The last couple of weeks we have been on the river every week testing our equipment. On Wednesdays we did our test runs looking for the methods to sample litter from the river. The thing we really wanted to find out was the mesh of the nets we should use in the winter season.
Basically we sample on two places:
a) skimming the surface with a surface net and
b) sampling the water at a depth of 10 – 60 cm with the suspension net.
So far we have been sampling with improvised nets made from do-it-yourself materials to get some clues about the mesh we should choose.
The surface net has a width of 1.35 m and the suspension net has an opening of 1,0 m x 0,5 m.
Test run 1 was done with only a suspension net with a mesh of 12,5 mm. After sampling more than 8000 m3, nothing was found.
That day was a beautiful day after a long period of dry weather. The throughput of the river was very low and there was no wind at all. All litter stayed peacefully on the banks or was trapped in bushes along the river. The water in the Meuse was or seemed “clean”.
One week later, test run 2 was done with both a surface net (mesh 6,25 mm) and the old suspension net with a mesh of 12.5 mm.The results were quite different from the results the week before. We caught quite a lot of litter on the surface net, not so much on the suspension net.
What was the difference with test run 1 the week before? First of all there was a lot of wind now and the water had sometimes waves of about 50 cm high.
Litter resting on the banks was washed in the river together with a lot of leaves and branches. We skimmed about 22.000 m2 and the result is shown here.
The suspension net (having a mesh of around 12 mm) showed results different from the week before.We caught mainly flexible pieces of plastics which were floating under the surface, pulled there by the turbulence of the wind-agitated water. Comparing the catch with the surface net was not possible because of the different mesh-sizes.
The third week we tried on both nets a mesh size of 2 mm, but here we had no success. The surface net collapsed by the pressure of the water and the suspension net was completely clogged with algae and silt.
- It makes a lot of sense to distinguish between measuring the surface and to measure below the surface. Plastics float in calm waters, but are pulled below the surface as soon as turbulence occurs. The division between “floating” and “in suspension” is very much related to the turbulence in the water.
- The nets should have the same mesh and the optimum seems to be somewhere around 3-4 mm. We can then sample microplastics (pellets and little fragments, without the nets having clogged too soon with algae and silt).
- It is vital to take into account the different conditions when we do the sample runs. The influence of the wind, the throughput of the river and the meteorological situation in the days or weeks before in the entire catchment area will have a big influence.
- One other observation is the effect of lakes connected to the river. We noticed a kind of plume of litter coming out of the lake pushed out by the wind.
We will now concentrate on building the final nets and prepare for measuring campaigns in winter where there will be water speeds of up to 15 km/hr. So far we have been measuring at maximum water speeds of only 1 or 2 km/hr!