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.
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