As a retired biogeochemist I can tell you that it is a matter of the source of acidity or alkalinity in the soil and water. Soils can be basic for example if they have limestone in them. Acid soils usually have organic matter which contributes organic acids however there can also be acidic minerals, such as sulfides and others, that form acids when they oxidize or break down. When you apply an alkaline water (pH>7) it will react with the acidic components and whichever is in greater abundance will tend to win out. Because we tend to water a lot, the balance is usually shifted in favor of the water and whether it is acidic or alkaline. So in your case your water is probably greatly tipping the soil towards alkaline conditions. Eric Schrader has a good YouTube video on the subject on checking pH and treating alkaline water (see below). Public water systems add limestone (CaCO3) to their water to help prevent leaching of lead and other trace elements from copper pipes and old lead solder so in those cases expect your water to be in the pH 7-8 range. The same is true if your drinking water comes from wells in limestone bedrock However if you use organic fertilizer it will usually add acidity but not likely as much as we’d like. In this cases I am talking about soils in pots however both natural and agricultural soils can undergo different acidification processes.
