Low pH of the rain water is ...

Low pH of the rain water is usually caused by air pollution. We had the same problems in the 80s and 90s of the last century here in Germany. We called it "acid rain". It can destroy trees and whole forests. The reasons are sulphur oxides and nitrogen oxides from traffic, industries and power plants who don´t have sufficient exhaust gas treatment plants. The gaseous pollutants react with the humidity in the air to sulphurous acid and nitrous acid which leads to a low pH in the rain water. The low pH in the rain water can be adjusted by using Calcium Carbonate or Dolomite filters. They remineralize the rain water with Calcium and / or Magnesium and increase the pH at the same time. But you should be aware that the sulfate and sulfite as well as nitrate and nitrite anions are still remaining in the water. Especially nitrite is toxic and for nitrate there are limit values given for the use as drinking water. So please make water analysis before you use this water as potable water. If there is nitrite in the water you cannot drink it. I don´t know the limit value for nitrate in drinking water in your country. Here in Germany the limit value is 50 mg/l. Lots of experts think that this value is still too high and should be lowered.