After the surprise of the Weerwater being rather alkaline, I set up a map of new sample points for different lakes. Biking together with my eldest son, it took 50 km and a couple of hours to collect them all, but let me show you the map before we go on.
The Weerwater was in again, but only with two sample points now. It was clear that the pH was not very different at the sample locations last time, so it made sense to reduce the number of sample locations. The other large lake (Noorderplassen) got two sample locations as well. The water around Almere is formally divided in three lakes with different names, but all these are remainders of the former sea (Zuiderzee), which became a sweet-water lake after closing the dam (Afsluitdijk) 90 years ago. The Markermeer and IJmeer are not separated by any construction, but got different names. The Gooimeer, to the South is connected to the IJmeer, but the connection is rather narrow.
Now the question is, will these lakes be more acidic or at least less alkaline than the Weerwater?
The same (clean) sample bottles were used and rinsed with the lake-water first before collecting the actual sample. Because of the long trip, the measurement was done a couple of hours after sampling, but again we will check what happens to the pH during the days after sampling. The weather remained nice. About 25 oC and still no rain!
The results were completely different from what I expected – again. More alkaline samples and this time even more extreme! The Markermeer scored 8.92, but I still had this issue with the pH meter being slightly off. That’s why I applied a correction formula and presented the corrected values. For the Markermeer the corrected value became 8.69. Still a very high pH. Although I had some trouble getting my pH meter right (fortunately the next time everything became stable), this value is confirmed by an article (although a couple of years old). It says that the pH of the Markermeer is around 8.7.
The good news about the measurements is that the days after I finally managed to get my pH meter stable (rather accurate and precise, using the calibration buffers). After this new attempt the measurements could vary a couple of hundreds, but the average was stable and close to the buffer’s value. I calibrated once and then the values remained stable. The 4.00 buffer was checked less frequently because the samples were in the region above 7.
Seeing the stable results, I was confident to measure the change of the pH of the samples over time again. Like the previous set the samples were kept in a closed bottle, at room temperature and in the dark. During a couple of days after sampling they were measured and stored again.
For most samples the pH will drop slowly, meaning the sample is getting less alkaline. Most likely this is the result of some biological processes. Either the alkaline factor is processed, or some acid is formed (neutralising the sample) or both. Since I don’t have chemical details (yet), we can only guess. By the way: all data are available in a spreadsheet and the original logs will be kept. Please reach out at email@example.com if you are interested.
More lakes to come!