Water quality refers to the
chemical, physical, biological, and radiological characteristics of water. It
is a measure of the condition of water relative to the requirements of one or
more biotic species and or to any human need or purpose. It is most frequently
used by reference to a set of standards against which compliance can be
assessed. The most common standards used to assess water quality relate to
health of ecosystems, safety of human contact and drinking water.
Standards
In the setting of standards,
agencies make political and technical/scientific decisions about how the water
will be used. In the case of natural water bodies, they also make some
reasonable estimate of pristine conditions. Different uses raise different
concerns and therefore different standards are considered. Natural water bodies
will vary in response to environmental conditions. Environmental scientists
work to understand how these systems function, which in turn helps to identify
the sources and fates of contaminants. Environmental lawyers and policymakers
work to define legislation with the intention that water is maintained at an
appropriate quality for its identified use.
The vast majority of surface water
on the planet is neither potable nor toxic. This remains true when seawater in
the oceans (which is too salty to drink) is not counted. Another general
perception of water quality is that of a simple property that tells whether
water is polluted or not. In fact, water quality is a complex subject, in part
because water is a complex medium intrinsically tied to the ecology of the
Earth. Industrial and commercial activities (e.g. manufacturing, mining,
construction, transport) are a major cause of water pollution as are runoff
from agricultural areas, urban runoff and discharge of treated and untreated
sewage.
Categories
The parameters for water quality
are determined by the intended use. Work in the area of water quality tends to
be focused on water that is treated for human consumption, industrial use, or
in the environment.
Human consumption
Contaminants that may be in
untreated water include microorganisms such as viruses, protozoa and bacteria;
inorganic contaminants such as salts and metals; organic chemical contaminants
from industrial processes and petroleum use; pesticides and herbicides; and
radioactive contaminants. Water quality depends on the local geology and
ecosystem, as well as human uses such as sewage dispersion, industrial
pollution, use of water bodies as a heat sink, and overuse (which may lower the
level of the water).
The United States Environmental
Protection Agency (EPA) limits the amounts of certain contaminants in tap water
provided by US public water systems. The Safe Drinking Water Act authorizes EPA
to issue two types of standards: primary standards regulate substances that
potentially affect human health, and secondary standards prescribe aesthetic
qualities, those that affect taste, odor, or appearance. The U.S. Food and Drug
Administration (FDA) regulations establish limits for contaminants in bottled
water that must provide the same protection for public health. Drinking water,
including bottled water, may reasonably be expected to contain at least small
amounts of some contaminants. The presence of these contaminants does not necessarily
indicate that the water poses a health risk.
In urbanized areas around the
world, water purification technology is used in municipal water systems to
remove contaminants from the source water (surface water or groundwater) before
it is distributed to homes, businesses, schools and other users. Water drawn
directly from a stream, lake, or aquifer and that has no treatment will be of
uncertain quality.
Industrial and domestic use
Dissolved minerals may affect
suitability of water for a range of industrial and domestic purposes. The most
familiar of these is probably the presence of ions of calcium and magnesium
which interfere with the cleaning action of soap, and can form hard sulfate and
soft carbonate deposits in water heaters or boilers. Hard water may be softened
to remove these ions. The softening process often substitutes sodium cations.
Hard water may be preferable to soft water for human consumption, since health
problems have been associated with excess sodium and with calcium and magnesium
deficiencies. Softening decreases nutrition and may increase cleaning
effectiveness.
Environmental water quality
Urban runoff discharging to
coastal waters
Environmental water quality, also
called ambient water quality, relates to water bodies such as lakes, rivers,
and oceans. Water quality standards for surface waters vary significantly due
to different environmental conditions, ecosystems, and intended human uses.
Toxic substances and high populations of certain microorganisms can present a
health hazard for non-drinking purposes such as irrigation, swimming, fishing,
rafting, boating, and industrial uses. These conditions may also affect
wildlife, which use the water for drinking or as a habitat. Modern water
quality laws generally specify protection of fisheries and recreational use and
require, as a minimum, retention of current quality standards.
