A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 3 - Streams


pH in Streams

Why Is It Important?

The pH of a sample of water is a measure of the concentration of hydrogen ions. The term pH was derived from the manner in which the hydrogen ion concentration is calculated – it is the negative logarithm of the hydrogen ion (H+) concentration. What this means to those of us who are not mathematicians, is that at higher pH there are fewer free hydrogen ions, and that a change of one pH unit means there is a tenfold change in the concentration of the hydrogen ion. For example, there are ten times more hydrogen ions available at a pH of 7 than at a pH of 8. The pH scale ranges from 0 to 14. A pH of 7 is considered to be neutral. Substances with pH less than 7 are acidic, while substances with pH greater than 7 are basic. The pH of most natural waters ranges between 6.5 and 8.5.

The pH of water determines the solubility (amount that can be dissolved in the water) and biological availability (amount that can be utilized by aquatic life) of chemical constituents such as nutrients (e.g., phosphorus, nitrogen, and carbon) and heavy metals (e.g., lead, cadmium, copper). For example, in addition to determining how much and what form of phosphorus is most abundant in the water, pH also determines whether aquatic life can use it. Heavy metals tend to be more toxic at lower pH because they are more soluble and more bioavailable.

Reasons for Natural Variation

Geology of the watershed and the original source of the water determine the initial pH of the water. The greatest natural cause for change in pH in a stream is the seasonal and daily variation in photosynthesis. Photosynthesis uses up hydrogen molecules, which causes the concentration of hydrogen ions to decrease and therefore the pH to increase. Respiration and decomposition processes lower pH. For this reason, pH is higher during daylight hours and during the growing season, when photosynthesis is at its peak.

Although pH may be constantly changing, the amount of change remains fairly small. Natural waters are complex, containing many chemical "shock absorbers" that prevent major changes in pH. Small or localized changes in pH are quickly modified by various chemical reactions so little or no change may be measured. This ability to resist change in pH is called buffering capacity. No only does the buffering capacity control would-be localized changes in pH, it controls the overall range of pH change under natural conditions. The pH scale may go from 0 to 14, but the pH of natural waters hovers between 6.5 and 8.5.

Expected Impact of Pollution

Because polluted conditions typically correspond with increased photosynthesis in a stream, pollution may cause a long-term increase in pH. The more common concern is changes in pH caused by discharge of municipal or industrial effluents. However, most effluent pH is fairly easy to control, and all discharges in Washington State are required to have a pH between 6.0 and 9.0 standard pH units, a range that protects most aquatic life. So, although these discharges may have a measurable impact on pH, it would be unusual (except in the case of treatment plant malfunction) for pH to extend beyond the range for safety of aquatic life. However, since pH greatly influences the availability and solubility of all chemical forms in the stream, small changes in pH can have many indirect impacts on a stream.

pH is expressed in terms of pH units. The Washington State water quality standard for pH in Class AA, A, and B streams states that pH must fall within a range of 6.5 to 8.5. For Class C streams, pH must fall within a range of 6.5 to 9.0. Because pH represents the antilog of a number it is not mathematically correct to calculate simple averages or other summary statistics. pH should be reported as a median or range of values. The table at the right summarizes pH data to provide a comparison for three Western Washington streams.

Summary of pH Data (pH units) Collected from Three Western Washington Streams During 1988-89

  Yearly Median

Summer Range (May-Oct.)

Winter Range
(Nov.- Apr.)

Cedar River 7.6 7.4 - 7.9 7.2 - 7.5
Newaukum Creek 7.7 7.8 - 8.0 7.4 - 7.6
Springbrook Creek 7.0 6.9 - 7.2 6.7 - 7.0


Revised From: Metro 1990. Quality of Local Lakes and Streams 1988-89 Status Report. Municipality of Metropolitan Seattle, Water Resources Section.

The next section discusses nutrients in streams.

Chapter Four provides information about how to measure pH in streams.


Stream Temperature | Stream Dissolved Oxygen | Stream pH | Stream Nutrients | Stream TSS and Turbidity | Stream Fecal Coliforms | Return to Table of Contents