Sand: Tracer Sediment

I have spent many months holed up in the laboratory counting green grains of sand. Last year we dumped over 1 ton of fluorescent, magnetic tracer sand into the North Sea, where the waves and tides then scattered them along the coast. We then spent the following weeks circling around on a boat to try and find it all again. We scooped up over 200 samples of sand from the seabed, then brought them back to the lab for analysis. We used a super strong magnet and blue UV light to separate the tracer (bright green) from the normal sand (looks grey or purple under UV light). This part is REALLY boring because most samples don’t have any tracer but we still have to look hard for it . But then we get to look at all the sand under a fancy microscope, which is my new favourite toy! Under the UV light, the tracer reminds me of little green constellations of stars in a purple night sky.

TracerSediment

The tracers glow bright green under ultraviolet lights.  On the top left we have a jar of normal beach sand sitting next to a jar of tracer.  The distinction becomes clear once we place them under UV illumination (top right). This is especially important when we analyze samples taken from the seabed, where there may be only a few grains of tracer (bottom left). The fluorescent properties of the tracer help it stand out from normal sand (bottom right), which lets us count the individual grains.

Fortunately, we can use computers to count the individual grains and tell us their size.  With this information, we can estimate how the size of a sand grain determines how far and fast it will travel.  This is important for planning sand nourishments to protect the coast.

G001-2017-08-30_000006

If we can figure out where the green tracer did (or didn’t) go, that will tell us how normal sand moves around on the Dutch coast. And this will hopefully keep our feet dry here in Delft for a long time to come!

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