By Stephen Wiman
In addition to its effects on biological and chemical processes, the variability of pH affects our decisions in domestic water usage. High-pH water often tastes bitter and may be an indication of the scaling potential of the water. Low-pH water may lead to the dissolution of pipes, particularly copper pipes. The EPA classifies pH under unregulated Secondary Drinking Water Standards and recommends a range between 6.5 and 8.5 pH units.
The term "pH" refers to the potential of hydrogen. The scale measures the logarithmic concentration of hydrogen ions (H+) and hydroxyl ions (OH-), which constitute H2O. Because the scale is logarithmic, a change in pH by a factor of 10 results in a change of one unit on the pH scale - the pH scale ranges from 0 to 14 and is a measure of the acidity/basicity of water. A neutral solution, with a pH of 7.0, is achieved when the activity ofH+ and OH- is balanced. Water that has more free hydrogen ions (over 7) is acidic and water that has more free hydroxyl ions (less than 7) is basic or alkaline. Acids lower the pH of a solution and bases raise the pH.
Well water is typically high in ions (both positive cations and negative anions) such as calcium and magnesium (the "hardness" minerals), sodium, potassium, nitrate, chloride and sulfate. The presence of these ions decreases the activity of the H + ion and increases the activity of the OH- ions, causing the water to be higher pH. Well-water pH is a function of the minerals taken into solution as the water moves through rock strata. In Santa Fe, our municipal water, which is commonly a blend of sources, has a pH ranging from 7.04 to 8.21, with both the low and high ranges occurring in Buckman Well Field water (2010 Water Report, Sangre de Cristo Water Division).
The term "alkaline" should not be confused with the term "alkalinity;' which refers to the "buffering" capacity of water, or its ability to resist or "buffer" changes that would make the water more acidic. The main sources of natural alkalinity, which limits swings in pH levels, are rocks containing carbonate, bicarbonate, and hydroxide compounds. Borates, silicates, and phosphates may also contribute to alkalinity.
Conversely, granite, which is a common aquifer in the Santa Fe foothills, has few minerals that contribute to alkalinity. Areas rich in granite have generally low alkalinity and therefore poor buffering capacity. We sometimes see low pH in water produced from fractured granite aquifers.
In general, reverse osmosis (RO) water, although extremely pure, has inherently low pH. This is not because of the RO process per se, but is a function of the fact that RO water has such low total dissolved solids, or mineral ions, that it has little or no buffering capacity. The easiest way to raise the pH of RO water to a more palatable, and less corrosive, pH level above 7.0, is to pass it through a food- grade, NSF-certified calcium carbonate (calcite) media filter. If you buy bottled water, most of which is mass-produced by RO, you can bet that the pH has been adjusted upward.
Stephen Wiman has a background in earth science (Ph.D. in geology) and is the owner of Good Water Company in Santa Fe. He can be reached at 505-471-9036 and skwiman @ goodwatercompany.com.Related Links:
What is pH and How is it Measured
Good Water Company
Is Rainwater Really Safe?
The Drinking Water Book - How to Eliminate Harmful Toxins from Your Water