Electrical
conductivity (EC) estimates the amount of total dissolved salts (TDS),
or the total amount of dissolved ions in the water. EC is controlled
by:
- 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:
- Wastewater
from sewage treatment plants (point source pollutants; see:
links)
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- Wastewater
from septic systems and drainfield on-sitewastewater treatment
and disposal systems (nonpoint source pollutants; see: links )
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- Urban
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 other links) agricultural runoff of water
draining agricultural fields typically has extremely
high levels of dissolved salts (another major nonpoint source
of pollutants; see: links).
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- 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).
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- Atmospheric
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.
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- 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.
- Bacterial
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).
What
in the world are microSiemens per centimeter (µS/cm)?
These 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.
Up until about the late 1970's the units of EC were micromhos
per centimeter (µmhos/cm) after which they were changed to microSiemens/cm
(1µS/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 Ohm’s 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.
All Lake Access 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 salt concentration).
How much salt is there in lakewater?
The image below was developed to give you an idea of how much
salt (dissolved solids and ions) is present in some of the Lake Access
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.
Water
Body |
EC
(uS/cm) |
TDS
(mg/L) |
Divide
Lake |
10 |
4.6 |
Lake
Superior |
97 |
63 |
Lake
Tahoe |
92 |
64 |
Grindstone
Lake |
95 |
65 |
Ice
Lake |
110 |
79 |
Lake
Independence |
316 |
213 |
Lake
Mead |
850 |
640 |
Atlantic
Ocean |
43,000 |
35,000 |
Great
Salt Lake |
158,000 |
230,000 |
Dead
Sea |
? |
? |
Independence
is, of course, a Lake Access Lake; Divide is a softwater, acid-rain
sensitive lake in northeastern Minnesota; Superior and Tahoe are ultraoligotrophic
lakes; Ice and Grindstone 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.
REFERENCES
Michaud, 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, M.L. 1989. NALMS management guide for lakes and reservoirs.
NorthAmerican Lake Management Society, P.O. Box 5443, Madison, WI,
53705-5443, USA (http://www.nalms.org).