One of the most basic concepts associated with water and life on earth is pH. Four billion years ago, the entire surface of the earth was covered with water, and today 71% of its surface is still under water. Life evolved in water, and living cells require a water medium in which to carry out the reactions that sustain life. Whether on the earth's surface or in living cells, water contains two ions: hydrogen (H+), which is acidic, and hydroxyl (OH-), which is basic. These two ions strongly associate with each other, and are used to measure the acidity of water on a scale of 0 (acidic) to 14 (alkaline) pH units. The pH determines the rate of many biochemical reactions in streams and rivers. Also, pH affects the availability of certain constituents in the water (e.g. zinc toxicity generally has a larger effect on aquatic organisms at a low pH while ammonia is more toxic at a higher pH). Aquatic organisms each have an optimal pH range for functioning, so extremes on either end of the pH scale can be toxic.

Normal cellular functions take place in the pH range of 7.2 to 7.5. Organisms living in water can tolerate a wider pH range than can the interior of cells. This is because cell membranes maintain the internal pH at a constant value while natural aquatic habitats tend to fluctuate. Brown trout adults can tolerate pH values from 5.0 to 9.5, but prefer the narrower range of 6.8 to 7.0. Aquatic insect survival is optimal in the narrower range of 6.5 to 8.5. There isn't much change in pH from upstream to downstream outside of the neutral range. However, pH will change daily depending on biological activity and sun exposure. The state standard for pH in the watershed is 6.5 to 9.0.

Sources of acidic discharges into streams, such as mine drainage or acid rain produced by fuel combustion, can lower the pH (increase the H+ ion concentrations) to a point at which aquatic life either becomes stressed or can no longer survive. Additionally, waste water treatment plants may introduce ammonia to stream ecosystems. Ammonia in its unionized form is toxic to aquatic life at high concentrations. Both temperature and pH affect the levels of ammonia and are important factors in determining the ability of the stream to assimilate ammonia up to the instream standard.

pH Scale