American Journal of Water Resources, 2013, Vol. 1, No. 3, 34-38
Available online at http://pubs.sciepub.com/ajwr/1/3/3
© Science and Education Publishing
DOI:10.12691/ajwr-1-3-3
Water Quality Assessment in Terms of Water Quality
Index
Shweta Tyagi1, Bhavtosh Sharma2,*, Prashant Singh1, Rajendra Dobhal2,3
1
Department of Chemistry, DAV Post Graduate College, Dehradun, Uttarakhand, India
2
Uttarakhand Science Education and Research Center, Dehradun, Uttarakhand, India
3
Uttarakhand Council of Science and Technology, Dehradun, Uttarakhand, India
*Corresponding author: bhavtoshchem@gmail.com
Received July 19, 2013; Revised August 05, 2013; Accepted August 07, 2013
Abstract Water quality index (WQI) is valuable and unique rating to depict the overall water quality status in a
single term that is helpful for the selection of appropriate treatment technique to meet the concerned issues. However,
WQI depicts the composite influence of different water quality parameters and communicates water quality
information to the public and legislative decision makers. In spite of absence of a globally accepted composite index
of water quality, some countries have used and are using aggregated water quality data in the development of water
quality indices. Attempts have been made to review the WQI criteria for the appropriateness of drinking water
sources. Besides, the present article also highlights and draws attention towards the development of a new and
globally accepted “Water Quality Index” in a simplified format, which may be used at large and could represent the
reliable picture of water quality.
Keywords: ground water, surface, water quality, water quality index
Cite This Article: Shweta Tyagi, Bhavtosh Sharma, Prashant Singh, and Rajendra Dobhal, “Water Quality
Assessment in Terms of Water Quality Index.” American Journal of Water Resources 1, no. 3 (2013): 34-38. doi:
10.12691/ajwr-1-3-3.
1. Introduction
Water, a prime natural resource and precious national
asset, forms the chief constituent of ecosystem. Water
sources may be mainly in the form of rivers, lakes,
glaciers, rain water, ground water etc. Besides the need of
water for drinking, water resources play a vital role in
various sectors of economy such as agriculture, livestock
production, forestry, industrial activities, hydropower
generation, fisheries and other creative activities. The
availability and quality of water either surface or ground,
have been deteriorated due to some important factors like
increasing population, industrialization, urbanization etc.
Water quality of any specific area or specific source can
be assessed using physical, chemical and biological
parameters. The values of these parameters are harmful
for human health if they occurred more than defined limits
[1,2,3,4]. Therefore, the suitability of water sources for
human consumption has been described in terms of Water
quality index (WQI), which is one of the most effective
ways to describe the quality of water. WQI utilizes the
water quality data and helps in the modification of the
policies, which are formulated by various environmental
monitoring agencies. It has been realized that the use of
individual water quality variable in order to describe the
water quality for common public is not easily
understandable [5,6]. That's why, WQI has the capability
to reduce the bulk of the information into a single value to
express the data in a simplified and logical form [7]. It
takes information from a number of sources and combines
them to develop an overall status of a water system [8-25].
They increase the understanding ability of highlighted
water quality issues by the policy makers as well as for the
general public as users of the water resources [26]. The
present study reviews some of the important water quality
indices used in water quality assessment and provides
their mathematical structure, set of parameters and
calculations along with their merits and demerits, which
are being used worldwide.
2. Water Quality Index
Initially, WQI was developed by Horton (1965) [27] in
United States by selecting 10 most commonly used water
quality variables like dissolved oxygen (DO), pH, coliforms,
specific conductance, alkalinity and chloride etc. and has
been widely applied and accepted in European, African and
Asian countries. The assigned weight reflected significance
of a parameter for a particular use and has considerable
impact on the index. Furthermore, a new WQI similar to
Horton’s index has also been developed by the group of
Brown in 1970 [28], which was based on weights to individual
parameter. Recently, many modifications have been considered
for WQI concept through various scientists and experts [29,30].
A general WQI approach [31] is based on the most
common factors, which are described in the following
three steps:
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American Journal of Water Resources
1. Parameter Selection: This is carried out by judgment
of professional experts, agencies or government
institutions that is determined in the legislative area.
The selection of the variables from the 5 classes
namely oxygen level, eutrophication, health aspects,
physical characteristics and dissolved substances,
which have the considerable impact on water quality,
are recommended [32].
2. Determination of Quality Function (curve) for Each
Parameter Considered as the Sub-Index: Sub-indices
transform to non-dimensional scale values from the
variables of its different units (ppm, saturation
percentage, counts/volume etc.).
