Depending on the chemicals in the soil, the pH of water passing through it increases, decreases or remain the same. In areas where water passes through soil containing limestone, dissolving the calcium carbonate in water creates a buffer solution, tying up the hydrogen or hydroxyl ions in the water so the pH becomes less likely to change.
The pH of pure water is 7.0. This indicates it has an equal concentration of hydrogen and hydroxyl ions. However, water is a powerful solvent, dissolving many ionic compounds that affect the concentration of hydrogen ions. The scale used in measuring pH is logarithmic, so a drop in pH from 7 to 6 indicates a tenfold increase in hydrogen ions, creating an acetic solution. Pine needles create an acidic soil, so the compounds affecting the acidity of the soil dissolve in water and lower its pH. Wood ash is basic in nature; streams near the remains of forest fires have a higher pH due to water dissolving the lye from the ash as it passes through the area. The air also affects the pH of rain. Excess carbon dioxide in the air due to human pollution causes the pH of water to drop; however, acid rain dissolves more limestone and increases the buffering of the water.