The amount of salts carried in streams increases during storm events, as sediments and salts are washed from roads and parking lots. How much salt is a problem?
Snowmelt causes increased solids and salts in Duluth streams. Jump ahead to learn more and to view some real data.
Electrical Conductivity (EC25)
Water is called the universal
solvent because of its ability to dissolve so many substances.
As water moves across the rocks and soils in a watershed, and
then down the stream channel it picks up a variety of dissolved
and particulate materials. The dissolved, or soluble fraction
of the water's total solids load is referred
to as total dissolved solids (or salts),
abbreviated as TDS. To measure it, a known
volume of a water sample is sucked through a fine filter that
retains virtually all of the particulate matter. The remaining
filtrate that has passed through the filter is then heated to
evaporate off the water, leaving behind a residue of dissolved
The TDS is the weight of this material per unit volume (usually
given as milligrams per liter). Although simple, the analysis
requires an expensive drying oven, a very sensitive and expensive
weighing scale (analytical balance) and a lot of space and
time. However, the water quality parameter electrical
conductivity (EC) provides a simple, inexpensive
measure of TDS that can be determined precisely and accurately
in the field using automated electronic sensors.
You can estimate the total dissolved salt concentration of
a water sample by multiplying its temperature normalized electrical
conductivity by a factor of between 0.5 and 1.0 for natural
waters. The value of this factor depends upon the type of
dissolved solids. A widely accepted value to use for a ballpark
guestimate is 0.67.
TDS (in mg/L or ppm) = 0.67 x EC25 (in uS/cm or micromhos/cm)
Is fairly accurate for most natural waters.
Conductivity is a measure of water's ability
to conduct an electric current and is directly related to
the total dissolved salt content of the water. This is because
the salts dissolve into positive and negative ions that can
conduct an electrical current proportionately to their concentration.
It is called EC, for electrical conductivity, and is reported
in micromhos per centimeter (umhos/cm) which has been recently
renamed as uS/cm (microSiemens per centimeter). EC is temperature
sensitive and increases with increasing temperature. Most
modern probes automatically correct for temperature, standardize
all readings to 25°C and then refer to the data as specific
EC which is labeled EC25 throughout this website.
Most of the TDS of natural waters is comprised
of inorganic compounds - mineral as opposed to the organic
compounds derived from organisms. Although there are at least
traces of many elements, the great majority of the TDS load
is from four negative ions (bicarbonate, carbonate, chloride,
sulfate) and four positive ions (calcium, magnesium, sodium
and potassium). Why is it important?
Aquatic organisms require a relatively
constant concentration of the major dissolved ions in the water,
much as we require relatively constant concentration of certain
dissolved ions in our blood and other bodily fluids. Levels
too high or too low may limit survival, growth or reproduction.
EC25 is also one of a number of general indicators of the
overall “health” of a stream and variations from
its normal range may indicate sources of pollution such as:
Water mass tracer
- wastewater from sewage treatment
plants and industrial discharges. These are point sources
of pollutants. Domestic sewage is enriched by human wastes
in addition to food, laundry and other materials that find
their way down household drains. Depending on the municipality,
a variety of industrial wastewaters that have been pre-treated
to varying degrees, are then mixed with the domestic wastewater
prior to treatment. However, treatment at this stage usually
has little effect on TDS since the primary goals are to
break down organic matter, remove particulate materials,
remove nutrients (phosphorus and nitrogen) and disinfection.
Some industrial wastes are extremely salty, to the point
of being called “brines”, and require expensive
pre-treatment to prevent the high TDS levels from harming
the microorganisms that are the main sewage treatment process
(see also WLSSD).
- wastewater from on-site wastewater
treatment and dispersal systems (septic systems and drainfields)
- urban runoff from roads and
construction sites (especially road salt; see winter
storm graph from Chester Creek, November 2002). 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. Road de-icing salts can be quite varied
but typically are mostly sodium chloride (NaCl) and magnesium
chloride (MgCl2). Duluth road salts
are mostly NaCl. Application rates of salt on Duluth city
streets are available here.
- agricultural runoff of water
draining agricultural fields typically has extremely high
levels of dissolved salts (another major nonpoint
source of pollutants). Although nutrients (ammonium-nitrogen,
nitrate-nitrogen and phosphate from fertilizers) and pesticides
(insecticides and herbicides mostly) comprise a minor fraction
of the total dissolved salts, their concentrations are greatly
elevated relative to natural ecosystems and typically cause
significant negative impacts on streams and lakes receiving
agricultural drainage water. High EC25 values are also often
associated with increased soil erosion. Soils washed into
receiving waters also add oxygen depleting organic matter
in addition to nutrients and pesticides.
- acid mine drainage - drainage
from operating and abandoned mine sites can contribute iron,
sulfate, copper, nickel, cadmium, arsenic, and other compounds
if minerals containing these constituents are present and
are exposed to air and water. The high TDS of mine drainage
in coal and metal mines in particular is well known to cause
serious ecological damage in some parts of the U.S.
