Water Clarity

Seagrasses are important in removing fine particles from water. These particles can scatter light and decrease the ability of light to penetrate the water. To do this experiment, you will need a sample of sediment that contains a range of particle sizes. You can use beach sand. Obtain two pint jars. Place equal amounts of your sediment in each. Add water to the top in both jars. To one jar, add two drops of dish detergent. Place lids on both jars and gently invert 5 or 6 times to mix the sediment with the water. You do not want to shake vigorously because it will cause the water to foam. The water should be somewhat opaque in both jars. Record what you see. Now remove the lid from each and take a sample of the water one centimeter from the surface using a dropper. Place 2 droppers of water onto a piece of filter paper (a white coffee filter can be used if filter paper is not available). Allow the paper to dry. When the paper is dry, look at it using your hand lens. Do you see fine particles? Rub the area gently with your finger. Do you feel any sediment? Record your results as instructed by your teacher. Continue to watch the jars every day, noting when the water starts to clear. Be sure to note the size of the clear area each day. Does one jar clear faster than the other? [teacher note: Likely the jar with the soap added will clear last. The soap will break apart clumps that have formed from clay size particles adhering to larger particles. It generally takes longer for the clay particles to come out of suspension. However, if there is only a small amount of clay size particles in the sample, the difference may not be obvious in this type of experiment.] Continue to sample and observe the water in the same way each day until the area above the sediment at the bottom of the jar is completely clear.

Water clarity experiment – understanding fine particles, part II
For this experiment, you will need the same materials as above except you also must have a piece of artificial grass and a piece of wax paper. You do not need to use the soap. You can use a clean door mat that has plastic “grass” blades. Place the sediment into the bottoms of the jars and fill the jars with water as described above. Gently invert the jars 5 or 6 times. You will pour the water slowly over the surface of the mat and collect the water to place in the jar again. For the second jar, you will pour the water over the surface of the wax paper and collect the water to place in the jar again. Replace the lids and repeat the observations that were made for the first experiment. Did you see any difference in the outcomes? [teacher note: the plastic grass should have collected some of the finer particles, so it is likely that this jar cleared faster. Again, if there was little fine material in the sample, students may not see a difference using this method of observation. Geological oceanographers use very precise instruments to measure the settling velocities of particles. One of the newest techniques being used commonly is LISST or Laser In-Situ Scattering and Transmissometry. Click here to learn more.]

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