Last updated: September 19, 2016
Parts Per Million
- Grade Level:
- Upper Elementary: Third Grade through Fifth Grade
- State Standards:
- ESS3.A, ESS3.C
- Two 100 or 250 ml beakers
- Water mixed with food coloring
- Clear water
- 10 ml graduated cylinder
- See-through cups
Background: Scientists measure and report water contaminants in parts per million (ppm) and parts per billion (ppb) because very small amounts of contaminants can cause large problems. Many pollutants are undetectable by the human mouth or nose at low concentration. The concentration at which a person can detect (though not identify) a compound is called the “odor or flavor detection threshold.”
Water pollution comes from a variety of sources. Animal feedlots leech numerous pollutants into surface and groundwater including ammonia and hydrogen sulfides. Pharmaceutical companies release a wide range of chemicals including phenol and aldehydes. Even wastewater treatment plants designed to mitigate pollution still release some pollutants such as hydrogen sulfides and mercaptan. Mercaptan is the substance added to natural gas to give it an odor. US law dictates that it must be detectable at a concentration 5 times lower than natural gas needs to ignite. Source: http://www.sipcotcuddalore.com/downloads/odour_research.pdf
Each substance has a unique threshold but the threshold can vary from individual to individual. Acetic acid (main component in vinegar after water) is detectable by the human mouth at 900 ppm. Formaldehyde is detectable at a little over 1 ppm. Hydrogen sulfide is detectable at about 30 ppm but begins to have detrimental effects under 10 ppm. Most bacteria will be flavorless at dangerous levels. The Environmental Protection Agency mandates the testing of numerous chemicals, bacteria, and compounds. Click here for a list of these chemicals.
Note: mg/L or mg/kg is the same as ppm.
1. Lead students on a discussion of what are some dangerous substances that can be in water. For instance how does bacteria, pesticides, industrial waste, radioactive particles affect water quality. Discuss what they think is a safe amount for these compounds.
2. Show students a glass of water. Ask how they know it is safe to drink. Have them list things they would like to know about the water before they drink it. Would they drink water from a bottle? From a faucet? From a stream? From a puddle?
3. Measure 100ml of water mixed with several drops of food coloring (until color is very obvious) and state that this represents polluted water. Ask if anyone would want to drink it.
4. Take 10ml of the polluted water (put some in a see-through cup for later comparison) and combine it with 90ml of clear water in a beaker. This is one part per 10. Would they drink it? What if they were thirsty or in the desert?
5. Now repeat the process with 10ml of this new solution. (Again put some of remaining liquid into the next see through cup for comparison and reuse the beakers.) This represents 1 part per 100. Would they drink the water now? Repeat the process and dilute the polluted water one more time, 10ml to 90ml clear water. This is now 1 part per thousand (ppt). Repeat this three more times until you get to 1 part per million (ppm). Would they drink the water now? Why or why not?
6. Have a discussion about why water is treated. Is it necessary to clean it? It is possible to clean all containments out of water? How do we come to an acceptable standard fro clean water? What amount of containments can be present in water without being harmful?
7. Why is it important to have clean water? People need it, does anything else? Make a list of other things affected by polluted water (animals, plants, air quality, etc).
8. Have the students brainstorm a list of things can be done to keep water clean and a list of things that people do that pollute water. Write these on the board in two categories.
What do treatment plants do when there is too much water to treat? Do they keep the water or do they dump it into the nearest water source?
Sewage treatment plants can have a system overload due to equipment malfunction or a high water volume (heavy rains). When this happens a treatment plant is often forced to release untreated or semi-treated sewage into the environment.
Is it possible to clean all water?
It is impractical and expensive to completely filter all water.
How important is it to keep caves clean? Should you use caves as dumping grounds? What would happen to the quality of water in the cave environment if things like parking lot run off of gasoline and oil were getting into a cave?
Fifty-one percent of water runs through caves before it is cleaned up and used by humans. Pollutants can not only affect the water quality of the cave but the living organisms that call the cave home.
Can animals build up a tolerance to unclean water? What are examples?
Acid rain for example affects different fish species. Florida water is naturally acidic so those species are less affected than those elsewhere.
Pollutants can become concentrated at different levels of the food chain. For example, DDT affected birds of prey more than other species because it was concentrated at the top of food chain.
How does nature help clean ground water?
Groundwater filtration, vegetation, dillution.