pH changes in well water

Asked September 2, 2016, 5:12 PM EDT

The well water that comes out of my tap has a pH of 7.4. However as it sits (for example in a jar) and the surface is exposed to the air the pH will drop over a period of about a week until it stabilizes at 8.3. This is problematic because I'm having a difficult time acclimating tropical fish to the aquarium as the fish cannot adjust to the abrupt pH change. My well is 180 feet deep an drilled through the Precambrian shield (granite). What causes this dramatic pH change?

Outside United States

4 Responses

My background is in drinking water, not aquariums. I have seen that pH can change in water exposed to air. Based on the type of bedrock you describe, I am actually surprised your pH isn't lower to begin with. Are you treating the water at all before adding it to the tank? I am also surprised that your water is increasing in pH, rather than decreasing as it is exposed to air. This site, while not from our program, of course, seems to have some suggestions: Sorry I can't be of more help!
Perhaps try talking to an aquaculturist?

Thanks for your prompt reply. However, I'm still at a loss to why the pH changes as the water sits undisturbed in a clean glass container. It does take time – about one week before the change begins, but within a a few days after the pH stabilizes at pH 8.3 This is water straight from the well with no treatment.There must be something in the well water that reacts with air to change the pH so dramatically. As for the fish I'm aware of several methods to adjust pH, which are not always simple or satisfactory. I'm married to an aquaculture specialist who has taught university level courses on aquaculture and limnology, but he also can't explain the pH change. I think I need a water chemistry/geology expert.

Normally, pH in water decreases with exposure to air because of the carbon dioxide present in the air. displaystyle ce CO2H2OHCO3-H
I am not sure why yours would be increasing. Is this only after it's been added to the tank, or does it happen outside of the tank as well?
This is the geologist I would recommend, who would have experience with similar bedrock to what you have:

Good luck!

The pH change doesn't seem to have anything to do with the tank. I took a sample of water and added it to a clean,glass jar and the pH changed within the week. So who Knows what's happening?