Credits
Cindy Grindy developed this lesson.

Goals
Students will encounter the complexity of identifying the cause of unusual variations in WOW conductivity data. Students will propose theories on why there was a sudden increase in conductivity in Ice Lake, Minnesota during July of 1998.

Introduction
In July of 1998, the conductivity data for all depths of Ice Lake showed surprising increases. Students need to hypothesize what might cause such increases: natural changes in the surroundings during this period, human activity, or a faulty sensor. It is interesting to note that the culverts surrounding Ice Lake were flushed by city workers in the weeks before the conductivity spiked. One possible culprit for the conductivity increase is the salt applied during winter road de-icing that accumulated in the culverts. Perhaps the salt was flushed into the lake by the flushing process and increased the conductivity. This lesson is an excellent activity to help students begin to understand the complexity of options when trying to identify the cause of sudden fluctuations in water chemistry data. Scientists and resource managers struggle every day with complex problems like this.

Outcomes
Students will:

1. Identify factors that affect conductivity in aquatic ecosystems.
2. Collect and analyze WOW conductivity data for different dates.
3. Calculate the mass of salt required to explain the increase in conductivity in Ice Lake on
   July 4, 1998.
4. Suggest possible causes of the sudden increase in conductivity, including natural changes, human activity, a faulty sensor, or problems in data transmission.
5. Propose experiments for further research to solve the mystery of the weird conductivity data.

Keywords
Conductivity, bicarbonate equilibrium, concentration, siemens, microsiemens, standard curve, Molarity, millimolarity, ions, conductivity profile, epilimnion, metalimnion, hypolimnion

WOW Curriculum Links
Water on the Web Tutorial, Conductivity, Thermal Stratification

Chemistry - solubility, electrolytes, equilibrium, standard curve, conductivity Biology - stratification in lakes, photosynthesis, respiration, bicarbonate equilibrium Math - graphing, standard curve, proportion, regression

Materials/Resources/Software
This activity assumes students have been introduced to Water on the Web, have Internet access, and are able to collect data from the Web site. Students should be able to construct and use graphs. It is helpful if the students have already completed the WOW conductivity lesson.

Each student group will need:

• internet access to WOW site
• graph paper or plotting program
• conductivity probe
• beaker, stirring rod, distilled water for each small group
• 1.0 M standard NaCl solution
• mystery solutions - 50 ml beakers of distilled water to which you have added known quantities of NaCl
• worksheets 1-4 for student completing the directed study lesson

Time Required
Four 50-minute class periods with Internet access available

Procedure

Knowledge Base

Class Period 2:
Discuss students' ideas about the relationship between conductivity and concentration of sodium chloride (salt, NaCl). Conductivity measures the ability of an aqueous solution to conduct electricity and is determined by the presence of electrolytes (sodium chloride in this experiment). As the salt concentration increases, conductivity increases. Each student team should complete Worksheet 2 and produce a graph.

Class Period 3:
When the conductivity of a lake or river changes, it indicates there has been some sort of disturbance. The disturbance might result from natural or human activities. It might be caused by physical, chemical, or biological factors. In this activity, students will use the standard curve developed in class period 2 to identify the conductivity and salt concentration of a mystery brine solution.

Discuss students' ideas about what might happen when a lake experiences a distinct change in conductivity resulting from human activity. In this experimental "lake" investigation, students find that the Teeny-Weeny Brine Shrimp Processing Facility has diverted salt water from their processing effluent into the nearby storm sewer. You will add a water sample ("effluent") with a known amount of salt to each team's "lake." Each team's job is to calculate just how much salt the Brine Shrimp facility released into their lake.

Class Period 4:
In July of 1998, the conductivity data for the all levels of Ice Lake showed a surprising increase. Researchers are attempting to explain the increase. Review the factors that affect conductivity. Remind students that the increase might be the result of natural changes or human activity, or it might be a technical problem with the sensor or data transmission. In the weeks before the conductivity increase, city workers flushed the storm sewers and culverts. Could this have something to do with the change?

Student Inquiry
Students will be playing the role of a Water Quality Specialist. They are charged with creating a report about the increased conductivity in Ice Lake. Begin by reviewing the factors that affect conductivity. Remind students that they will need to refer to their list of factors during their investigation. Some factors will require close scrutiny ; others can be eliminated if they do not relate to this incident.

