pH is a measurement using a scale of 0-14 to represent how acidic (0-6) or basic (8-14) water is, with 7 being neutral. pH stands for "potential Hydrogen" and is actually the measure of the relative amount of free hydrogen ions (H+) and free hydroxyl ions (OH-) available in water. A water molecule (H2O) is made up of one H+ ion and one OH- ion, so these ions are always present in water as water molecules collide, split into these two ions, then reform into a water molecule. If these two ions are equal as they are in pure water, then the water will be neutral (pH of 7 SU). If there are other ions present which bond with either an H+ ion or a OH- ion, the ratio changes causing a shift in pH. Water that has more unbonded H+ ions is acidic and water that has more unbonded OH- ions is basic. The pH scale is in logarithmic units, which means that for each whole number on the pH scale there is a 10-fold change in acidity. For example, water that has a pH of 6 is 10 times as acidic as water with a pH of 7, and water with a pH of 5 would be 10 times 10 (two steps down on the scale) or 100 times as acidic as water with a pH of 7. The logarithmic pH scale does not have a typical unit of measurement like many of the other parameters we measure. Instead the units of measure for pH are referred to as Standard Units (SU).
pH affects and is affected by most chemical and biological processes occurring in the water, and can be monitored to detect when chemical changes occur in the water whether due to pollution or other naturally occurring environmental processes. pH determines the solubility (amount that can be dissolved in water), transport (movement through the environment), and bioavailability (amount available for plants and animals to use) of many chemical substances such as nutrients and heavy metals. The state of some of these chemicals can be changed by pH, and increase exposure of aquatic plants and animals to these chemicals which can cause harm at particular levels. pH is also one of the critical factors limiting the distribution of aquatic organisms. Although some organisms are specialized and can live in more extreme pH environments, most aquatic organisms require water with a pH between 6.5 and 8.5 SU to be healthy. Frequent changes in pH or sustained pH levels outside an organism's preferred range can cause health issues, reproductive problems, and eventually death. Even small changes in pH can change the diversity or community composition of organisms in aquatic habitats.
The buffering capacity of water (the ability of water to resist changes in pH) has a lot to do with the presence and concentration of carbon dioxide (CO2), bicarbonate (HCO3-), and carbonate (CO32-) dissolved in the water. Rainwater is naturally slightly acidic (pH ~5 SU) due to the atmospheric CO2 combining with the water (H2O) and forming carbonic acid (H2CO3) and lowering the pH. Acid rain occurs when the atmosphere contains higher than normal amounts of nitric and sulfuric acids primarily from emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) resulting from burning fossil fuels. Rain is considered acidic when it reaches a pH of around 4 SU. Surface water runoff will also pick up both natural and man-made chemicals along the way that can change the pH. One of the naturally occurring processes is the production of tannins or tannic acid from decaying plants (organics) which when dissolved in water causes the water to turn a dark, brown tea color and lowers the pH. The pH of groundwater, is affected by the rocks and chemicals it contacts beneath the ground, as well as the quality of the water being recharged or percolating down into the aquifer from the surface. In Florida, the limestone in our aquifer is made mostly of calcium carbonate (CaCO3). Acidic water will react with the carbonate in the limestone, dissolving the limestone, and neutralizing the acid to a pH around 7 SU. This is one of the processes that help create cave and cavern systems of our springs. Plants growing in the water also play a role in affecting the pH. During the day plants photosynthesize producing both oxygen and bicarbonate ions which shift the pH upward. At night those same plants respire and release carbon dioxide which lower the pH.