Satirical cartoon by William
Heath, showing a woman observing monsters in a drop of London water (at the
time of the Commission on the London Water Supply report, 1828)
There is some desire among the
public to return water bodies to pristine, or pre-industrial conditions. Most
current environmental laws focus on the designation of particular uses of a
water body. In some countries these designations allow for some water
contamination as long as the particular type of contamination is not harmful to
the designated uses. Given the landscape changes (e.g., land development,
urbanization, clearcutting in forested areas) in the watersheds of many
freshwater bodies, returning to pristine conditions would be a significant
challenge. In these cases, environmental scientists focus on achieving goals
for maintaining healthy ecosystems and may concentrate on the protection of
populations of endangered species and protecting human health.
Sampling and measurement
The complexity of water quality as
a subject is reflected in the many types of measurements of water quality
indicators. The most accurate measurements of water quality are made on-site,
because water exists in equilibrium with its surroundings. Measurements
commonly made on-site and in direct contact with the water source in question
include temperature, pH, dissolved oxygen, conductivity, oxygen reduction
potential (ORP), turbidity, and Secchi disk depth.
Sample collection
An automated sampling station
installed along the East Branch Milwaukee River, New Fane, Wisconsin. The cover
of the 24-bottle autosampler (center) is partially raised, showing the sample
bottles inside. The autosampler was programmed to collect samples at time
intervals, or proportionate to flow over a specified period. The data logger
(white cabinet) recorded temperature, specific conductance, and dissolved
oxygen levels.
More complex measurements are
often made in a laboratory requiring a water sample to be collected, preserved,
transported, and analyzed at another location. The process of water sampling
introduces two significant problems. The first problem is the extent to which
the sample may be representative of the water source of interest. Many water
sources vary with time and with location. The measurement of interest may vary
seasonally or from day to night or in response to some activity of man or
natural populations of aquatic plants and animals. The measurement of interest
may vary with distances from the water boundary with overlying atmosphere and
underlying or confining soil. The sampler must determine if a single time and
location meets the needs of the investigation, or if the water use of interest
can be satisfactorily assessed by averaged values with time and/or location, or
if critical maxima and minima require individual measurements over a range of
times, locations and/or events. The sample collection procedure must assure
correct weighting of individual sampling times and locations where averaging is
appropriate.Where critical maximum or minimum values exist, statistical methods
must be applied to observed variation to determine an adequate number of
samples to assess probability of exceeding those critical values.
The second problem occurs as the
sample is removed from the water source and begins to establish chemical
equilibrium with its new surroundings - the sample container. Sample containers
must be made of materials with minimal reactivity with substances to be
measured; and pre-cleaning of sample containers is important. The water sample
may dissolve part of the sample container and any residue on that container, or
chemicals dissolved in the water sample may sorb onto the sample container and
remain there when the water is poured out for analysis. Similar physical and
chemical interactions may take place with any pumps, piping, or intermediate
devices used to transfer the water sample into the sample container. Water
collected from depths below the surface will normally be held at the reduced
pressure of the atmosphere; so gas dissolved in the water may escape into
unfilled space at the top of the container. Atmospheric gas present in that air
space may also dissolve into the water sample. Other chemical reaction
equilibria may change if the water sample changes temperature. Finely divided
solid particles formerly suspended by water turbulence may settle to the bottom
of the sample container, or a solid phase may form from biological growth or
chemical precipitation. Microorganisms within the water sample may
biochemically alter concentrations of oxygen, carbon dioxide, and organic
compounds. Changing carbon dioxide concentrations may alter pH and change
solubility of chemicals of interest. These problems are of special concern
during measurement of chemicals assumed to be significant at very low concentrations.
Filtering a manually collected
water sample (grab sample) for analysis
Sample preservation may partially
resolve the second problem. A common procedure is keeping samples cold to slow
the rate of chemical reactions and phase change, and analyzing the sample as
soon as possible; but this merely minimizes the changes rather than preventing
them. A useful procedure for determining influence of sample containers during
delay between sample collection and analysis involves preparation for two
artificial samples in advance of the sampling event. One sample container is
filled with water known from previous analysis to contain no detectable amount
of the chemical of interest. This sample, called a "blank," is opened
for exposure to the atmosphere when the sample of interest is collected, then
resealed and transported to the laboratory with the sample for analysis to
determine if sample holding procedures introduced any measurable amount of the
chemical of interest. The second artificial sample is collected with the sample
of interest, but then "spiked" with a measured additional amount of
the chemical of interest at the time of collection. The blank and spiked
samples are carried with the sample of interest and analyzed by the same
methods at the same times to determine any changes indicating gains or losses
during the elapsed time between collection and analysis.
