Geology Reference
In-Depth Information
EXERCISE
13
Water Quality Data
and Pollution Sources
INTRODUCTION
Water quality in a stream, lake, well, or spring is a
product of the quality of the precipitation and any
changes that have occurred to the water at or below
the surface of the earth. As water moves through the
hydrologic cycle, it changes chemically, physically,
and biologically (Hem, 1985). Some changes cause
the quality to deteriorate and if the inputs are signifi-
cant from human sources, the water is considered to
be contaminated.
Major sources of contamination include indus-
trial, municipal, agricultural, and mining activities. The
contaminants may be physical, chemical, or biologi-
cal. Some constituents affect taste, others may be
corrosive, and a few are health hazards. All natural
water, however, contains some impurities. These nat-
urally occurring background concentrations reflect
the soluble products in the soil and rocks through
which the water moved.
Under the 1986 Amendments to the Safe Drinking
Water Act, the U.S. Environmental Protection Agency
(USEPA) established
National Primary Drinking Water
Regulations
for specific contaminants that may have
any adverse effect on human health and could be
expected in public water systems (Table 13.1). These
regulations indicate MCLGs and MCLs. MCLGs
(Maximum Contaminant Level Goals) are recom-
mended maximum limits of contaminants in drinking
water. They are nonenforceable health goals of conta-
minant levels at which no known or anticipated
adverse health effects occur and that allow an ade-
quate margin of safety. MCLs (Maximum Contaminant
Levels) are the maximum permissible levels. They are
set as close to the MCLGs as is feasible using the
best technology and techniques available. Additional
organic chemicals and their potential health effects
from exposure above the MCLs are available online
from the USEPA.
In addition to health concerns addressed under
the primary regulations, the USEPA has developed
National Secondary Drinking Water Regulations
that
cover aesthetic qualities, such as taste and odor, of
drinking water (Table 13.2). Under this section, Sec-
ondary Maximum Contaminant Levels (SMCLs) have
been set for contaminants. The SMCLs are federally
nonenforceable and establish limits for contaminants
in drinking water that may affect aesthetic qualities
and public acceptance of drinking water. Standards for
drinking water in the United States continue to evolve.
The concentrations of substances dissolved in
water are normally reported as milligrams per liter
(mg/L), parts per million (ppm), parts per billion (ppb),
or grains per gallon (gpg). Milligrams per liter and parts
per million are practically the same for waters with less
than 7,000 ppm total dissolved solids (TDS). The two
terms are often used interchangeably. One mg/L means
that a 1 -liter sample of water contains 1 milligram of
substance dissolved in it, or that in a million total units
1 unit consists of a dissolved substance. To correctly
interpret the significance of water-quality data, it is
necessary to understand something about the origin
and reactions of certain compounds.
In this exercise we investigate the importance of
selected inorganic chemicals in water and their
potential sources. An estimate of the quantity of
inorganic chemicals in water is provided by specific
conductance. The capacity for water to conduct an
electric current is determined by the temperature and
the degree of ionization of the elements and com-
pounds in the water. This capacity is known as
specific conductance,
and by measuring it we can
determine the quantity of total dissolved solids (TDS)
in the water. The concentration of dissolved solids (in
ppm) is usually estimated by multiplying the specific
conductance, which is given in umhos/cm, or
microsiemens, by 0.67. A mho, which is a unit for con-
ductivity, is the reverse of an ohm, the unit for resistivity.
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