As stated previously, the pH of pure water at 25° is 7.0, Rain water normally has a fairly low pH because of its high content of carbon dioxide and because sulfate is often present as free acid. Usually the pH of rain is raised as the water percolates through the soil and picks up calcium and magnesium ions.
However, in boggy areas and in some types of rain forests, the rain water enters the streams channels without much contact with mineral soils. This produces streamwater with pH less than 7. River and stream water in areas not influenced by pollution generally has a pH between about 6.5 and 8.5.
Most groundwaters found in the United States have pH values ranging from around 6.0 to8.5, but water having lower pH is not uncommon in thermal springs. Water with a pH much over 9.0 is unusual, but by no means unknown.
Where photosynthesis by aquatic organisms takes up dissolved carbon dioxide during the daylight hours, a diurnal pH fluctuation may occur and the maximum pH value may sometimes reach as high as 9.0.
The pH range of lakes having some degree of flow through the basin is generally about 6 to 9. In limestone regions, the dissolved carbonates may extend the pH range considerably beyond 9. In basins lacking outlets, evaporation may concentrate alkaline substances, resulting in pH readings of over 12. At the other extreme, accumulations of acids such as sulfuric acid in volcanic lakes, gives a pH as low as 1.7.
Natural waters whose pH is below 4.5 contain a low proportion of disassociated to undisassociated carbon dioxide species and many such waters have other sources of hydrogen ions. Most commonly the source is a reaction that involves oxidation of some form of sulfur. Waters whose pH is controlled by equilibria involving carbon dioxide species and solid calcium carbonate are susceptible to increases in pH of a full unit.
The pH of a water sample can also be affected by oxidation of ferrous ions.
The hydrogen ion concentration of a raw water source for domestic water is important in that it affects taste, corrosivity, efficiency of chlorination and treatment processes.
The optimum pH for irrigation water depends on the types of crops being grown and the physical and chemical properties of the soil. In the Western United States where alkaline soils predominate, waters with low pH values are desirable. In Eastern acidic soils, where liming is practiced, waters of moderately high pH are not detrimental to most crops.
The pH of most inland waters that support fish range between 6.7 and 8.6 with extremes of 6.3 and 9.0. Of the U.S. waters that have a good fish fauna, only 5% have a pH less than 6.7, 50% have a pH less than 7.6 and in 95%, the pH is less than 8.3.
The permissible range of pH for fish depends upon many other factors such as temperature, dissolved oxygen, prior acclimatization, and the content of various anions and cations. The pH value determines the degree of disassociation of weak acids and bases, some of which may be more toxic in the molecular than ionic form. Some of the minimum and maximum pH values and recommended ranges in pH as reported in the literature are given in the following tabulation for fish. For other aquatic organisms, ranges were reported as follows:
TABLE GOES HERE
When the pH falls below 5.0, specialized flora and fauna develop and some species have been observed at pH values as low as 2.9.
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