I have a question rather than a specific change, I do not understand the rationale of eliminating the effect of contamination
The authors use figure c and make a statement that the drift in surface pressure does not decrease upon changing to a fresh filter paper in the same trough of water. Since one does not see such a change it is not an effect of contamination.
This is true only if we assume they are stripped away by the first filter paper completely. This is a very big assumption as we do not even know which contaminant is the contributing factor and if its concentration is negligible enough to make this assumption. If the contaminant were not stripped away completely by the first paper,The first filter paper would reach a saturation hence the plateau but with the new filter paper, it is a fresh start all over so the graphs I would expect should look similar as it does in figure c. Since it is hard to point out which contaminant is really the contributing factor it is in my belief difficult to make a claim based on this graph alone.
The other results are convincing enough for me and work without the contamination data.