What is pH?

pH is the measurement of the concentration of hydrogen ions in the water. pH is measured on a logarithmic scale (0-14 pH units), with values on the lower end of the scale considered acidic (high levels of H+ ions) and higher values considered basic (lower levels of H+ ions). The carbonate system is a natural process that maintains pH equilibrium in waters impacted by the photosynthesis and respiration of aquatic plants and decomposition by bacteria. Pollution from sewage overflows, stormwater runoff, carbon dioxide emissions, chemical spills and acid precipitation can negatively impact healthy pH levels.

Why is pH important?

Consistent and moderate levels of pH are critical for the survival of aquatic life, because pH affects the availability of essential nutrients and minerals, as well as the presence of toxic substances in the water. Acidic waters (low pH) impede the ability of freshwater invertebrates to build their calcium carbonate shells. Low pH levels also increase the availability of phosphorus, driving algae blooms that can ultimately result in harmful decreases of dissolved oxygen. Basic waters (high pH) also increase the availability of ammonia, which is toxic to aquatic life.

How do we measure pH?

pH is measured with a probe that is lowered by staff or volunteers into the water at each station location. pH readings are recorded half-way between the water’s surface and the river bottom. We assess the pH data using the State of Maryland’s instantaneous numeric threshold range of 6.5 – 8.5 (SU pH), which ensures the protection of the ecological health of animal and plant populations in the State’s streams and rivers from harmful acidic or basic pH conditions.