3. Sub-Indices Aggregation with Mathematical
Expression: This is frequently utilized through
arithmetic or geometric averages.
However, a huge number of water quality indices viz.
Weight Arithmetic Water Quality Index (WAWQI),
National Sanitation Foundation Water Quality Index
(NSFWQI), Canadian Council of Ministers of the
Environment Water Quality Index (CCMEWQI), Oregon
Water Quality Index (OWQI) etc. have been formulated
by several national and international organizations. These
WQI have been applied for evaluation of water quality in
a particular area [33,34]. Moreover, these indices are often
based on the varying number and types of water quality
parameters as compared with respective standards of a
particular region. Water quality indices are accredited to
demonstrate annual cycles, spatial and temporal variations
in water quality and trends in water quality even at low
concentrations in an efficient and timely manner. On the
basis of reviewed literature, available indices have many
variations and limitations based on number of water
quality variables used and not accepted worldwide [35].
Hence, it needs worldwide acceptability with varying
number of water quality variables. Various WQI
determination methods have been described herein.
2.1. National Sanitation Foundation Water
Quality Index (NSFWQI)
A usual water quality index method was developed by
paying great rigor in selecting parameters, developing a
common scale and assigning weights. The attempt was
supported by the National Sanitation Foundation (NSF)
and therefore as NSFWQI in order to calculate WQI of
various water bodies critically polluted. The proposed
method for comparing the water quality of various water
sources is based upon nine water quality parameters such
as temperature, pH, turbidity, fecal coliform, dissolved
oxygen, biochemical oxygen demand, total phosphates,
nitrates and total solids [28,36]. The water quality data are
recorded and transferred to a weighting curve chart, where
a numerical value of Qi is obtained. The mathematical
expression for NSF WQI is given by
WQI = ∑ i =1 QiWi
n
Where,
Qi = sub-index for ith water quality parameter;
Wi = weight associated with ith water quality parameter;
n = number of water quality parameters.
For this NSFWQI method, the ratings of water quality
have been defined by using following Table 1:
Table 1. Water Quality Rating as per different Water Quality Index methods
National Sanitation Foundation Water Quality Index (NSFWQI)
WQI Value
Rating of Water Quality
91-100
Excellent water quality
71-90
Good water quality
51-70
Medium water quality
26-50
Bad water quality
0-25
Very bad water quality
Canadian Council of Ministers of the Environment Water Quality Index (CCME WQI)
95-100
Excellent water quality
80-94
Good water quality
60-79
Fair water quality
45-59
Marginal water quality
0-44
Poor water quality
Oregon Water Quality Index (OWQI)
90-100
Excellent water quality
85-89
Good water quality
80-84
Fair water quality
60-79
Poor water quality
0-59
Very poor water quality
2.2. Canadian Council of Ministers of the
Environment Water Quality Index (CCME
WQI)
CCME WQI provides a consistent method, which was
formulated by Canadian jurisdictions to convey the water
quality information for both management and the public.
Moreover, a committee established under the Canadian
Council of Ministers of the Environment (CCME) has
developed WQI, which can be applied by many water
agencies in various countries with slight modification
[37,38,39]. This method has been developed to evaluate
surface water for protection of aquatic life in accordance
to specific guidelines. The parameters related with various
measurements may vary from one station to the other and
sampling protocol requires atleast four parameters,
sampled atleast four times [40,41]. The calculation of
index scores in CCME WQI method can be obtained by
using the following relation:
WQI
= 100 −
F12 + F22 + F32
1.732
Where,
Scope (F1) = Number of variables, whose objectives are
not met.
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American Journal of Water Resources
F1= [No. of failed variables /Total no. of variables]*100
Frequency (F2) = Number of times by which the
objectives are not met.
F2 = [No. of failed tests/Total no. of tests]*100
Amplitude (F3) = Amount by which the objectives are not
met.
(a) excursioni = [Failed test valuei /Objectivej ]-1
n
(b) normalized sum of excursions (nse) = ∑ excursionsi
i=1
/No of tests
(c) F3 = [nse/0.01nse+0.01]
Therefore, five categories have been suggested to
categorize the water qualities which are summarized in
Table 1.
Vo is the ideal value of this parameter in pure water
Vo = 0 (except pH =7.0 and DO = 14.6 mg/l)
Si is recommended standard value of ith parameter
The unit weight (Wi) for each water quality parameter is
calculated by using the following formula:
Wi = K Si
Where,
K = proportionality constant and can also be calculated
by using the following equation:
K=
1
(1
∑ Si)
The rating of water quality according to this WQI is given
in Table 2.
