Pure water with no impurities not in has a pH of 7. At pH values below 7 we have excess acid, and at pH levels above 7 we have excess base in the water. Natural, unpolluted rain has a pH between 5 and 6, so even rain water from the least polluted place on Earth has some natural acidity. The most acidic rain has a pH of about 4. Most lakes and streams have pH's in the range of 6 to 8.
Some water is more acidic in areas with minerals in the soil.
Mining can also release acid-causing minerals to a stream. There is a great amount of limestone in our area. It can be the reason for some acidity it our water. The pH of a water body has a strong influence on what can live in it. Amphibian life can be sensitive to low pH. Most insects, amphibians, and fish are no found in water bodies with pH below 4.