the Atlantic
Ocean overlies the lost Kingdom of Atlantis and possibly Jimmy Hoffa;
the Great Salt Lake is an enormous hypersaline lake near Salt Lake
City, Utah - it is the relict of what was once a huge inland freshwater
sea that dried up, thereby concentrating the remaining salts after
the water evaporated.
J.P. 1991. A citizen's guide to understanding and monitoring lakes
and streams. Publ. #94-149. Washington State Dept. of Ecology, Publications
Office, Olympia, WA, USA (360) 407-7472.Moore,
NALMS management guide for lakes and reservoirs. North
American Lake Management Society, P.O. Box 5443, Madison, WI, 53705-5443,