Experimental Design

Directed Study
Class Period 1:
Students will establish a "typical" midsummer profile for conductivity by averaging conductivity values in Ice Lake in Grand Rapids, Minnesota. Before students create the profile, discuss their ideas about what the profile will look like.

Class Period 2:
Students should form teams of 3-4 people. Each team will create one graph showing the standard curve for sodium chloride concentration vs. conductivity.

Teams need the following supplies:

• clean, dry beaker, 150 or 200 ml
• 100 ml of distilled water
• stirring rod
• pipette or dropper to dispense single drops of the 1.0 M NaCl solution
• conductivity probe

Class Period 3:
Students should work in the same teams they did for class period 2. They will need the following supplies:

• clean, dry beaker, at least 250 ml
• beaker of 1.0 M NaCl solution
• stirring rod
• pipette or dropper to dispense single drops of the 1.0 M NaCl solution
• 200 ml of tap water
• conductivity probe
• a mystery brine solution from the effluent of the Teeny-Weeny Brine Shrimp Facility

Ask students to create an experimental "lake" in a beaker using at least 200 ml of tap water and enough NaCl solution to establish a 1.5 mM initial salt concentration in the "lake." (They may want to refer to your procedures used for the Class Period 2 activity.)

Class Period 4: Students should form teams of 3-4 people. Each team should produce one graph and complete one copy of Worksheet 4.

Student Inquiry
Students need to prepare an experimental plan for their investigation. They need to carefully think about the data they have available. They also need to plan how they will present their results. You may want to advise students to create a standard curve for conductivity using salt concentrations in water. This could be a useful reference as they consider whether road salt from the culverts may have caused the conductivity increase in Ice Lake.

Data Collection

Directed Study
Class Period 1:
Students should work in teams of 3-4 members. They should download conductivity data for Ice Lake from June 15, June 30, July 15, July 30, August 15, August 30 for 1998 and for the same dates in 1999. They will need to the use the data to create a graph establishing a "typical" profile.

Class Period 2:
Students should pour 100 ml of distilled water into the clean dry beaker and measure the conductivity. Next, they should add one drop of 1.0 M NaCl, stir, and measure the conductivity. Adding one drop of 1.0 M NaCl makes the concentration of the solution 0.0005 M or 0.5 mM as shown by the calculation below. (This assumes that one drop is approximately 1/20 of a ml. It is also assumed that the addition of each drop does not significantly increase the total volume of the solution.)
V1 Mx = V2M2
(100 ml) Mx = (0.05 ml) (1.0 M)
Mx = 0.0005 M = 0.5 mM
Students should continue to add the 1.0 M NaCl one drop at a time, stirring and measuring conductivity values. They should record their conductivity measurements on Worksheet 2.

Class Period 3:
Students should measure conductivity and the volume of water in their experimental "lake." Next, they should add the mystery brine effluent solution to the lake and measure the volume and conductivity again. Students can record their observations on Question 1 of Worksheet 3.

Class Period 4:
Students should visit the WOW Web site at http://wow.nrri.umn.edu and look in the "Understanding" section to learn more about conductivity. After locating the main factors that affect conductivity, students should use the "Data" section of the Web site to find the data they need for Worksheet 4.

Student Inquiry
Students should follow their plan for data collection. Remind them to write out their methods and sources in detail and to create accurate tables.

Data Management and Analysis

Directed Study
Class Period 1:
Students will create and analyze a data table of conductivity (columns) for those 12 dates vs. depth (rows). The last column averages conductivity values. Each team will hand in one completed worksheet and two graphs (one plotting 1998 data and one plotting 1999 data).

Class Period 2:
Students should graph their data using concentration as the independent variable and the conductivity as the dependent variable. They should use computer software or a calculator to graph the data and find the "best fit" line or a line of regression.

Class Period 3:
Students will determine the increase in conductivity and the increase in salt concentration.

Class Period 4:
Students should graph conductivity vs. depth, creating a separate line for each day and use WOW's color plotter to observe changes in conductivity during the two week period.

