developed this lesson.
will develop an understanding of the concept of conductivity
in lakes and its relationship to productivity in aquatic systems.
a measure of waters ability to conduct electrical current.
Measurements of conductivity provide a general indication of water quality.
The geology of a lakes watershed establishes the normal ranges
for conductivity in a lake. Some pollution discharges and polluted runoff
into lakes can cause changes in conductivity especially if the pollutants
include inorganic dissolved solids such as ions: bicarbonate, sulfate,
chloride, calcium, magnesium, sodium, potassium, and phosphate.
can meet the goals for this lesson by completing a directed study or
a student inquiry lesson.
study lesson guides students through analyzing conductivity of water
samples in the laboratory and in a lake. Students need to print a copy
of the student lesson.
study lesson is found in the student section of WOW under the title: "Studying Conductivity."
inquiry lesson asks students to create a presentation about conductivity
for chemical company executives. The instructor specifies the format
for the final presentation: written paper, oral report, poster, or multi-media
inquiry lesson is found in the student section of WOW under the title: "Investigating Conductivity."
- Identify the most likely variables that influence conductivity measurements
within a lake.
- Graph conductivity profile measurements within a lake.
- Describe and compare conductivity measurements from different depths
within a lake.
- Explain potential impacts of contaminants on conductivity measurements.
- Explain the relationship of conductivity to lake productivity.
group will need:
I - Conductivity in the Laboratory
students knowledge about materials that are good conductors and
poor conductors of electricity. How does conductivity relate to lakes?
How might material in lakes affect conductivity?
to develop an introduction to their presentation. They should write
a paragraph that explains how conductivity relates to lakes and how
pollutants in lakes may affect conductivity.
into pairs and supply each group with a copy of the lesson. Based on
the class discussion, ask the groups to rank predictions for different
samples of water. Students should record explanations for their rankings
in their lab journals.
are at their workstations, designate one student as the "runner/recorder"
whose responsibility it will be to get the water samples tested and
to record the results. The other student will be the "tester" and responsible
for testing each sample with the conductivity pen. Students can switch
roles after completing half of the conductivity tests. It is important
that students rinse the sample beaker with distilled water between
need to analyze the conductivity of tap water, tap water with baking
soda, tap water with table salt, tap water with granite chips, and
water with nitrate-rich fertilizer. They need to describe how they
will set up a laboratory experiment to test these samples. They also
to record explanations for their predicted results.
should select four samples they want to test at warmer temperatures.
After testing those samples at room temperature, the samples should
be placed in a hot water bath until they have increased in temperature
by 10 degrees. (Students need to rinse the thermometer each time it
is used to check the sample temperature).
should test the samples at room temperature and record the measurements.
Next, they should place the samples in a hot water bath until the samples
have increased in temperature by 10 degrees. (Be sure students rinse
the thermometer each time it is used to check the sample temperature).
Students should record the measurements.
Management and Analysis
should record their results on Worksheet
1 and Worksheet
groups should present their results to the class. Create a class table
of results for each sample. Then discuss the following questions with
- What variables
might affect the differences in student observations for each sample?
- What might
be inferred about conductivity readings in lakes?
- How does
the temperature of the sample affect its conductivity?
need to create a table or form to record their measurements. They need
to consider the variables that might have affected the results. How
can they prove their results are valid?
the students' results and how these observations might relate to conductivity
measurements in lakes.
these readings relate to conductivity readings in lakes?
Several variables may influence students' results, including: calibration
differences between conductivity
pens, researcher error, conductivity pens and/or beakers that have not
been properly cleaned, temperature of the samples, and possible uneven
distribution of materials in the solution or mixture. Lake conductivity
measurements vary with density layers and temperatures in lakes. Uneven distribution of materials
in lakes also causes conductivity measurements to fluctuate. As temperature
increases in a solution, the conductivity will also increase.
should turn in their worksheet after they have completed Part II of
use the introductory paragraph about conductivity, items recorded in
the lab journal, and WOW data analysis to prepare a final presentation.