Acid mine drainage, often referred to
as AMD, results when the mineral pyrite (FeS2)
is exposed to air and water, resulting in the formation
of sulfuric acid and iron hydroxide. The combination of
high acidity, high TDS (sulfate usually) and iron coatings
can be devastating to stream communities. Pyrite is usually
present in coal-mining and many metal mining areas. AMD
becomes a problem when the overlying rocks are exposed and
removed during surface mining to get to the coal. Minnesota's
Iron Range iron mining area has had little impact from AMD
except for mineralized (sulfide-bearing rock) Duluth Complex
waste rock piles at the Dunka Pit iron mine near Babbitt,
MN which have required a variety of treatment methods to
protect downstream water resources.
- atmospheric inputs of ions
are typically small except near seashores where ocean water
increases the salt load ( "salinity" ) of precipitation.
Sea spray can also be important and this oceanic effect
can extend inland about 50-100 kilometers and be predicted
with reasonable accuracy.
EC25 is also very useful for simply identifying the various
sources of water responsible for flow at a particular site.
For instance, groundwater typically has a higher TDS than surface
water and so a sudden increase in EC25 along a streamcourse
may indicate the presence of springs or seeps. In coastal marine
estuaries, EC25 is a good indicator of tidal effects where saltwater
may “intrude” far upstream on a regular basis. During
floods, the opposite effect may be seen where low EC25 freshwater
pushes farther offshore, actually “floating” on
top of the much denser saltwater.
Expected Impact of Pollution
Fish and other aquatic organisms
The major direct concern associated with high dissolved salt
concentrations relates to direct effects of increased salinity
on the health of aquatic organisms. A vast scientific literature
exists on this subject and the table below (coming soon) summarizes
some of these effects.
Also remember that electrical conductivity and TDS may indicate
the potential for other, more toxic pollutants to be present
that are extremely expensive to monitor.
Drinking water and irrigation issues
Although not a major issue in Duluth where our drinking water
comes from Lake Superior that has relatively low TDS (see
figure below), high dissolved solids is a major problem in
parts of the western and southwestern U.S. For instance the
Colorado River picks up salts as it flows from pristine mountain
watersheds through arid lands where evaporation acts to concentrate
salts, through erodible desert soils high in carbonate and
sulfate minerals, and then through vast agricultural areas
where irrigation acts to further leach salts from soils and
added fertilizers. By the time the river flows into Mexico
its TDS is high enough to create an unpleasant taste and be
unfit for irrigating many plants.
High levels can also create excessive deposits
in plumbing fixtures and water pipes and has also been reported
to cause laxative effects (usually where sulfate is particularly
high). Since, by itself, high levels of TDS in drinking water
do not represent a major human health risk, there is no Primary
Drinking Water Standard. TDS is however, included in the federal
list of Secondary Drinking Water Standards
TDS ………… 500 mg/L
Effects……… hardness; deposits; colored water;
staining; salty taste
Reasons for Natural Variation:
What controls the concentration of salts and the level
of electrical conductivity?
EC25 values in streams reflect primarily a combination
of watershed sources of salts and the hydrology of the system.
The underlying geology (rock types) of the basin determines
the chemistry of the watershed soil and ultimately its streams
For example, limestone leads to higher EC25
because of the dissolution of calcium and carbonate minerals
as water flows over them. Other rocks, such as granite and
quartz are more resistant and watersheds where these rocks
dominate will have lower TDS and EC25 values unless other
factors are involved. Adding to these natural sources of salts
are the various pollutant loads described above.
The hydrology, or flow regime, of the stream
controls the amount of water in the system as well as the
delivery of soluble compounds to the stream. So when it rains
hard, a lot of dissolved (as well as particulate) solids are
washed into the stream. But the actual concentration, the
TDS (as indicated by the EC25 value), may decrease because
of dilution by all that rainwater. The same situation can
occur in the springtime during snowmelt runoff. The first
flush of runoff usually will produce high TDS and EC25 values
because of all the accumulated road salt, sand and automobile
“grime” from the winter. However, as the snowpack
melts, flow increases and the large amount of relatively “clean”
meltwater that is released acts to decrease the TDS.
graph summarizes some of the results from a snowmelt
runoff study conducted by MPCA-Duluth staff in
1999 for Kingsbury Creek, Amity, Keene and Miller
Creeks (MPCA 2000). Their first sample on March
25 was collected when the spring runoff had just
begun and flow was still relatively low (9 cfs).
Both EC25 and TDS were at their highest levels
in this study due to road salt loads that washed
into the stream with the first flush of snowmelt.
Four days later these levels had decreased sharply
due to dilution when streamflow jumped from 9
to over 90 cfs due to warm weather. After another
4 days, flows had dropped, but were still high
and so EC25 and TDS remained relatively low. A
final set of samples was collected in late September
during the very low base-flow period and EC25
and TDS were higher since groundwater seepage
comprised most of the flow at this time. Similar
patterns were observed at the other streams. Additional
sampling at Miller Creek by the South St. Louis
County Soil and Water Conservation District (SSLSWCD)
prior to the peak spring runoff showed much higher
salt levels. This clearly demonstrated that the
large load of urban pollutants that can accumulates
over the winter when the stream is mostly frozen,
can be suddenly released and potentially ”shock”
fish and other aquatic organisms.