Testing in response to natural
disasters and other emergencies
Inevitably after events such as
earthquakes and tsunamis, there is an immediate response by the aid agencies as
relief operations get underway to try and restore basic infrastructure and
provide the basic fundamental items that are necessary for survival and
subsequent recovery. Access to clean drinking water and adequate sanitation is
a priority at times like this. The threat of disease increases hugely due to
the large numbers of people living close together, often in squalid conditions,
and without proper sanitation.
After a natural disaster, as far
as water quality testing is concerned there are widespread views on the best
course of action to take and a variety of methods can be employed. The key
basic water quality parameters that need to be addressed in an emergency are
bacteriological indicators of fecal contamination, free chlorine residual, pH,
turbidity and possibly conductivity/total dissolved solids. There are a number
of portable water test kits on the market widely used by aid and relief
agencies for carrying out such testing.
After major natural disasters, a
considerable length of time might pass before water quality returns to
pre-disaster levels. For example, following the 2004 Indian Ocean Tsunami the
Colombo-based International Water Management Institute (IWMI) monitored the
effects of saltwater and concluded that the wells recovered to pre-tsunami
drinking water quality one and a half years after the event. IWMI developed
protocols for cleaning wells contaminated by saltwater; these were subsequently
officially endorsed by the World Health Organization as part of its series of
Emergency Guidelines.
Chemical analysis
A gas chromatograph-
mass spectrometer measures
pesticides and other organic pollutants
The simplest methods of chemical
analysis are those measuring chemical elements without respect to their form.
Elemental analysis for dissolved oxygen, as an example, would indicate a
concentration of 890,000 milligrams per litre (mg/L) of water sample because
water is made of oxygen. The method selected to measure dissolved oxygen should
differentiate between diatomic oxygen and oxygen combined with other elements.
The comparative simplicity of elemental analysis has produced a large amount of
sample data and water quality criteria for elements sometimes identified as
heavy metals. Water analysis for heavy metals must consider soil particles
suspended in the water sample. These suspended soil particles may contain
measurable amounts of metal. Although the particles are not dissolved in the
water, they may be consumed by people drinking the water. Adding acid to a
water sample to prevent loss of dissolved metals onto the sample container may
dissolve more metals from suspended soil particles. Filtration of soil
particles from the water sample before acid addition, however, may cause loss
of dissolved metals onto the filter. The complexities of differentiating
similar organic molecules are even more challenging.
Making these complex measurements
can be expensive. Because direct measurements of water quality can be
expensive, ongoing monitoring programs are typically conducted by government
agencies. However, there are local volunteer programs and resources available
for some general assessment. Tools available to the general public include
on-site test kits, commonly used for home fish tanks, and biological assessment
procedures.
Real-time monitoring
Although water quality is usually
sampled and analyzed at laboratories, nowadays, citizens demand real-time
information about the water they are drinking. During the last years, several
companies are deploying worldwide
real-time remote monitoring systems for measuring water pH, turbidity or
dissolved oxygen levels.
Drinking water indicators
An electrical conductivity meter
is used to measure total dissolved solids
The following is a list of indicators
often measured by situational category:
Alkalinity
Color of water
pH
Taste and odor (geosmin, 2-Methylisoborneol (MIB), etc.)
Dissolved metals and salts (sodium, chloride, potassium, calcium,
manganese, magnesium)
Microorganisms such as fecal coliform bacteria (Escherichia coli),
Cryptosporidium, and Giardia lamblia; see Bacteriological water analysis
Dissolved metals and metalloids (lead, mercury, arsenic, etc.)
Dissolved organics: colored dissolved organic matter (CDOM), dissolved
organic carbon (DOC)
Radon
Heavy metals
Pharmaceuticals
Hormone analogs
Environmental indicators
Physical indicators
Water Temperature
Specifics Conductance or EC, Electrical Conductance, Conductivity
Total suspended solids (TSS)
Transparency or Turbidity
Total dissolved solids (TDS)
Odour of water
Color of water
Taste of water
Chemical indicators
pH
Biochemical oxygen demand (BOD)
Chemical oxygen demand (COD)
Dissolved oxygen (DO)
Total hardness (TH)
Heavy metals
Nitrate
Orthophosphates
Pesticides
Surfactants
Biological indicators
Ephemeroptera
Plecoptera
Mollusca
Trichoptera
Escherichia coli (E. coli)
Coliform bacteria
Biological monitoring metrics have
been developed in many places, and one widely used measure is the presence and
abundance of members of the insect orders Ephemeroptera, Plecoptera and
Trichoptera. (Common names are, respectively, Mayfly, Stonefly and Caddisfly.)
