Water Quality Terms
pH (HYDROGEN ION CONCENTRATION)
(1) A convenient method of expressing the acidity or basicity of
a solution in terms of the logarithm of the reciprocal (or negative
logarithm) of the hydrogen ion concentration. The pH scale runs from
0 to 14; a pH value of 7.0 indicates a neutral solution. Values
above 7.0 pH indicate basicity (basic solutions); those below 7.0 pH
indicate acidity (acidic solutions). Natural waters usually have a
pH between 6.5 and 8.5. Because the units are derived from common
logarithms, a difference of one pH unit indicates a tenfold (101)
difference in acidity; similarly, a difference of two units
indicates a hundredfold (102) difference in acidity. The
term originally derived from "potential of hydrogen," or
hydrogen power. (2) A term indicating the hydrogen ion concentration
of a solution, i.e., a measure of the solution's acidity. The term
(from French, pouvoir hydrogène, or literally, "hydrogen
power") is defined as the negative logarithm of the
concentration of H+ ions (protons): pH = -log10 [H+],
where [H+] is the concentration of H+ ions in
moles per liter (see Mole). Because H+ ions associate
with water molecules to form hydronium (H3O+)
ions (see Acid and Base), pH also is often
expressed in terms of the concentration of hydronium ions. In pure
water at 22 C (72 F),H3O+ and hydroxyl (OH-)
ions exist in equal quantities; the concentration of each is 0.107
moles/liter. Consequently, the pH of pure water is -log (0.107),
which equals log 107, or 7. If an acid is added to water, however,
an excess of H3O+ ions is formed; their
concentration can range between 0.106 and 0.10 moles/liter,
depending on the strength and amount of the acid. Therefore, acid
solutions have a pH ranging from 6 (for a weak acid) to 1 (for a
strong acid). Inversely, a basic solution has a low concentration of
H3O+ ions and an excess of OH-
ions, and the pH ranges from 8 (for a weak base) to 14 (for a strong
base).
HARDNESS—A
property of water which causes an increase in the amount of soap
that is needed to produce foam or lather and that also produces
scale in hot water pipes, heaters, boilers and other units in which
the temperature of water is increased materially. Hardness is
produced almost completely by the presence of calcium and magnesium
salts in solution. The following scale may assist in appraising
water hardness, measured by weight of dissolved salts (in
milligrams) per unit (in liters) of water:
[1] Soft—0-60
milligrams/liter (mg/l);
[2] Moderately Hard—61-120 mg/l;
[3] Hard—121-180 mg/l; and
[4] Very Hard—over 180 mg/l.
To convert from milligrams per liter (mg/L)
to grains per gallon, use the following formula:
hardness in mg/L / 17.1 = hardness in grains
per gallon
ALKALINITY
The capacity of water for neutralizing an acid solution.
Alkalinity of natural waters is due primarily to the presence of
hydroxides, bicarbonates, carbonates and occasionally borates,
silicates and phosphates. It is expressed in units of milligrams per
liter (mg/l) of CaCO3 (calcium carbonate) or as
microequivalents per liter (ueq/l) 20 ueq/l = 1 mg/l of CaCO3.
A solution having a pH below 4.5 contains no alkalinity. Low
alkalinity is the main indicator of susceptibility to acid rain.
Increasing alkalinity is often related to increased algal
productivity. Lakes with watersheds that have sedimentary carbonate
rocks are high in dissolved carbonates (hard-water lakes). Whereas
lakes in granite or igneous rocks are low in dissolved carbonates
(soft water lakes).
IRON
A silvery white or gray, soft, ductile, malleable, somewhat
magnetic metal. Oxidizes to rust under certain
conditions. Can cause discoloration of concrete or structures
if water used for irrigation is high in Iron.
SALT
A chemical class of ionic compounds formed by the combination of
an acid and a base. Most salts are the result of a reaction between
a metal and one or more nonmetals.
BACTERIA
Microscopic unicellular organisms, typically spherical, rod-like, or
spiral and threadlike in shape, often clumped into colonies. Some
bacteria cause disease, while others perform an essential role in
nature in the recycling of materials, for example, decomposing
organic matter into a form available for reuse by plants. Some forms
of bacteria are used to stabilize organic wastes in wastewater
treatment plants, oil spills, or other pollutants. Disease-causing
forms of bacteria are termed "pathogenic." Some forms of
bacteria harmful to man include:
[1] Total Coliform Bacteria—A
particular group of bacteria that are used as indicators of
possible sewage pollution. They are characterized as aerobic or
facultative anaerobic, gram-negative, nonspore-forming, rod-shaped
bacteria which ferment lactose with gas formation within 48 hours
at 3C. In the laboratory these bacteria are defined as all the
organisms that produce colonies with a golden-green metallic sheen
within 24 hours when incubated at 35C plus or minus 1.0C on M-Endo
medium (nutrient medium for bacterial growth). Their
concentrations are expressed as numbers of colonies per 100
milliliter (ml)l of sample.
[2] Fecal Coliform Bacteria—Bacteria that are
present in the intestine or feces of warm-blooded animals. They
are often used as indicators of the sanitary quality of the water.
In the laboratory they are defined as all the organisms that
produce blue colonies within 24 hours when incubated at 44.5C plus
or minus 0.2C on M-FC medium (nutrient medium for bacterial
growth). Their concentrations are expressed as numbers of colonies
per 100 ml of sample.
[3] Fecal Streptococcal Bacteria—Bacteria found
also in the intestine of warm-blooded animals. Their presence in
water is considered to verify fecal pollution. They are
characterized as gram-positive, cocci bacteria which are capable
of growth in brain-heart infusion broth. In the laboratory they
are defined as all the organisms that produce colonies which
produce red or pink colonies within 24 hours at 35C plus or minus
1.0C on KF-streptococcus medium (nutrient medium for bacterial
growth). Their concentrations are expressed as numbers of colonies
per 100 ml of sample.
TURBIDITY
The term "turbid" is applied
to waters containing suspended matter that interferes with the
passage of light through the water or in which visual depth is
restricted. The turbidity may be caused by a wide variety of
suspended materials, such as clay, silt, finely divided organic and
inorganic matter, soluble colored organic compounds, plankton and
other microscopic organisms and similar substances. Turbidity in
water has public health implications due to the possibilities of
pathogenic bacteria encased in the particles and thus escaping
disinfection processes. Turbidity interferes with water treatment
(filtration), and affects aquatic life. Excessive amounts of
turbidity also make water aesthetically objectionable. The degree of
the turbidity of water is measured by a Turbidimeter.
COLOR
(1) Measured in units that relate to a standard. A
yellow-brown natural color is associated with lakes or rivers
receiving wetland drainage. The average color value for Wisconsin
lakes is 39 units, with the color of state lakes ranging from zero
to 320 units. Color also affects light penetration and therefore the
depth at which plants can grow. (2) One control of light
transmission through water. High color values in many lakes result
from the decomposition of vegetation, which gives the water a brown,
tea-like color. Determined by a comparison with standardized
colored-glass discs and reported in platinum-cobalt (Pt-Co) units.
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