Credits
Bob Fedeler developed this lesson.

Goals
Students will develop an understanding of the concept of conductivity in lakes and its relationship to productivity in aquatic systems.

Introduction
Conductivity is a measure of water’s ability to conduct electrical current. Measurements of conductivity provide a general indication of water quality. The geology of a lake’s 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.

Students can meet the goals for this lesson by completing a directed study or a student inquiry lesson.

The directed 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.

The directed study lesson is found in the student section of WOW under the title: "Studying Conductivity."

The student 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 presentation.

The student inquiry lesson is found in the student section of WOW under the title: "Investigating Conductivity."

Outcomes
Students will:

1. Identify the most likely variables that influence conductivity measurements within a lake.
2. Graph conductivity profile measurements within a lake.
3. Describe and compare conductivity measurements from different depths within a lake.
4. Explain potential impacts of contaminants on conductivity measurements.
5. Explain the relationship of conductivity to lake productivity.

Keywords
Conductivity, solutes, ions, solutions, mixtures

WOW Curriculum Links
Thermal Stratification

Materials/Resources/Software
Each student group will need:

• a hot water bath for samples
• Conductivity of Water Samples Worksheet 1 and Conductivity of Water Samples Worksheet 2 for directed study students
• lab journal for student inquiry students
• conductivity pen (or other conductivity meter)
• five 50 ml beakers or small clear glasses
• a squirt bottle filled with distilled water
• a thermometer or temperature probe

Time Required
2 class periods

Procedure

Part I - Conductivity in the Laboratory

Knowledge Base

Directed Study
Discuss 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?

Student Inquiry
Ask students 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.

Experimental Design

Directed Study
Divide students 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.

When students 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 each test.

Student Inquiry
Students need to analyze the conductivity of tap water, tap water with baking soda, tap water with table salt, tap water with granite chips, and tap water with nitrate-rich fertilizer. They need to describe how they will set up a laboratory experiment to test these samples. They also need to record explanations for their predicted results.

Data Collection

Directed Study
Each group 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).

Student Inquiry
Students 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.

Data Management and Analysis

Directed Study
Students should record their results on Worksheet 1 and Worksheet 2.

Student groups should present their results to the class. Create a class table of results for each sample. Then discuss the following questions with the students:

• 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?

Student Inquiry
Students 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?

Interpretation of Results

Directed Study
Discuss the students' results and how these observations might relate to conductivity measurements in lakes.

Student Inquiry
How might these readings relate to conductivity readings in lakes?

Notes: 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.

Reporting Results

Directed Study
Students should turn in their worksheet after they have completed Part II of the lesson.

Student Inquiry
Students 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.

Part II - Investigating Conductivity in Lakes

Knowledge Base
The WOW 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 either 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.

Figure 1: Ice Lake Conductivity Profile

Figure 2: Lake Independence Conductivity Profile

Directed Study
Students should begin by making predictions for ranking conductivity levels within the lake based on their hands-on experiences with conductivity.

Student Inquiry
Ask students 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 they analyze WOW data? Why?

Experimental Design

Directed Study
Each group should select a day(s) from within the summer season to create a conductivity profile.

Student Inquiry
Students 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 measurements.

Data Collection

Directed Study
Have students access the conductivity data for a WOW lake from the website. (This data could also be provided through handouts copied from the website).

Student Inquiry
Students should select several days during the summer and access the conductivity data for a WOW lake from the website.

Data Management and Analysis

Directed Study
Students should complete the worksheet. When students are done, suggest they review conductivity data for the lake on other dates.

Student Inquiry
Students 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?

Interpretation of Results

Directed Study and Student Inquiry
Discuss the data collected. Ask students to reflect on the following questions:

• What might cause the conductivity readings to vary within different layers of the lake?
• What are the relationships between time of year or time of day and conductivity values?
• Is it reasonable to suspect lake pollutants based on conductivity measurements?

Notes: 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).

The temperature algorithm is :

EC (specific, i.e. at 25°C) = 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 inflows (could be higher or lower than the lake). Also note that many conductivity pens and water quality instruments are NOT temp compensated.

When conductivity readings for a lake suddenly increase, pollutants that dissociate into ions in water, such as salts, may be entering the lake from point or non-point sources.

Conclude by noting that conductivity measurements relate to the amount of total dissolved solids (TDS) 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.

Reporting Results

Directed Study
Ask each group to come to the front of the room to post their graph. (Place the graphs in chronological order for all to see.)

Student Inquiry
Students 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.

Resources

1. Behar, S., Dates, G., Byrne, J. (1996). Testing the Waters. River Watch Network: Montpelier, VT.
2. Cole, G.C. (1988). Textbook of Limnology. Waveland Press: Prospect Heights, IL.
3. Hach Company (1989). Water Analysis Handbook. Hach Company: Loveland, CO.
4. Stednick, J. D. (1991). Wildland Water Quality Sampling and Analysis. Academic Press, Inc.: New York.

Extensions

1. Compare the conductivity values for lakes to their susceptibility to acid rain.
2. Use graphs to compare conductivity with TDS and alkalinity.
3. Compare conductivity for lakes that stratify and lakes that do not stratify.
4. Explore the relationships between pH and conductivity in lakes.
5. Explore storm effects on lake conductivity, particularly in the surface waters of a lake.
6. Investigate the possibility of road salt affecting conductivity in lakes.
7. Investigate the use of the morphoedapthic index for estimating lake productivity for fish.