Water Clarity (Natural Swimming Pool)

For the first 9 months we were seriously wondering whether our whole concept was fatally flawed. The water in the filtration pond immediately filled with silt from the clay and this did not settle out at all. Month after month we waited for the water to show signs of clearing, but every slight wind or rain shower stirred up the pond again and we could not see more than 30cm depth (our homemade Secchi Disk reading in June was less than 15cm!). We tried various experiments with the water to see what would work best at clearing it. We took jars of water and gave them different treatments, such as alum powder, extra organic matter, pH adjustment, but the main factor for clearing the water appeared to be that in the jars the water was not disturbed and the silt settled out within a short space of time. This was clearly never going to happen in the filtration pond, so we could only bide out time and hope that things changed. Amazingly, after 9 months we noticed a change as the water went from cloudy to clear almost overnight. Evidently once the clay started to flocculate, the process was self-perpetuating and all through that summer we had a clear filtration pond. We were then, at last, able to start the circulation and turned on the pool pump 9 months after it had first been installed.

Silt filled water just after filling the pond in March 2020
Using a homemade Secchi Disk to measure water clarity. It started to become obscured after just 15cm in June 2020.
The water is still filled with silt 6 months later in September 2020. Note how little growth there has been on the margins despite planting with seeds and cuttings.
Filtration pond by March 2021. The pipe is clearly visible on the bottom at 1.5m depth.
The fine filamentous structure of Nitella hyalina may well have helped the silt to flocculate out of the water

It is hard to explain what caused this change in the water. It could have been the plants had finally started to grow as the water temperature warmed up, but it could equally have been due to some unseen organic elements in the water. One thought is that it is around this time that we first saw the Nitella and it began to increase. One cannot say with certainty that it was the Nitella (or possibly some other algae) that caused the clay to flocculate as opposed to the Nitella was able to appear because the water started to become clearer. However, it is clear that patience is vital to establishing a pond if using natural earth surrounds as opposed to a concrete casement for the filtration pond where we would not have had this particular difficulty.

The clarity of the water has mostly persisted from that point. There was abnormally heavy rain in May 2021 and this brought in a lot of excess silt from the edge of the pond as the plantings were still growing and much of the ground was bare. The water remained cloudy for most of the winter after that but cleared much quicker than before once it warmed up. We expect as the marginal plantings grow, the water will be less susceptible to disruption from heavy downpours. In summer, when we have allowed the water level in the filtration pond to drop too much, we have noticed a slight green coloration coming in due to algal growth. We presume this is due to the increased heating of a shallower pond. Restoring the water level has also restored the balance to the pool. The only other maintenance we have had to carry out is occasional vacuuming of the swimming pool area to remove organic matter from the bottom. This organic matter does not harm the water quality but aesthetically it is nicer to not have it floating around when people are swimming. We also have to be alert for any frogs that decide it is nicer to spawn in the swimming pool rather than the filtration pond and remove the frogspawn with a net when it appears, and returning it to where we would have preferred it had been laid in the first place.

Filtration Pond
The water in the filtration pond is now staying clear as this photo shows taken in January 2022
Frogs are welcome in our filtration pond, and occasionally we find them in the swimming pool too.

Altogether the natural swimming pool has been a huge success and we look forward to seeing it develop as time goes on. The pond plantings will only improve the water quality over time but already the water is beautiful for swimming in. Management of the pond will be required to ensure no one plant dominates, but hopefully a balance will be achieved in time. Wildlife is also coming into the pond already, with lots of frogs and dragonflies, as well as other pond life. The surrounds will also grow up so that it all blends in with the landscape (as well as reducing silt run-off into the pond). And we look forward to the succession of flowers that that will bring and will frame that beautiful view across the Overberg. We hope you will enjoy it during your stay at Phillipskop as well.

Natural swimming pool at Phillipskop
Clear water in the natural swimming pool

More information on the design and construction of our pool can be found in these posts:

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