the lower fox river watershed monitoring program    
University of Wisconsin Green Bay
University of Wisconsin Milwaukee


pH measures the hydrogen ion concentration of a solution, which controls whether a solution is acidic or alkaline. The pH scale is logarithmic, meaning that every one unit of change in pH is a ten-fold increase or decrease in acidity. The pH scale in the environment ranges from 0 (acids) to 14 (bases), with 7 considered neutral. pH must be measured directly in the stream, since changes in temperature can affect the pH value.

The pH of stream water is influenced by many sources. First, natural rainwater has a pH of approximately 5.6, attributable to mixing with carbon dioxide in the atmosphere to form carbonic acid. Nitrous oxides or sulfur oxides from fossil fuel consumption also can mix with water in the atmosphere to form acid rain, which can substantially lower the pH of streams. Acid rain may have more or less of an effect due to local geology. Calcium and magnesium ions dissolved from the surrounding limestone bedrock in the Fox River Basin increases the buffering capacity of the stream to resist changes in pH. However, volcanic bedrock weathers more slowly, so streams in igneous regions have less buffering capacity and acid rain has a greater impact on stream or lake chemistry. Photosynthesis also influences stream pH because carbon dioxide is used by that process during daylight hours and given off by respiration at night. Because carbon dioxide dissociates in water to create carbonic acid, decreased levels of CO2 during the day results in alkaline water conditions, and increased levels of CO2 at night create acidic pH values. This pH dynamic is most visible during algal blooms, with pH in some cases exceeding 9.0.

The optimum pH for most aquatic organisms is between 6.5 and 8.5. pH values outside this range may first affect reproductive processes, and then species survival. As pH controls the dissociation of substances into dissolved ions, low pH values increase the availability of certain toxic chemicals, such as mercury, lead, iron, chromium, and other heavy metals. Also, as pH increases, the ammonium ion (NH4) changes to ammonia (NH3), which is very toxic to aquatic life.

Streams in Northeast Wisconsin typically have pH values between 7.0 and 9.0, depending on the time of year that sampling occurs. The water quality standard for most surface waters in Wisconsin to support fish and aquatic life is a pH between 6.0 and 9.0, with no change greater than 0.5 units outside the estimated natural seasonal maximum and minimum values.

Because pH is measured on a logarithmic scale, to correctly calculate an average value for replicate measurements, the values would have to be transformed logarithmically into hydrogen ion concentrations, averaged, and then transformed back into pH. Rather than perform these calculations, the median pH measurement should be reported to the LFRWMP database. pH results measured with the pH Testr 3+ should be reported to 2 decimal places.