pH Levels of Stormwater FAQs
pH is a key water quality parameter influencing a wide array of chemical interactions in water. The solubility of metals, the ionization of ammonium, and boundary layer kinetics are all affected by pH. pH is a measure of the hydrogen ion concentration in solution indicating the relative presence of acids and bases. Since pH is measured on a logarithmic scale, a shift in pH by 1 unit indicates a ten-fold increase or decrease in the concentration.
The pH of stormwater is slightly higher than that of rainwater due to the alkalinity that stormwater picks up when encountering paved surfaces. Rainwater has a pH of 5.6. A common range of pH values for stormwater is 6.5 – 7.2.[vii] Carbon dioxide and sulfur dioxide are prevalent emissions that reduce the pH of surface water.
pH should be neutralized in stormwater runoff because stormwater runoff ends up in waterways used by fish and for drinking water, bringing its pH level with it. Extremely high or low pH substances can damage human tissue on contact.[ii] Chronic pH imbalances in the human body can cause effects ranging from muscle weakness to death.[iii] From an aesthetic standpoint, both high pH (over 8.5) and low pH (under 6.5) water have an unpleasant taste.[i]
Negative effects of pH imbalance on the environment include:
- aquatic invertebrates with exoskeletons or shells having sensitivity to pH lower than 7[iv] which can impede shell formation;[v]
- trout eggs and larvae only tolerating a narrower pH range around neutral;[vi]
adult fish having non-lethal adverse effects at the high and low ends of this range;[vii] and
- low pH within this range possibly killing fish if metals are also present in the water, because low pH can bring metals to a more dangerous dissolved state.[viii]