The instructor specifies the format for the final presentation: written
paper, oral report, poster, or multi-media presentation.
II - Investigating Conductivity in Lakes
data visualization tools can help illustrate changes in conductivity
during an extended period of sampling (see Figure 1). A conductivity
profile for a single sampling period could also be demonstrated by
using the Profile Plotter
or by creating a profile in Excel (see Figure 2). You may want to display
profiles such as these for the students. This could be done either during
your initial discussions for this lesson, or as part of the discussion
and closure for the lesson.
1: Ice Lake Conductivity Profile
2: Lake Independence Conductivity Profile
should begin by making predictions for ranking conductivity levels
within the lake based on their hands-on experiences with conductivity.
to reflect on how what they learned about conductivity in a water sample
might apply to a lake setting. Do they expect similar results when
analyze WOW data? Why?
should select a day(s) from within the summer season to create a conductivity
should identify the WOW lake they will investigate. They need to describe
how they will use WOW data to resolve the charges against the chemical
company. Discuss which depths would have the highest and lowest conductivity
access the conductivity data for a WOW lake from the website. (This
data could also be provided through handouts copied from the website).
should select several days during the summer and access the conductivity
data for a WOW lake from the website.
Management and Analysis
should complete the worksheet. When students are done, suggest they
review conductivity data for the lake on other dates.
should complete a table or graph. Remind students to label axes and
include a title and legend when creating a graph. Ask them to reflect
on their results. Were the results what they expected?
Study and Student Inquiry
the data collected. Ask students to reflect on the following questions:
- What might
cause the conductivity readings to vary within different layers of the
are the relationships between time of year or time of day and conductivity
- Is it reasonable
to suspect lake pollutants based on conductivity measurements?
All WOW 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).
algorithm is :
EC (specific, i.e.
= EC(t) /[1+ 0.019*(t-25)]
Since the RUSS
EC sensors are temperature compensated we expect to see increased EC
with depth during the summer in stratified
systems due to increased respiration
in the hypolimnion
which produces bicarbonate ion. When the lake turns over and mixes uniformly,
surface water readings will then increase relative to late summer. Hypolimnion
EC would decrease due to it being diluted by epilimnetic water. In the
summer epilimnetic EC may increase due to evaporation
(this is very noticeable in the arid southwestern US) but may also be
affected by direct precipiation (usually low EC) and by groundwater
(could be higher or lower than the lake). Also note that many conductivity
pens and water quality instruments are NOT temp compensated.
readings for a lake suddenly increase, pollutants that dissociate into
ions in water, such as salts, may be entering the lake from point or
by noting that conductivity measurements relate to the amount of total dissolved solids
in a lake. TDS values can be estimated by multiplying the conductivity
values by a factor that is unique to each lake. In Minnesota the factor
ranges from 0.55 to 0.90.
group to come to the front of the room to post their graph. (Place
the graphs in chronological order for all to see.)
should use the data from the laboratory study and WOW to create a presentation
for the chemical company. It can be an oral presentation, written paper,
poster, or multi-media presentation. They should include suggestions
about how to mitigate the problem if a pollution problem was identified.
- Behar, S., Dates, G., Byrne, J. (1996). Testing the Waters. River
Watch Network: Montpelier, VT.
- Cole, G.C. (1988). Textbook of Limnology. Waveland Press: Prospect
- Hach Company (1989). Water Analysis Handbook. Hach Company: Loveland,
- Stednick, J. D. (1991). Wildland Water Quality Sampling and Analysis.
Academic Press, Inc.: New York.
- Compare the conductivity values for lakes to their susceptibility
to acid rain.
- Use graphs to compare conductivity with TDS and alkalinity.
- Compare conductivity for lakes that stratify and lakes that do not
- Explore the relationships between pH and conductivity in lakes.
- Explore storm effects on lake conductivity, particularly in the
surface waters of a lake.
- Investigate the possibility of road salt affecting conductivity
- Investigate the use of the morphoedapthic
index for estimating lake productivity for fish.