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108

Toxic! Or Is It?

One of the big questions about any type of pollution is, “Will it hurt plants, 

animals, or humans?”

How can we fi nd out? We don’t want to risk harming lots of living organisms to 

see whether a substance is toxic, especially if the organisms are humans! 

Scientists investigating pollution often use a technique called “bioassay” to 

test for toxicity. A bioassay is a measurement of the eff ects of a substance on 

living organisms. Tests for toxic substances use certain species called “indicator species,” which may be juvenile fi shes, plant seeds, microscopic animals, 

or even bacteria! Several individuals of an indicator species are exposed to 

the substance being tested for a certain period of time, and then compared 

to another group of the same species that was not exposed to the substance 

(the second group is called the “control group”). If the organisms in the test 

group show eff ects such as slow growth, reduced movement, or death and these 

eff ects are not seen in the control group, this is considered to be an indication 

that the substance being tested is toxic.

Bioassays usually do not indicate which specifi c substances cause toxicity, or 

the amounts of toxic substances present. But they do provide a good indication 

of the total toxicity of samples that may contain more than one toxic substance, 

and they are a quick and inexpensive way to test for toxicity in many samples.

Here is a simple bioassay that you can use to test for toxicity in runoff water 

that you suspect may be contaminated.

What You Will Do Use a radish seed bioassay to test for toxicity

Beach pollution at Bay St. Louis, MS after Hurricane Katrina. Courtesy NOAA.

What You Will Need

❒ 12 or more ziplock plastic bags

❒ One paper towel for each plastic bag

❒ Ten radish seeds for each plastic bag

❒ Permanent marker for labeling sample jars and plastic bags

❒ Household bleach, about 1/4-cup

❒ Distilled water, about one quart

❒ Ruler graduated in millimeters (mm)

❒ Tweezers or forceps

❒ Glass containers for collecting samples; baby food jars or similar size are 

perfect

❒ Clean glass jar for mixing bleach solution, about one pint

❒ Measuring cup that is not used for food

❒ Protective gloves and safety glasses, one set for each person participating 

in this activity

Beach pollution at Bay St. Louis, MS after Hurricane Katrina. Courtesy NOAA.
National Oceanic and Atmospheric Administration (NOAA) 109

Science | Service | STEWARDSHIP Protect the Eart

Warnings

 Do this activity with an adult partner!

 Always check with an adult before collecting 

samples that may be contaminated with 

harmful substances. 

 Wear protective gloves and safety glasses when 

handling sample solutions!

 Do not use glasses, dishes, or any container that 

is used for food!

 Wash your hands after handling sample solutions, paper towels, and seeds!

How to Do It

1. Collect water samples to be tested. You may 

want to test water from puddles, streams, or 

ponds. Do this ONLY with an adult partner! 

Wear protective gloves and safety glasses! 

Collect each sample in a clean glass jar, and 

label the jar with a number.

2. Put on protective gloves and safety glasses, 

then mix one ounce of household bleach with 

nine ounces of distilled water in a clean glass 

jar.

3. Put the radish seeds in a clean glass container, and add about two ounces of the bleach 

solution from Step 2. The bleach solution 

kills fungi, which could interfere with seed 

germination.

4. Bioassays are carried out in zip-lock plastic 

bags containing a paper towel saturated with 

the test solution. Saturated means that the 

towel is damp all over, but isn’t dripping. 

Three separate tests (called “replicates”) 

will be done for each solution. Prepare three 

plastic bags for each sample to be tested, 

plus three more bags for the control solution (distilled water). Label each bag with 

the sample number, plus a letter to identify 

the three replicates (so for sample number 

1, there would be three bags labeled “1-A,” 

“1-B,” and “1-C”). Put a folded paper towel in 

each bag.

5. Pour enough sample solution into each bag 

to saturate the filter paper. Use the same 

amount of sample in all tests. Saturate the 

paper towels in the control bags with the 

same amount of distilled water.

6. Using tweezers or forceps, place ten seeds on 

the paper, evenly spaced, in each bag. Seal 

the top of the bag.

7. Put the bags on a tray or shallow box where 

they can remain for five days. The bags 

should be at room temperature, but do not 

place them in direct sunlight! Each day, 

record the number of seeds that germinated 

(split open) in each bag, and measure (to the 

nearest mm) the length of the root that has 

emerged from each germinated seed. 

8. At the end of five days, calculate the average 

number of seeds that germinated in the three 

bags for each sample and for the controls. 

Also, calculate the average length of the 

roots among the germinated seeds in the 

three bags. If fewer seeds germinated in bags 

containing test samples, or if the roots grew 

more slowly than in the control bags, this is 

an indication that the samples may contain 

toxic substances.

Want to Do More?

1. For more information about simple bioassays, 

see

www.wwoa.org/resources/public-education/

lettuce-seed-bioassay/

2. Visit www.epa.gov/nutrientpollution for more 

information about polluted runoff.

3. Visit oceanservice.noaa.gov/education/lessons/the_seeds.html for a lesson on nonpoint source pollution called The Seeds Tell 

the Story.