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pH is a measure of a water's acid/base content and is measured in pH units on a scale of zero to 14. A pH of seven is neutral (distilled water), while a pH greater than seven is basic/alkaline and a pH less than seven is acidic.The pH level of surface water is influenced by the concentration of acids in rain and the types of soils and bedrock in an area. The typical pH of rainfall in the U.S. is slightly acidic, ranging from 5.0 to 5.6. As rainwater falls, it dissolves carbon dioxide from the atmosphere, thus forming a weak carbonic acid and lowering the pH of the precipitation. Low pH levels (acidic) can have a harmful impact on the health of aquatic communities. Very acidic water or acid rain can directly harm aquatic life and can also allow toxic substances, such as ammonia and heavy metals, to leach from our soils and possibly be taken up by aquatic plants and animals (bioaccumulation).Even with the natural inputs of acidic water,the pH of Iowa surface waters generally range from 8.0 to 8.4. The presence of alkaline (basic) soils and limestone bedrock in many areas of the state help neutralize the effect acidic precipitation might have on Iowa's streams and lakes. This is quite fortunate for Iowa since pH can influence many chemical and biological processes. Most aquatic organisms require habitats with a pH of 6.5 to 9.0.Extremely high or low pH values are rare in Iowa. Most values that exceed 9.0 (basic) are caused by excessive algal growth, a sign of nutrient enrichment. Very low (acidic) pH readings are generally near point sources of pollution.