How much turbidity is too much?

Teacher note:
While not essential for this unit, this lesson is a hands-on activity that provides students with a way to understand the impact of turbidity pollution on aquatic organisms. We have tested this experiment and saw results within 4-6 hours, allowing it to run for three days.  Bentonite is cheap and easy to use; however, you may decide to split up the class so that each group only does one or two concentrations (instead of having each group run a range of concentrations).  This would reduce the amount of equipment you need. 

Purchase a Daphnia culture kit at least 2 weeks before you plan to do the experiment, to ensure that you have a healthy culture.  You should try and use newly hatched Daphnia in order to minimize the differences in the sizes of the animals.  Because the appearance of resting eggs indicates a poor culture environment, do not use Daphnia with resting eggs. Students can do these next steps to minimize the preparation time.  Twenty-four hours before beginning the experiment, remove all the females bearing embryos from the stock culture and place them in 400-mL beakers containing 300 mL of spring or stream water and the appropriate amount of food.  Five beakers, each containing 10 adults, usually will supply enough young individuals for one toxicity test. When you are ready to begin your bioassay, choose young (small) Daphnia from these cultures.  Introduce the same number of neonates (at least 5-10) into each test vessel and control using a plastic, disposable pipette with a 5-mm diameter. Be sure to release the young below the surface to avoid killing them by trapping air under their carapaces. Record the time and number of young introduced into each labeled vessel.

1. Pass out the student worksheets. 
2. Engage:  Hold up a jar of water that has a lot of sediment in it, and then ask students what they think would happen if you put a goldfish in this jar.  How long would the goldfish survive?  What, besides death, would indicate stress on the animal?   
3. Explore:  Students will receive the materials and begin working on the lab, following the directions on the lab sheet.  You will need to leave the Daphnia in the turbid solution overnight in order to see a response (4-6 hours is usually enough).  Additional information on conducting a bioassay can be found on Cornell's Environmental Inquiry website.
4. Make sure that students write down THEIR turbidity values - or do this part as a class.  Turbidity values will be different depending on the background levels in your water sample. 
5. You could ask the students to measure other types of responses in the organisms – heart rate, for example, might be more useful as a measure of stress instead of waiting for death. 
6. Explain:  Turbidity is reported by in nephelometric units (NTUs) which refers to the type of instrument (turbidimeter or nephelometer) used for estimating light scattering from suspended particulate material.  The term Nephelometric refers to the way the instrument estimates how light is scattered by suspended particulate material in the water. This measurement generally provides a very good correlation with the concentration of particles in the water that affect clarity.  High turbidity levels also adds cost to the treatment of drinking water since the turbidity must be virtually eliminated for effective disinfection (usually by chlorine in a variety of forms) to occur. Another concern is that particulates also provide attachment sites for heavy metals such as cadmium, mercury and lead, and many toxic organic contaminants such as PCBs, PAHs and many pesticides.
7. With respect to hydrofracking, turbidity values in nearby streams are expected to increase as a result of the construction of the wells and the associated infrastructure. 
8. Focus on the discussion of variability in the student worksheet to highlight the importance of getting more data, how finding an average affects your ability to "see" all of your data, etc. 
9. Extend: The last question on the worksheet asks students to view the results of turbidity changes as a result of large storms in the Hudson River estuary.  Use the HRECOS website to view these changes – we suggest looking at the impacts of Hurricane Irene and Tropical Storm Sandy. We have also embedded graphs of these data in the answer key in case you want to show students the graphs instead of having them look for the data themselves. 
10. Evaluate: Students will submit their lab reports.