The SSLSWCD data for 6 weeks
prior to the first MPCA sampling at Miller Creek in March
1999 recorded 10 out of 11 samples to have chloride (salt)
concentrations that EXCEEDED the federal and state water quality
standard of 230 mg/L for protection of aquatic life.
Sudden inputs of concentrated pollutant, especially during
low flow periods can cause significant negative impacts to
See an example of how EC25 responds to an early
winter snowstorm in Duluth streams that still had their
automated samplers in place!
Notice that the sensors picked up the big spike in conductivity
with a very small blip in flow (stream stage height) and
almost no change in turbidity or temperature. Adding all
these facts together points to roadsalt washing into the
Go here to see what happened on November 13, 2002 when Duluth
had its second icy snowstorm.
You can create your own animated graphs yourself with our Dataviewer applets.
How much salt is there in stream and lakewater?
Pure water would theoretically
have an EC25 value of less than 0.01 µS/cm. In practice,
expensive distillers and de-ionizing systems used in water quality
analytical laboratories, such as at NRRI, produce water as low
as about 0.05 – 0.10 µS/cm. Rain water generally
has considerably higher values because it has accumulated various
particles of dust, soil and other airborne aerosols that at
least partially dissolve before deposition.
The image below was developed to give you an
idea of how much salt (dissolved solids and ions) is present
in the DuluthStreams streams and in Minnesota's Water-on-the-Web
(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
(values are approximated annual means)
|Great Salt Lake
|| ~ 330,000
Rivers & Streams*
|St. Louis River at Highway 23
STORET Database means
|St. Louis River at Duluth Lift Bridge
|Tischer Creek: mean (range)
(96 - 2689)
|Chester Creek: mean (range)
(2000) Yr 1999
DuluthStreams Yr 2002
|Split Rock River at Lake Superior
Air Quality Division 1988
|Blind Temperance River at Lake Superior
Air Quality Division 1988-1990
|Cloquet River at St. Louis River
STORET Database means
|Upper Mississippi River at Minneapolis, MN
Region 5 summary: mean (range)
|Lower Mississippi River near New Orelans,
|Minnesota R. near Mississippi R. **
STORET Database means)
* TDS and EC25 are highly variable and these values
provide only a general range
** Note-the Minnesota River watershed is dominated by
agriculture and has been identified as degraded by nonpoint
source pollution from this landuse
Divide is a softwater, acid rain sensitive lake in northeastern
Lake Superior and Lake Tahoe are
ultra-oligotrophic (for more information about how lakes
behave go to http://WaterOntheWeb.org
Ice and Independence are Water-on-the-Web (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.
The St. Louis River, Tischer
Creek, Chester Creek, Kingsbury Creek, Amity Creek, Miller
Creek and Cloquet R (near its confluence with the St. Louis
River) are all Duluth area streams. The Split Rock and Blind
Temperance River daa were included because they were considered
to be potentially "acid rain-sensitive" streams
as per Divide Lake by the Minnesota Pollution Control Agency
and along with some other northshore of Lake Superior streams,
were sampled intensively from 1988-1991. They drain largely
granitic watersheds with thin, low fertility soils, and have
experienced few impacts associated with human development.
Data are presented for the Mississippi
River in its upper region near Minneapolis- St. Paul and in
its lower region in Louisiana near where it empties into the
Gulf of Mexico. Although one might expect dramatic EC25/TDS
differences due to water quality changes as the river drains
more and more land that has been modified by human development,
the changes are in fact not large. This is because of the
mixing of higher salinity waters from midwestern agricultural
land drainage (MN, IA, WI, IL, MO, AR) with lower salinity
waters from southeastern and middle Atlantic areas (IN, OH,
KY, TN). These areas also receive higher annual precipitation
that further acts to dilute agricultural drainage waters (a
good summary of Mississippi River water quality may be found
at the USGS website: http://water.usgs.gov/pubs/circ/circ1133/)
The Minnesota River is a major tributary of the Mississippi
River immediately downstream of the Minneapolis-St. Paul metro
area. Its watershed is dominated by agriculture and the river
has been identified as degraded by nonpoint source pollution
from this landuse. It is the focus of a major restoration
effort by local, state and federal agencies.
MPCA. 2000. Duluth Metropolitan Area Streams Snowmelt Runoff
Study (J. Anderson, T. Estabrooks and J. McDonnell, March
2000, Duluth Regional Office). Minnesota Pollution Control
Agency, St. Paul, MN 55155.
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.
North American Lake Management Society, P.O. Box 5443, Madison,
USGS. 1995. Contaminants in the Mississippi River, 1987-92.
Edited by Robert H. Meade. U.S. GEOLOGICAL SURVEY CIRCULAR
1133. Reston, Virginia, 1995