Student Inquiry
Students should analyze the data they collected. Does it eliminate any possible causes for the increased conductivity? Does it point to a possible culprit?

Interpretation of Results

Directed Study
Class Period 1:
Students will consider the accuracy of the data, the relationship between conductivity and depth, and what other lake conductivity profiles might look like.

Class Period 2:
Students will reflect on the "fit" of their data to the development of a standard curve for conductivity using salt concentrations in water. They will also reflect on the possibilities for experimental error affecting the results. Their curve will be used to predict conductivity values for concentrations of salt water that have not been tested with a probe.

Class Period 3:
Students will consider the possible effects that an increase in the salt concentration of the lake could have on plants, animals, and humans.

Class Period 4:
The students apply what they have learned to provide a rational hypothesis for the increased conductivity in Ice Lake during early July of 1998. Student Inquiry Students use their data and results to hypothesize what has caused the increased conductivity in Ice Lake during early July of 1998. They are asked to consider their level of confidence in their results and decide what additional information would be useful.

Reporting Results

Directed Study
(for all class periods, 1-4)
Students should complete and hand in their worksheets and graphs for each of the class periods. You may want to get the students to debate their hypotheses for the increased conductivity in Ice Lake during early July, 1998. (Students may find the idea of salt pollution from the flushing of culverts so appealing that they ignore the tremendous quantities of salt required to make salt pollution a possible explanation for increased conductivity.)

Evaluate if students can:

1. Identify at least five factors affecting conductivity in lakes and briefly explain how each influences the conductivity of aquatic ecosystems.
2. Read a standard solution graph and interpret salt concentration from conductivity measurements.
3. Describe at least three possible sources/causes for the sudden increase in the conductivity of Ice Lake in July 1998.
4. Propose one field or lab experiment and one strategy for accessing WOW data to investigate the source of elevated conductivity.

Student Inquiry
Students should get a chance to present their reports orally or in written form for others to review. Having your results reviewed and challenged is an important part of the scientific process.

Notes: Soluble salts, commonly called electrolytes, dissolve in water and produce ions in solution. The concentration of ions is directly related to conductivity. The more abundant the ions, the greater the transmission of electricity and the higher the conductivity. Concentration of soluble salts depends on:

1. evaporation and precipitation,
2. formation of insoluble precipitates,
3. mixing of water masses having different salinities,
4. diffusion of dissolved materials from one water mass to another,
5. movement of water masses through the lake, or
6. freezing and thawing.

One principle electrolyte in aquatic ecosystems is the bicarbonate ion (HCO31-(aq)). The concentration of bicarbonate ion is generally related to the relative amounts of photosynthesis and respiration (see WOW lesson on the effects of pH at http://waterontheweb.org/teacher/ph/teaching.html). In the epilimnion in summer, photosynthetic activity is relatively high and dissolved carbon dioxide is consumed, reducing the concentration of bicarbonate ions (HCO31-(aq)). Conversely, in the hypolimnion, respiration produces excess carbon dioxide and the concentration of bicarbonate increases (see Figure 1). Thus in the summer, the conductivity of non-polluted lakes increases as depth increases, largely due to the decrease in photosynthesis with depth and the relative increase in respiration with depth.

Figure 1

Extensions
Have students:

1. Compare data from other WOW lakes for the same time period and identify similar peaks in conductivity.
2. Compare midsummer data from Ice Lake for other years and determine whether there are similar peaks in conductivity.
3. Collect samples of spring runoff along city streets that are salted during the winter and from areas where no salt is applied. How do the conductivity levels compare?
4. Collect samples from a nearby storm sewer outfall every other day before, during, and after spring melt and runoff (about one month). Predict how conductivity will change during the melt. Graph conductivity data over time. Do the patterns match their predictions?
5. Research how increased salt levels in streams, lakes, wetlands, and ditches affect plant and animal life.
6. Research alternatives to traditional road salt; compare effectiveness, cost, availability, and reluctance on the part of highway departments to use alternatives. Consider environmental impacts, human safety, cost, and time required to use alternatives and identify which road management practices they think local units of government should follow.
7. Interview local highway departments to find out what they use for reducing ice on roadways. Make a presentation to the city or county board if they think road management practices should be changed.