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Sensor Parameters
and TDS
turbidity and sediments

manual parameters
dissolved oxygen
total suspended solids
fecal coliform


Conversion Tables

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) and TDS

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 solids.

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.
The equation:
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:

  • 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.
Water mass tracer
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 and lakes.

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.

This 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 aquatic organisms.

(Note: this image is not from Duluth!)
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 streams.

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.

photo of salt piles
for different lakes

(values are approximated annual means)
Lakes EC25
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 ? ~ 330,000

Rivers & Streams*


St. Louis River at Highway 23
     EPA STORET Database means
150 140
St. Louis River at Duluth Lift Bridge
     DuluthStreams Yr 2002
98-203 ?
Tischer Creek: mean (range)
     DuluthStreams Yr 2002
(96 - 2689)
Chester Creek: mean (range)
     DuluthStreams Yr 2002
Kingsbury Creek
    MPCA (2000) Yr 1999
     DuluthStreams Yr 2002



Amity Creek
     MPCA 2000


Miller Creek
     MPCA 2000
159-887 120-510
Split Rock River at Lake Superior
     MPCA Air Quality Division 1988
52-155 48-130
Blind Temperance River at Lake Superior
     MPCA Air Quality Division 1988-1990
35-130 22-111
Cloquet River at St. Louis River
     EPA STORET Database means
100 100
Upper Mississippi River at Minneapolis, MN
     (EPA Region 5 summary: mean (range)
Lower Mississippi River near New Orelans, LA
     (www.deq.state.la.us/surveillance; 1986-1991)
~300-500 ~200-300
Minnesota R. near Mississippi R. **
     (EPA STORET Database means)
750 ?

* 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 Minnesota;

Lake Superior and Lake Tahoe are ultra-oligotrophic (for more information about how lakes behave go to http://WaterOntheWeb.org and http://LakeAccess.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/)

     figure from http://water.usgs.gov/pubs/circ/circ1133/images/fig12.jpeg

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, WI, 53705-5443,

USGS. 1995. Contaminants in the Mississippi River, 1987-92. Edited by Robert H. Meade. U.S. GEOLOGICAL SURVEY CIRCULAR 1133. Reston, Virginia, 1995
( http://water.usgs.gov/pubs/circ/circ1133/)