It should be acidic. The modern industrial revolution, starting with the steam engine, the boiler burns coal, produces steam and drives the machine; As a result, the coal consumption of thermal power plants has increased rapidly. Unfortunately, coal contains impurity sulfur, about 1%, which will release acid gas SO2 when burning. The high temperature produced by combustion can also promote some chemical changes in the combustion-supporting air. Oxygen combines with nitrogen and releases acid gas nitrogen oxides. They were washed and dissolved by rain and snow in the sky, and the rain turned into acid rain; These acidic gases become impurities such as sulfate, nitrate and ammonium ions in rainwater. Rural rainwater contains ammonium carbonate, which is not acidic; The rainwater in the suburbs contains ammonium sulfate, which is slightly acidic; Urban rainwater contains sulfuric acid or acidic sulfate, which is acidic, so river water should also be acidic.