EPT indexes will naturally vary from region to region, but generally, within a
region, the greater the number of taxa from these orders, the better the water
quality. Organisations in the United States, such as EPA offer guidance on
developing a monitoring program and identifying members of these and other
aquatic insect orders.
Individuals interested in
monitoring water quality who cannot afford or manage lab scale analysis can
also use biological indicators to get a general reading of water quality. One
example is the IOWATER volunteer water monitoring program, which includes a
benthic macroinvertebrate indicator key.
Bivalve molluscs are largely used
as bioindicators to monitor the health of aquatic environments in both fresh
water and the marine environments. Their population status or structure,
physiology, behaviour or the level of contamination with elements or compounds
can indicate the state of contamination status of the ecosystem. They are
particularly useful since they are sessile so that they are representative of
the environment where they are sampled or placed. A typical project is the
Mussel Watch Programme, but today they are used worldwide.
The Southern African Scoring
System (SASS) method is a biological water quality monitoring system based on
the presence of benthic macroinvertebrates. The SASS aquatic biomonitoring tool
has been refined over the past 30 years and is now on the fifth version (SASS5)
which has been specifically modified in accordance with international
standards, namely the ISO/IEC 17025 protocol. The SASS5 method is used by the
South African Department of Water Affairs as a standard method for River Health
Assessment, which feeds the national River Health Programme and the national
Rivers Database.
Water quality standards and
reports
World Health Organisation
guideline
World Health Organisation (WHO) guideline for Drinking Water Standards.
Indian Council of Medical Research
standards
Indian Council of Medical Research (ICMR) Standards for Drinking Water.
International standards
Water quality regulated by the
International Organization for Standardization (ISO) is covered in the section
of ICS 13.060, ranging from water sampling, drinking water, industrial class
water, sewage water, and examination of water for chemical, physical or
biological properties. ICS 91.140.60 covers the standards of water supply
systems.
National specification for
drinking water
European Union
Further information: Water supply
and sanitation in the European Union
The water policy of the European
Union is primarily codified in three directives:
Directive on Urban Waste Water Treatment (91/271/EEC) of 21 May 1991
concerning discharges of municipal and some industrial wastewaters;
The Drinking Water Directive (98/83/EC) of 3 November 1998 concerning
potable water quality;
Water Framework Directive (2000/60/EC) of 23 October 2000 concerning
water resources management.
United Kingdom
In England and Wales acceptable
levels for drinking water supply are listed in the "Water Supply (Water
Quality) Regulations 2000."
South Africa
Further information: Water supply
and sanitation in South Africa
Water quality guidelines for South
Africa are grouped according to potential user types (e.g. domestic,
industrial) in the 1996 Water Quality Guidelines. Drinking water quality is
subject to the South African National Standard (SANS) 241 Drinking Water
Specification.
United States
In the United States, Water
Quality Standards are created by state agencies for different types of water
bodies and water body locations per desired uses.The Clean Water Act (CWA)
requires each governing jurisdiction (states, territories, and covered tribal
entities) to submit a set of biennial reports on the quality of water in their
area. These reports are known as the 303(d), 305(b) and 314 reports, named for
their respective CWA provisions, and are submitted to, and approved by, EPA.
These reports are completed by the governing jurisdiction, typically a state
environmental agency, and are available on the web. In coming years it is
expected that the governing jurisdictions will submit all three reports as a
single document, called the "Integrated Report." The 305(b) report
(National Water Quality Inventory Report to Congress) is a general report on
water quality, providing overall information about the number of miles of
streams and rivers and their aggregate condition. The 314 report has provided
similar information for lakes. The CWA requires states to adopt water quality
standards for each of the possible designated uses that they assign to their
waters. Should evidence suggest or document that a stream, river or lake has
failed to meet the water quality criteria for one or more of its designated
uses, it is placed on the 303(d) list of impaired waters. Once a state has
placed a water body on the 303(d) list, it must develop a management plan
establishing Total Maximum Daily Loads for the pollutant(s) impairing the use
of the water. These TMDLs establish the reductions needed to fully support the
designated uses.
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