Are Culverts the Culprits?

Introduction
In this series of activities you will explore strange changes in conductivity in Ice Lake during July 1998 after reviewing the sources of conductivity and measurement of conductivity. The complete lesson has four activities. In the lab components of the lesson, you will create a standard curve and use it to determine how much salt has been added to your experimental lake. You will also use the WOW site to find background material on conductivity and download data from Ice Lake. You will need Internet access, and should be able to download data into a table and then graph the relationship between conductivity and depth. Each student will complete and hand in two worksheets as part of this lesson. Each team will complete 3 graphs.

Day 1: Conductivity & Stratification in Lakes

Knowledge Base
It may help to review the WOW lessons on stratification and conductivity.
Answer Question 1 on Worksheet 1.

Experimental Design
Establish a typical midsummer profile for the conductivity of Ice Lake in Grand Rapids, Minnesota. Consider what the profile might look like and participate in the class discussion about the profile.

Data Collection
Form a team of 3-4 people to complete Worksheet 1. Go online and retrieve the conductivity data for Ice Lake for June 15, June 30, July 15, July 30, August 15, August 30 for 1998 and for the same dates in 2000.

Data Management and Analysis
Complete the data table of conductivity (columns) for the 12 dates. In the last column, average the conductivity values for each depth.

Use the data from your table to create two graphs (one for each year)plotting conductivity vs. depth. Use a separate line for each date. Plot the average values for each depth as a separate line on each graph. Remember to label the axes and identify each line. Each team should hand in one copy of worksheet and two graphs. Answer Questions 3 ¬ 6 on Worksheet 1.

Interpretation of Results
Consider how conductivity may be related to water depth. Answer Questions 7-11 on Worksheet 1.

Reporting Results
Answer Questions 11 and 12 on the worksheet. Hand in your team's worksheet and the two graphs your team created. Be sure all team members' names are on them.

Day 2: Conductivity and a Standard Curve

Knowledge Base
In this activity you will determine 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. In this activity and the next, each student will complete Worksheet 2, and the team will produce one graph.

Experimental Design
Form teams of 3-4 people. Each team will create one graph showing the standard curve for sodium chloride concentration vs. conductivity.
Collect these 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

Data Collection
Pour 100 ml of distilled water into a clean dry beaker. Measure the conductivity. Add one drop of 1.0 M NaCl and stir. 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.

V1Mx = V2M2
(100 ml) Mx = (0.05 ml) (1.0 M)
Mx = 0.0005M = 0.5 mM

Continue to add the 1.0 M NaCl one drop at a time, stirring and measuring the conductivity after adding each drop. Record your measurements in the table on Worksheet 2.

Data Management and Analysis
Graph the data. Concentration is the independent variable and the conductivity is the dependent variable. Use computer software or a calculator to graph the data and find the "best fit" line or a line of regression.

Use this standard curve to answer Questions 6-8 on Worksheet 2.

Interpretation of Data
Reflect on your laboratory techniques and skills as you complete Worksheet 2. Use the information from your standard curve to address item 9.

Reporting Results
Compare your graph with other teams. Using a transparency and overhead projector or overlay mylar, plot the standard curves from all teams.

Keep the standard curve for the next activity.

Day 3: Brine Solution - Pollution Simulation

Knowledge Base
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, or might be caused by physical, chemical, or biological factors. In this activity you will use the standard curve you developed in the previous activity to identify the concentration of a mystery brine solution.

Consider a lake that experiences a distinct change in conductivity resulting from human activity in the watershed. In this experimental "lake" investigation you will find that the Teeny-Weeny Brine Shrimp Processing Facility has diverted salt water from their processing effluent into the nearby storm sewer. Your job is to calculate how much salt they released into your lake. Each small group will receive effluent with a different salt concentration.

Experimental Design
Work with the same team you used to complete the previous activity. Collect these 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

Create your experimental "lake" in your 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." (You may want to refer to your procedures used for the Day 2 activity.) Address points 1 and 2 on Worksheet 3 then have your teacher add the mystery brine effluent to your lake.

Data Collection
After the mystery brine solution has been added to your lake, measure the volume and conductivity again. Enter your observations point 3.

Data Management and Analysis
Use your standard curve graph or the regression equation from the previous activity (Worksheet 2) to determine the salt concentration before and after the effluent was added. Answer Questions 4-7 on the worksheet.

Data Interpretation
Reflect on your team's research skills and your confidence in the results. Hypothesize what effect a similar increase of salt would have on life in a real lake.

Reporting Results
Compare your results with other small groups and complete Worksheet 3. Hand in your team°s worksheet and the standard curve with everyone's name on it.

Day 4: Are Culverts the Culprits? An Ice Lake Mystery

Knowledge Base
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. Keep in mind that the increase might be the result of natural changes or human activity, or the increase 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? Reflect on what you have learned so far about conductivity. You may also want to review the information about conductivity.

Experimental Design and Data Collection
Form work teams of 3-4 people. Each group will complete Worksheet 4 and create one graph by using the WOW data to determine if culvert flushing has led to an increase in conductivity in Ice Lake during July 1998.

View, download, or copy the data necessary to answer Questions 2-6 on Worksheet 4.

Data Management and Analysis
Graph conductivity vs. depth, creating a separate line for each day and use the color plotter feature online to observe changes in conductivity during the two week period beginning June 28, 1998. Answer questions 7-10 on the worksheet as you analyze the increases in conductivity in Ice Lake during early July of 1998.

Data Interpretation
Consider how conductivity varies in Ice Lake in the summer compared to the winter. Consider the dramatic increase of conductivity in Ice Lake during early July of 1998. Discuss your theories as a team and answer Questions 11-15 on Worksheet 4.

Reporting Results Compare your theories with other groups. Write up some of the strengths and limitations of your team's theories for what caused the change in conductivity. Turn in your team°s worksheet and graph. Make sure everyone's name is on them.