Most North American rivers, lakes and streams fall within the range of pH 6.5-8.2. National guidelines recommend pH of 6.5-9.0 for the protection of aquatic life in freshwaters.

What is it?

pH measures the degree to which water is acidic (like lemon juice) or basic (like bleach or soap). pH is measured on a scale that ranges from 0 (strongly acidic) to 14 (strongly basic). In the middle is 7, where the pH is neutral (like in pure water).

Drawing of a pH scale showing pH values between 0 and 14. Neutral values are indicated near pH 7, increasingly acidic pH is indicated toward pH 0, and increasingly alkaline pH values are shown between pH 7 and 14. Various items (e.g., water, soap) as are depicted along the scale at their respective pH values.

Why does it matter?

pH in freshwater systems is affected by naturally occurring organic acids or by impacts from human activities, such as acid rain and acid rock drainage. When water is more acidic (has a lower pH) it can make certain chemicals and metals more toxic than normal. This is because acid waters make these elements more soluble – that is, they become more easily dissolved in the water and in this way become more biologically available to fish and other organisms.

Acid rain occurs when certain pollutants in the air (sulfur dioxide and nitrogen oxides) react with water to form acids. These acids are then deposited to the land, lakes, and rivers by rain and snow.

Acid rock drainage is a common problem in and around mines. Acidic conditions arise when sulphur-rich rocks are exposed to air and water (e.g., in tailings and waste rock piles). The acid often dissolves minerals, including metals, from the rock and both the acid and the metals can drain into waterways.

pH scale showing fish health at various stages between 0 (very acidic) and 14 (very basic). Unhealthy fish are shown at both ends of the spectrum with fish health improving at near-neutral pH levels.

Most fish prefer to live in water that ranges in pH from around 6.4 to 8.4. Fish eggs grow and survive best at a narrower range of pH; from 6.0-7.2.

How is it measured?

pH can be assessed using pH strips that change colour based on how acidic or basic the water is. The colour is evaluated against a guide to determine the approximate pH range of the water. The colour is evaluated against a guide to determine the approximate pH range of the water. pH can also be measured directly in a lake or river using a water quality meter with a pH sensor. Before use, this sensor is calibrated using standard solutions of known pH (typically pH 4, 7 and 10).

Hand drawn image of pH strips and handheld device with pH probe.

Related to pH: alkalinity, oxidation-reduction potential (ORP).

pH is influenced by: background water chemistry - especially carbonate, bicarbonate and naturally-occurring acids.

pH influences: dissolved concentrations of many metals and metalloids in water.