Treating Iron in Bore Water @Avowest via Oxygenation
The last block of land purchased by Avowest has a deep bore water supply. The bore, taps the Leederville aquifer and is nearly 300m deep. The water quality is better than most of the water I have to irrigate avocados, but at purchase, it contained about 10 mg/L of iron.
Typically, the iron in bore water exists as ferrous iron, which is soluble in water. As irrigation occurs and the water is exposed to oxygen the iron changes to its ferric form which is not soluble. The iron then precipitates and forms its characteristic brown stain on surfaces. It also precipitates inside pipes and sprinklers and blocks them up over time.
There must be a better way..
We decided it was better to try and deal with the iron problem up front than have to continually handle the results of iron build-up in the irrigation system.
Adrian and I visited a couple of farms where they were pumping water to a high point and letting it run through a channel to get oxygen exposure before settling in a dam. The systems were reasonable but there was still iron in the water being pumped through their irrigation systems.
Reviewing literature on treating iron made it apparent that there needs to be a lot of oxygen exposure to water with ferrous iron to get it to precipitate out rapidly. To achieve this, we devised a plan to spray the bore water in the air, collect it and drop it through three waterfalls then let it collect in a top dam. After two days hopefully all the iron has precipitated, and the water could be drained from the top dam into a bottom dam. This would be the water for irrigating our trees.
It seemed pretty simple, but between COVID, extreme weather and falling avocado prices it has taken nearly three years to get it up and running. This is the process and the results.
Step 1 – Build Dams
The first step was to build two 7.5 ML dams. The site at Gingin had plenty of slope so it was easy to build one above the other. The top dam is an iron settling dam. The bottom dam is water storage.
There is no clay in the subsoil, so the dams had to be plastic lined. The dams were built in the summer of 2021. There were so many hot days, which had bans on using machinery, that the contractor set up lights and built the dam at night.
There were also lots of shenanigans building a dam in the middle of mining boom and trying to get contractors to do the plastic lining. Patience was sorely tested.
But we ended up with two dams with some water in them.
Step 2 – Water Oxygenation
The next step was to set up the water oxygenation. Our first spray point became known as “The Candle”.
Step 2 – part 2 – setbacks
There were one or two minor setbacks on the way.
Step 3 – Setting up our waterfalls
Best said with pictures… and videos…
Step 4 – or should I say Step 4 thousand & one
Finally by May 2023 we were able to test to see how well it worked.
On the serious side,
I took the first sample from the bore before there was any exposure to air. The second sample came from the last waterfall as the water flowed into the settling dam. The third sample came from water that had been in the settling dam for three days and was being emptied into the bottom, storage dam. Measuring ferrous iron. The lower the value the better it is for irrigation.
- Bore sample = 8.2 mg/L ferrous iron
- Bottom waterfall = 7.9 mg/L ferrous iron
- Bottom dam = 0.06 mg/L ferrous iron
We have a heap of work to do to make it pretty and there is still some fine tuning to be done. A big thanks to Mike Healy for his unwavering support and enthusiasm.