Contaminated Sediments: Should we dredge? How?
|Introduction||This activity has two parts. Each part can be completed separately to study bioaccumulation and environmental pollution, or they can be done together as a comprehensive project. The following information will describe the lesson in detail and will provide you with activity sheets and data necessary to utilize these activities in your class.|
|Materials Needed||Part 1
Printable Data Sets
Bioaccumulation is the process by which toxins become more concentrated as you move up the food chain. Many substances can enter an aquatic ecosystem and become toxic to the flora and fauna. In low concentrations, these toxins often pose little threat, however, through the process of bioaccumulation, a small amount of a toxic substance can become a problem for larger predators and even humans.
Some toxins get suspended in the water column and remain there for days, months, and even years. Other heavy toxins may sink to the bottom of a river or lake and be mixed into the sediment. While in the sediment, they may remain harmless because no organisms are in direct contact with them. However, when this sediment becomes disturbed through wave and wind action, boating, and storms these toxins can become re-suspended in the water. When this happens, it increases the chances that an organism will come into contact with the toxin and absorb or ingest it.
Some scientists have argued that to eliminate the threat of the toxin, it is necessary to remove it all together from the system. To do this, they recommend dredging the contaminated river or lake bottom removing the contaminated sediment. Other scientists disagree, believing that the process of dredging leads to the re-suspension of toxins and that not all the toxin will be removed. They argue that what is already in the sediment should remain and that efforts should focus on reducing future toxins from entering the system.
There are three different types of dredges: clam shell, hydraulic, and bucket. In this activity, you will model one type of dredge and attempt to dredge a simulated lake bottom removing as much contaminated sediment as you can without re-suspending it into the water column.
The Ashtabula River is located in northeast Ohio. From the 1940s through the late '70s, hazardous waste caused the river's sediments to become seriously contaminated. Regular dredging is being prevented due to the contaminated sediments. Since 1983, a fish consumption advisory has been posted for the Ashtabula River. The advisory is based on PCB and mercury levels in fish tissue and suggests restricted consumption for smallmouth bass, largemouth bass, walleye, channel catfish and common carp in the Ashtabula River (OEPA website).
Part I: Dredging Contaminated Sediments
Can toxins covered by sediments be dredged without causing the toxin to be re-suspended in the water column?
1. Get a pan that has sediment on the bottom and water on the top. Answer Lab Question 1.
2a. Clam-Shell Dredge Group. Using both cups, carefully lower your cups into the pan and scoop up the sediment on the bottom. Your cups should come together as you raise them back towards the surface.
2b. Hydraulic Dredge Group. Using the pipette/baster, carefully suction out the sediment without disturbing the sediment on the bottom.
2c. Bucket Dredge Group. Using the large cup, carefully lower the cup into the pan and scoop up the sediment on the bottom.
3. Deposit the waste sediment into your plastic container and continue the dredging process in Step 2 until you have removed as much sediment as you can. Answer Lab Question 2.
4. When you are finished dredging, go around the room and compare your results to those in the other groups. Answer Lab Question 3.
5. Answer the conclusion questions and then return your materials.
1. Before you start dredging, does the water appear to be clear? Why is this?
2. Now that you are dredging, does the water look the same as it did before you started? Why is this?
3. Which method of dredging appeared to work the best?
1. Were you able to remove all of the toxic sediment?
2. Did some of the sediment get suspended into the water column? What does this probably mean about the toxin?
3. How could you improve your dredging method to make it cleaner or more effective? Could this be done in a river or lake?
4. Look at the plastic tub of toxic sediment. If this were really toxic, how would we safely dispose of it? What would be the negative effects of doing this?
5. If you were responsible for making the decision, would you rather dredge a contaminated lake bottom or leave it be. Explain your choice.
Part II: Should the Ashtabula River be Dredged?
The City of Ashtabula has contacted you to help them decide whether or not their river should be dredged. Use the data from a similar dredging project done in the Black River to make a decision on whether or not to dredge the Ashtabula River.
1. Examine Table 1 in your data set. Does it appear that the dredging operation reduced the amount of PAH compounds? What could account for the dramatic reduction of PAH compounds from 1980 1984?
2. Calculate the percent decrease of all PAH compounds from 1980 to 1984. Calculate the percent decrease of all PAH compounds from 1984 1992. Which year had the greatest percent decrease? Why might this be the case? Which year had the greatest overall reduction in numbers of PAH compounds? Why might this be the case?
3. Can you tell from the data whether or not dredging was successful? Explain your reasoning.
4. If you could have one more year of data, what year would you want? Explain your reasoning.
5. Examine Figure 1 in your data sets. What was the general trend in fish health from 1982-1987? What probably accounted for this change?
6. The incidence of cancer increased in bullheads between the 1992 and 1993 samples. Suggest a reason for this.
7. Use the data from Table 1 and Figure 1, as well as, your experience from the dredging activity to determine the best course of action for the Ashtabula River. Should the river be dredged or not? Meet with your group to develop a list of pros and cons for dredging. You should include the consequences for your decision.
8. Present your decision to the rest of the class.
9. Create a concept map for dredging toxic sediments based on the information you have learned in these activities. Make sure to include connections between the hydrosphere (water), lithosphere (sediments), and biosphere (living organisms). You will be graded using the evaluation rubric (below).
|Conclusion||Answer all lab and discussion Questions. Prepare yourself for a classroom discussion to explain your concept map and lab findings.|
Scoring Rubric for Concept Maps
Map is complete and reasonably arranged 0-4 points
All links have verbs describing the connections 0-4 points
Evidence of multiple earth subsystems exists 0-4 points
Connections in the map are logical and smooth 0-3 points
Map accurately represents the concepts related to dredging 0-3 points
|Extension Activity||Conduct a
web search for more data on the effects of dredging in
Ohio rivers and lakes to see if the new data would
influence your decision-making process.
Examine the arguments for and against dredging the Hudson River, at http://www.fishkillridge.org/water/hudson.htm
|References & Resources||Baumann,
P. and Harshbarger, J. (1996). Decline in liver neoplasms
in wild brown bullhead catfish after coking plant closes
and environmental PAHs plummet. Environmental
Health Perspectives. 103(2):168-170.
http://www.epa.gov/glnpo/sediment/Bullhead/report.html is the report on Bullhead catfish tumors.
Ashtabula River report updating toxin information: http://www.fws.gov/midwest/AshtabulaNRDA/
2004 update on PCBs and HCBs in the Ashtabula River, from US Fish and Wildlife Service [pdf file].
Ohio EPA Website: http://www.epa.gov/glnpo/aoc/ashtabula.html
King, Earth science teacher, Circleville, OH
Web assistance by Rosanne Fortner, The Ohio State University
From Decision Making Activities for the Great Lakes, Contaminated Sediments.
Developed by the Ohio Sea Grant Education Program, The Ohio State University. © 2003