A couple of posts ago I told you about my trip to the North and the three samples I took. It was only three samples because traffic was much slower than expected and I also had to make a detour. Because of this – although I left two hours early – I could not take the other three samples. It wasn’t a big issue, because nearly all samples until then showed a pH between 7.98 and 8.92. The first samples were at a small recreational lake in the suburbs of the city of Groningen, called “Kardingerplas”. The pH values for samples 1 to 3 were 8.28, 8.32, 8.31, so no surprise with an average of 8.30 and a standard deviation of 0.02. Quite normal. What surprise could the other lakes bring? Well, BIG surprises!
The next set of samples already brought completely unexpected results. To start with, the lake I selected is a bit complex. It has a section called “Hoornseplas” which is meant for swimming. The other part has to names: “Hoornsemeer” (Norhtern part) and Paterswoldsemeer” (Southern part). In the past (until about forty years ago) two lakes were in place, but taking away the barrier between them, just one lake remained still having two names. There is even another lake to the south (Friescheveenplas), but I didn’t know about it until I was finding out what I had been looking at, later when back at home. To provide an overview of the situation, my sampling points are plotted at the map below.
Coming at the “Hoornseplas” I observed a dam and I wondered whether this was not just a (legally) specified section of the “Hoornsemeer” as it looked like this small lake was completely separated from the larger one. To be sure, I sampled both sides of the dam and the pH was so different that there was no doubt: the two parts are not connected! The part called “Hoornseplas” showed pH values 9.56, 9.56 and 9.58 for the three samples taken, with an average of 9.57 and a standard deviation of 0.01. A pH of over 9.5 (!) is soapy water and perhaps that just what it is. If a lot of people are swimming there, bringing their sunscreens, lotions and shampoos, it might become more of a bath tub than a lake. It’s not dangerous to swim in (then a bathtub full of soap water would be as well), but surely plants and animals won’t love it.
Close to the other side of dam, there was a bridge and there the pH turned out to be only 8.17 (8.16, 8.15 and 8.18). To be sure I took a reference sample at a jetty about 10 metres away and it was 8.15, so we can be sure that the dam is a real dam, separating the two parts allowing them to differ 1.4 points in pH. Probably the dam is in place to save the real lake(s) from the devastating influence of the people bathing there. If so, it was a wise decision. The picture below shows the situation clearly – but there’s another surprise to come!
It’s not the end of the surprises. Sampling at another location, in the part called “Paterswoldsemeer”, I got very inconsistent results between my first and second sample. Standing on a jetty I took the first sample to the left and the second to the right. The jetty is hovering over the water and no barrier at all so I didn’t expect a pH of 8.55 to the left side and a pH of 7.95 to the right and thought this would be a mistake of some kind. Instead of taking a third sample only, I took another pair to the left and to the right and the results showed the same gap. Then I noticed the right side was close to a canal. The water coming from there could influence the water to the right of the jetty, leaving the water to the left more or less unchanged. There was a small bridge over the canal and I took a sample from there and indeed the pH there was only 7.70! At the end of the jetty, where the two flows would be mixed a bit more, the pH was 8.11. To the far left, far from the influence of the canal, the pH was 8.66. It is a very difficult story, but shown on a map it will be very clear what is going on there.
Originally I added an animated gif, to show the situation. Unfortunately then twitter wouldn’t show the link to this post. The animated gif had to be replaced by some pictures.