Water Quality 101: pH
“How Soluble is Your Water?”
By: Bryan Duggan
Coquille Indian Tribe, Water and Environmental Specialist
There is a lot of information out there on the world wide web about pH and water
quality; and they all pretty much say the same thing: pH is a measurement for determining if
a liquid is acidic or basic (alkaline) and pH is an important factor of water quality. You may
have encountered the term pH in relation to the water in a fish tank or a swimming pool.
“Bad” things can happen when pH levels are either too high or too low. What exactly
happens and why is it so “bad”? Well, lucky for you we have another wonderful opportunity
to talk about water quality and how the pH of water affects our lives and the life within our
greater ecology.
pH actually means “power of hydrogen” and refers to the concentration of hydrogen
ions (H+) in water or whatever liquid is being measured. The term pH was derived from the
manner in which the hydrogen ion concentration is calculated - it is the negative logarithm
of the hydrogen ion (H+) concentration. What this means to those of us who are not
mathematicians is that at higher pH, there are fewer free hydrogen ions and that a change of
one pH unit reflects a tenfold change in the concentrations of the hydrogen ion. For
example, there are 10 times as many hydrogen ions available at a pH of 7 than at a pH of 8.
The pH scale ranges from 0 to 14. A pH of 7 is considered to be neutral. Substances with a
pH of less that 7 are acidic; substances with a pH greater than 7 are basic.
Coquille Indian Tribe, Land Resources and Environmental Services (LRES) water
quality staff measure pH with a Hach pH meter, an electronic device with a specialized
probe. According to the Hach manufacturer “The probe contains an acidic aqueous solution
enclosed by a glass membrane that allows migration of H+ ions. The electrical potential of
the glass electrode depends on the difference in [H+] between the reference solution and the
solution into which the electrode is dipped.” As some of you (who are esteemed graduates
of middle school chemistry) know, pH can also be measured with pH litmus paper or by
adding a reagent (indicator solution) to the water sample and recording the color change.
So how does pH affect the beneficial uses of water? Well, an extraordinary property
of water is its ability to dissolve other substances. There is hardly a substance known which
has not been identified in solution in the earth's waters. Were it not for the solvent property
of water, life could not exist because water transfers nutrients vital to life in animals and
plants. The pH of water determines the solubility (amount that can be dissolved in the water)
and biological availability (amount that can be utilized by aquatic life) of chemical
constituents such as nutrients (phosphorus, nitrogen, and carbon) and heavy metals (lead,
copper, cadmium, etc.). For example, in addition to affecting how much and what form of
phosphorus is most abundant in the water, pH may also determine whether aquatic life can
use it. In the case of heavy metals, the degree to which they are soluble determines their
toxicity. Metals tend to be more toxic at lower pH because they are more soluble. A
changing pH in a stream can be an indicator of increasing pollution or some other
environmental factor.
Most aquatic animals and plants have adapted to life in water with a specific pH and
may suffer from even a slight change. Natural pH levels typically fall between 6.5 and 9.0,
depending on the surrounding soil and vegetation (more about this below). Even moderately
acidic waters (low pH) may reduce the hatching success of fish and amphibian eggs, irritate
fish and aquatic insect gills, and damage cell membranes. Water with extremely high or low
pH is deadly. A pH below 4 or above 10 will kill most fish and few aquatic organisms can
tolerate waters below a pH of 3 or above 11. Amphibians are particularly vulnerable to low
pH, probably because their skin is so sensitive to pollutants.
As would be expected in any ecological system, natural variables affect the pH of a
water body. There are three types of influences affecting the pH of a stream: 1.) watershed
effects, 2.) seasonal effects, and 3.) daily effects. Certain types of plant communities and
geological formations will influence the overall pH of watersheds. Pine or fir forests in your
watershed may lower the pH value for your stream; the decomposing needles of these trees
add to the acidity of soils and also influence the acidity of nearby streams. Certain dissolved
minerals, such as calcium carbonate (from the erosion of rocks), can combine with the extra
hydrogen or hydroxyl ions that alter water’s pH. When these minerals are present, the pH of
the water doesn’t change as much when acids or bases are added to the water. We call this
buffered water. Many soils in our part of the West contain these minerals. Not only does the
buffering capacity control would-be localized changes in pH, it also controls the overall
range of pH change under natural conditions. The pH scale may go from 0 to 14, but the
pH of natural waters hovers between 6.5 and 9.0. For seasonal effects, falling precipitation
dissolves atmospheric carbon dioxide (CO2) and forms a weak acid. During the daily cycle,
aquatic plants photosynthesize, and remove CO2 from the water; this can raise the pH in
your stream. Since photosynthesis occurs only when light is present, the highest pH in the
stream occurs in the late afternoon. Likewise, the lowest pH levels (acidic) will occur just
before sunrise.
When pollution results in higher algal and plant growth (e.g., from increased
temperature or excess fertilizers), pH levels may increase, as allowed by the buffering
capacity of the water body. Although these small changes in pH are not likely to have a
direct impact on aquatic life, they greatly influence the availability and solubility of all
chemical forms in the water and may further aggravate pollution problems.
What about the pH of the soil in my garden, you may wonder? The pH of the
irrigating water does not affect the pH of the growing media (i.e. your dirt); however,
irrigation water pH is an important factor as it relates to the solubility and the availability
of fertilizers in your soil.
Just as “degree” is a measure of temperature, pH is a measure of how acidic or
basic the water is. Water pH is critical to aquatic systems because the pH of water
determines the solubility and biological availability of chemical elements. We monitor
the pH of our Tribal waters to track the trends of the streams’ common parameters, so if
and when our values change we can determine the cause and extent of the changes and
then develop a mitigation plan, as appropriate.