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pH dGH question from water quality report

Andy Pierce

Member
Joined
27 Nov 2020
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672
Location
Cambridge, UK
@dw1305 is probably the right person for this one!

I'm looking at my 2021 water quality report for an area 8 miles southeast of Cambridge: Linton water quality zone. The water is very hard with dGH 17.8 at 317 ppm CaCO3. The water has mild salinity of 612 uS/cm which I convert approximately to 306 ppm overall. They list the pH as 7.40. So then from the pH and the dGH at a room temperature of 20C we can calculate dissolved CO2=15.664*dGH*10^(6.34-pH) which comes out to 24.3 ppm.

Can the dissolved CO2 at 24.3 ppm possibly be that high straight out of the tap? I don't see any fizzy carbonation bubbles... ;) How do they get the pH so low with such a lot of carbonate in the system; where am I going wrong with this one...?
 
For CO2 calculations one looks at the KH not the GH. The listed value of 317 is the General Hardness.

Edit: This link suggests the Alkalinity (Also KH/Carbonate Hardness) in Linton was high at 290mg/l in 2019 so your source CO2 is probably around 19.3ppm which is still quite high.
 
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Hi all,
The water is very hard with dGH 17.8 at 317 ppm CaCO3. The water has mild salinity of 612 uS/cm which I convert approximately to 306 ppm overall. They list the pH as 7.40.
The looks about right, the water is fully saturated with Ca++ and 2HCO3- ions from the CaCO3 picked up <"from the chalk aquifer">. The pH value is lower than the carbonate ~ CO2 equilibrium value, but the water sample may have been <"under pressure"> immediately before <"bottling for testing">. Water can hold an <"immense amount of CO2">, and CO2 is very soluble, so it only needs a little excess CO2 to lower the pH, but as soon as that is out-gassed to the atmosphere pH will rise.
Can the dissolved CO2 at 24.3 ppm possibly be that high straight out of the tap? I don't see any fizzy carbonation bubbles..
No, it may have been slightly higher than 3 ppm CO2, but it will be <"that sort of value">.

cheers Darrel
 
Hi all,
but I've thought they would release data for water in standard conditions. Or wouldn't they?
There will be a standard protocol, but I'm guessing that it isn't always rigorously followed.

In the UK water companies are now privately owned and very much run for profit. <"We used to visit the lab. at Wessex Water">, but when they were <"purchased by ENRON that stopped"> and I'm told that they now (under Malaysian ownership) they run a very tight ship and are purely interested in complying with the minimum statutory requirement.

cheers Darrel
 
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