National Waterways
Pollution Detective

Got the Dirt
- Dirty Rotten Sediment

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Approach with caution - renowned for dirty work in our streams; collapsing (stream) banks a favourite.

Water clarity can be measured using standard water monitoring equipment such as a water clarity tube, and a "black disk", or with a "homemade" clarity tube.

The instructions for operating the first two can be found at
www.ew.govt.nz/educationprogrammes/schools/teachers/classroomunits/streams.htm under "Conducting chemical/physical tests".

Water clarity measurement for waterway detectives

What you need:

		1m of 25mm clear plastic tubing
		Clear PET (soft drink) bottle with white cap
		Black permanent ink marker
		1m ruler, or strong stick with cm markings
		clear sticky tape
		1L container to collect sample in

 

Make your tube

1.		Cut off the top of your PET bottle about 6 mm below the collar, as shown on the diagram. (The cut must be above the point where the bottle top starts to flare out to get a good fit into the end of the tube.)
2.		Unscrew the cap and remove the washer (soft inner cap).
3.		Draw a cross on the white inside surface of the cap with the black marker pen. The cross should be "dark" enough to make the cap half black/half white. Screw the cap back on.
4.		Fit the cut off bottle top to the plastic tube.
5.		Attach the tube to the ruler (or stick), making the tube as straight as possible. The cap should be level/even with the 0 cm end of the ruler.

Sampling

1.		Collect your water sample, being very careful not to disturb bottom the sediment on the stream bottom. Taking your sample upstream of your feet may be helpful!
2.		Get into position to read your sample - be out of the shade of trees, but have your back to the sun, so that the tube is only shaded by your body.
3.		Shake your sample well.
4.		Slowly pour the sample into your tube while looking vertically down the tube. Tricky, but it can be done!
5.		Stop pouring at the point where the cross on the cap is "just" disappears.
6.		Note the reading on the ruler. This is the water clarity distance.
7.		Pour out some of the sample water and repeat steps 3 to 5 with another person to verify the result.
8.		Fill in the data sheet. Click here to go to the on-line data form.