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Re: Aquatic Plants Digest V3 #92
First I wrote:
> If your tap water comes out with a high pH (often over 9) and the pH drops
> after its exposed to air, then your public water system probably uses a
> bulk water-softening method (lime softening) that as a side effect strips
> all CO2 out of the water and increases the pH to high numbers - sometimes
> near 10.
>
And Dennis replied:
> This makes sense to some extent but I'm not on municiple water and own my
> own salt brine type softner.
Oops. Well, silly me.
> My out of the tap pH is usually between 8.4 and 9.0 which seems
> extremly hard to me. Aging the water brings is down to the mid 7's and CO2
> easily drops it to just under 7.0. Could the water softner be causing the
> increase?
> Also my total hardness measures 22 grains pr gallon while calcium hardness
> reads near 0. I'm thinking it is time for RO water.
>
I can't say much (accurately) without a lot more detail about your ground
water. Normally potable groundwater contains CO2 at or slightly above
atmospheric saturation levels and there is little change when it is
exposed to air.
Regular point-of-use water softeners don't change that. They use a
zeolite medium that is charged with sodium from the salt brine. When your
water is run through the charged-up zeolite, sodium from the zeolite is
traded for calcium and magnesium from the water. The zeolite is recharged
periodically with more sodium to keep it from "filling up" with calcium
and magnesium. This changes your hardness, but it doesn't alter the pH in
any way.
There are other softening methods, and modifications of this method and I
suppose that some of those could alter the pH. It should be fairly easy
to compare the pH of softened and unsoftened tap water to see if your
softener is somehow creating the effect.
The fact that your pH drops on exposure to air indicates that something
has acted to reduce the CO2 concentration below atmospheric saturation
levels. For instance, if carbonate minerals are forming from solution, or
very low oxidation potentials are allowing methanation, then that could
drop the CO2 concentration. But since your pH drops to 7.0 on exposure to
air it seems very unlikely that you have enough bicarbonate or carbonate
in the water to precipitate minerals. Methanation would be accompanied by
some other unusual conditions (nitrate and sulfate completely absent, iron
and manganese often in high enough concentration to discolor laundry,
often some pretty offensive odors).
Otherwise, there seems to be some confusion here. pH doesn't measure
hardness, and values of 8.4 to 9.0 can be found in very soft water.
Calcium hardness of 0 means that your softener is working. The "total
hardness" of 22 grains per gallon is confusing. That is about 380 ppm.
It would be extremely unusual for you to have no calcium but a total
hardness of 380 ppm, and even more unusual in softened water.
Perhaps your "total hardness" is actually total dissolved solids? If so
then your water is fairly fresh and it wouldn't be surprising for it to
reach a pH of 7.0 after it ages in the presence of air.
Your plants probably don't need an RO system, but they might actually need
the calcium that you're softening out of the tap water.
If you're interested, I wouldn't mind more discussion off the list, but it
really isn't on-topic here.
Roger Miller