FACTSHEET ON WATER QUALITY PARAMETERS
PH
p!~i is the concentration of hydrogen ions (H+) in a sample. plH is measured to determine the acidity of the water.
Why do we measure pH?
pH is an important indicator of chemical, physical, and
biological changes in a waterbody and plays a critical role
in chemical processes in natural waters. pH values are on
a scale from 0 to 14, with 7.0 considered neutral. Figure
1 shows typical pH values of common liquids. Solutions
with a pi I below 7.0 are considered acidic, and those
with a pH above 7.0 are considered basic. The pH scale
is logarithmic, meaning that every one-unit change in pH
represents a ten-fold change in acidity. In other words,
pH 6.0 is ten times more acidic than pH 7.0; pH 5.0 is one
hundred times mores acidic than ptrl 7.0.
Figure 1. Typical pH values of common liquids. Adapted from)
Water on the Web (2008)
pH is a key factor in water chemistry and toxicity. A
change in pH can alter the concentrations and forms
of toxic chemicals in water. Metals such as aluminum,
iead, mercury, copper, and arsenic are generally more
soluble at a lower pi I. Therefore, higher concentrations
can be absorbed into the tissues of organisms, rendering
these metals more toxic to aquatic life In more basic
waters (pH > 8.5), the conversion of the nontoxic form of
ammonia to the toxic form is increased.
pH also plays a key role in
aquatic health by affecting
biochemical processes and
the metabolism of aquatic
organisms. Generally, if
water is too acidic or too
basic, damage can occur
to an organism's gilis,
exoskeleton, fins, and other
critical components. Of
particular concern are pi I-
sensitive macroinvertebrates
(small organisms without a
backbone), fish eggs (most
fish eggs cannot hatch at a
pH I ess than 5), and juvenile
fish.
Organisms vary in the pi I
ranges they can tolerate.
Figure 2 illustrates the plH values at which key organisms
may experience die-off or avoidance. Furthermore, even
though an organism itself may tolerate a more extreme
plH, its food source may not.
ui very basic.
14.0
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-household lye
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12.0
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pH
What affects pH?
pH in a lake or river often fluctuates daily, with a higher pH
(more basic) during the daytime due to consumption of
CO2 by aquatic plants and algae during photosynthesis.
At night, aquatic plants and algae respire, giving off CO2
and lowering pH (more acidic). Therefore, pH tends to be
highest in late afternoon and lowest before sunrise. The pH
of a well buffered pond may fluctuate between 7.0 and 8.4
(Figure 3). Buffering capacity refers to the ability for water
to neutralize acids and bases and maintain a fairly stable
pH level.
9.0
8.5
X
Q- 7.5
7.0
6.5
6.01 1
Early Morning Late Afternoon Early Morning
Figure 3. Daily fluctuations in pH in a hypothetical well
buffered pond. Adapted from Wurts and Durburow (1992)
There are also seasonal changes in pH. Values are lower
during seasons with high rainfall and snowmelt. When
excess water runs directly into lakes or rivers without
passing through soil, the slight acidity naturally present
in precipitation does not get buffered. Additionally, pH in
streams or at the surface of lakes is higher during the
growing season (spring and summer) when there is more
photosynthetic activity by aquatic plants.
pH can also vary with depth in the water column. Near the
surface of lakes, pH is higher because light is available
for algae to photosynthesize. In thermally stratified lakes,
pH in the deeper water is often lower due to the lack
of photosynthesis as well as respiration by organisms
decomposing organic matter.
Both natural and human-induced factors can affect pH. For
example, pH near the surface of lakes can increase with
nutrient inputs and the growth of algae during algal blooms
which consume CO2. In addition, pH at the bottom of lakes
may decrease due to decomposition of excess algae,
plants, and other organic material.
pH can also decrease due to:
Acid rain, which is typically caused by nitrogen oxides
and sulfur dioxide from emissions of cars and coal-
fired power plants. Regulations imposed in the last
few decades to control acid rain have resulted in lake
recovery in some heavily affected areas.
Acid mine drainage from coal or sulfide mines, which
can reach streams.
Local geology helps control pH, with certain types of
bedrock such as limestone buffering the water against acid
inputs.
What are EPA's recommended ambient water quality criteria for pH?
In EPA's Quality Criteria for Water (1986), the recommended
water criteria for pH ranges from 6.5 to 9.0 depending on
what is protective of aquatic life and the particular system
(fresh vs saltwater).
2
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pH
How do we measure pH?
pH can be measured in the fieid using a water quality
probe, indicator tests or strips, or from grab samples.
Most commonly pH is measured in the fieid along with
other water quality parameter measurements including
temperature, dissolved oxygen, and specific conductance
using a single or multi-parameter probe. pH should also be
measured along with alkalinity, which is a measure of the
capacity of water to neutralize acids. Alkalinity is also
known as buffering capacity. Other parameters that may
inform pH results include nutrients and metals because pH
can indicate nutrient enrichment or impacts from metals.
pH can be measured at discrete times or continuously.
Optimally, monitoring should take place at the same
time every day if discrete measurements are being
taken. Monitoring sensors (continuous monitoring) allow
assessment of changes in pH throughout the day that
cannot be captured with a single sample.
pH can vary both horizontally and vertically in a waterbody.
Water samples should, therefore, be taken at regular
increments across a waterbody and at various depths (or
depth integrated, which is a sample that represents the
entire water column).
What are the challenges of using pH as a water quality parameter9
pH can be highly variable due to its sensitivity to natural
and human-induced factors. This sensitivity can make it
challenging to pinpoint the sources of long-term trends or
water quality exceedances; evaluation of trends in other
water quality parameters may help in identifying possible
causes.
EPA 841F21007C | July 2021
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