You might want to ...
Published by Tom Mathews, Engineering Manager
You might want to doublecheck your water analyzing procedures.
Rainwater should have a pH of 5.6 to 5.8, simply due to the presence of carbonic acid (H2CO3) from the atmosphere.
If the ph is between 4-5 units, that is indicative of a problem. The rainwater may be contaminated. The uric acid levels in bird droppings is high; reaching pH levels of somewhere between 3 to 4.5 – quite acidic.
Rainwater should have relatively low buffering capacity so that it should not take much of any alkaline chemical to raise the pH. Soda ash is an inexpensive and relatively harmless chemical for this purpose.
The water should also be disinfected.
Before treating the water, you should investigate further to determine the source of the contamination.