Bioassay Method for Detecting Toxic Materials in Water

Bioassay Method for Detecting Toxic Materials in Water


Toxic chemicals inhibit and retard the natural growth in plants and animals. Certain chemicals slow down the root growth in vegetables like the onion stem and prevent germination of lettuce seeds. The test given below is a method, which is based on root growth inhibition as an index of detecting toxicity of water. Plants like the hydra can also be used instead, which takes up a specific shape in the presence of toxic chemicals in water.


To detect the presence of toxic elements in water with the help of the sensitivity index of certain plant parts and aquatic life, which react to toxic water. This experiment will make the students understand how health of plants get affected and also detect toxicity in water bodies.

Materials Required

  1. Some amount of polluted water and clean and safe water in separate containers.
  2. Small pearl onions.
  3. Lettuce seeds
  4. Watch glass and test tube
  5. Glass beaker
  6. Hydra cultured in fresh water
  7. Graduated scale
  8. Blotting paper / tissue paper (for filtering water)


A. Onion bulb germination

Place six bulbs of small pearl onion over the mouths of test tubes containing polluted water. Perform similar control test with non-toxic water. Keep all 12 test tubes aside for 72 hours, after ensuring that the bottoms of the onions are in contact with the water in the test tubes. After the stipulated time, measure the length of the root on every onion bulb. Also, count the number of roots in each bulb. Compare the average length of root, as well as no. of roots in polluted and unpolluted water. If root growth inhibition is conspicuous, it can be denoted as an index to detect water toxicity.

B. Germination of lettuce seeds

Take a watch glass and place blotting paper or tissue paper moisten with known volume of polluted water and clean unpolluted water separately. Place 20 seeds of lettuce on it. Cover the container with glass so that no moisture is lost. After a stipulated time of 48- 72 hours, root emerging from each seed is measured. The inhibition of seed growth in this case as well, would be an index of tracking toxicity in water. Different types of seeds can also be tried and used and inference drawn.

C. Toxic effect on freshwater Hydra

Hydra is a solitary polyp of microscopic size living in clean fresh water ponds, attached to stones or waterweeds. The organism is highly sensitive to toxic elements. When the surface water becomes polluted and temperature rises as a result of the pollution, the animal retreats to the comparatively cool and clean bottom water, where it lives in a peculiar depressed state. The body shortens and gradually disintegrates.

This bioassay uses the reaction of freshwater hydra to toxic compounds as an index of indicating water toxicity (toxic materials in water). Hydra cultures are easy to maintain. A stereomicroscope would be necessary to perform this test.Place hydra in a group of four in small plastic containers and fill the first two containers with clean water and the next two with polluted water. Observe them regularly for four days. Record your observations and draw inference. How can Onion-bulbs, Lettuce seeds or Hydra be used for checking water pollution.Can you to develop a graded-scale
bio-assay method?

Related experimentation and implementation

Activated charcoal is known to absorb some toxic materials from water. Burnt coconut shell is a good source of activated carbon. Pass water containing toxic material through this charcoal and test again to see to what extent the toxic chemicals have been removed. Vary the flow rate, the depth of activated carbon bed etc, and observe results.


Source: Harness water resources for a better future - Activity Guide for the 13th National Children's Science Congress