2.3. Oregon Water Quality Index (OWQI)
OWQI creates a score to evaluate the general water
quality of Oregon’s stream and the application of this
method to other geographic regions, which combines eight
water quality variables into a single number. The
parameters covered in this method are temperature,
dissolved oxygen (DO), biochemical oxygen demand
(BOD), pH, ammonia and nitrate nitrogen, total
phosphorus, total solids and fecal coliform [32,42]. The
original OWQI was designed after the NSFWQI where the
Delphi method was used for variable selection. It
expresses water quality status and trends for the
legislatively mandated water quality status assessment.
The index is free from the arbitration in weighting the
parameters and employs the concept of harmonic
averaging. The mathematical expression of this WQI
method is given by
WQI =
n
1
∑ i =1 SI 2
n
i
Where,
n = number of subindices
SI = subindex of ith parameter
Furthermore, the rating scale of this OWQI has also been
categorized in various classes, which are given under
Table 1 [43].
2.4. Weighted Arithmetic Water Quality
Index Method
Weighted arithmetic water quality index method
classified the water quality according to the degree of
purity by using the most commonly measured water
quality variables. The method has been widely used by the
various scientists [44,45,46,47] and the calculation of
WQI was made [48] by using the following equation:
WQI = ∑ QiWi
∑Wi
The quality rating scale (Qi) for each parameter is
calculated by using this expression:
Qi = 100[(Vi − Vo Si − Vo)]
Where,
Vi is estimated concentration of ith parameter in the
analysed water
Table 2. Water Quality Rating as per Weight Arithmetic Water
Quality Index Method
WQI Value
Rating of Water Quality
Grading
0-25
Excellent water quality
A
26-50
Good water quality
B
51-75
Poor water quality
C
76-100
Very Poor water quality
D
Above 100
Unsuitable for drinking purpose
E
3. Merits and Demerits of Selected Water
Quality Index Methods
A comparison of all these water quality indices is also
performed under the study considering their merits and
demerits. Table 3 explains about the merits and demerits
of WQI methods.
4. Conclusions
After the study of different water quality indices, it may
be inferred that the aim of WQI is to give a single value to
water quality of a source alongwith reducing higher
number of parameters into a simple expression resulting
into easy interpretation of water quality monitoring data.
Moreover, this is an effort to review the important indices
used in water quality vulnerability assessment and also
provides information about indices composition and
mathematical forms. These indices utilize various
physico-chemical and biological parameters and have
been resulted as an outcome of efforts and research and
development carried out by different government agencies
and experts in this area globally. In spite of all the efforts
and different discussed indices being used globally, no
index has so far been universally accepted and search for
more useful and universal water quality index is still going
on, so that water agencies, users and water managers in
different countries may use and adopted it with little
modifications.
American Journal of Water Resources
37
Table 3. Merits and Demerits of Selected Water Quality Indices
National Sanitation Foundation (NSF) WQI
Merits
Demerits
References
1. Summarizes data in a single index value in an objective,
1. Represents general water quality, it does not
rapid and reproducible manner.
represent specific use of the water.
2. Evaluation between areas and identifying changes in
[49,50]
2. Loss of data during data handling.
water quality.
3. Lack of dealing with uncertainty and subjectivity
3. Index value relate to a potential water use.
present in complex environmental issues.
4. Facilitates communication with lay person.
Canadian Council of Ministers of the Environment (CCME) WQI
1. Represent measurements of a variety of variables in a
1. Loss of information on single variables.
single number.
2. Loss of information about the objectives specific to
2. Flexibility in the selection of input parameters and
each location and particular water use.
objectives.
3. Sensitivity of the results to the formulation of the
3. Adaptability to different legal requirements and different index.
water uses.
4. Loss of information on interactions between
4. Statistical simplification of complex multivariate data.
variables.
5. Clear and intelligible diagnostic for managers and the
5. Lack of portability of the index to different
general public.
ecosystem types.
[51,52]
6. Suitable tool for water quality evaluation in a specific
6. Easy to manipulate (biased).
7. The same importance is given to all variables.
location
8. No combination with other indicators or biological
7. Easy to calculate
data
8. Tolerance to missing data
9. Suitable for analysis of data coming from automated
9. Only partial diagnostic of the water quality.
sampling.
10. F1 not working appropriately when too few
10. Combine various measurements in a variety of different variables are considered or when too much
measurement units in a single metric.
covariance exists among them.
Oregon WQI
1. Does not consider changes in toxics
concentrations, habitat or biology.
1. Un-weighted harmonic square mean formula used to
2. To make inferences of water quality conditions
combine sub-indices allows the most impacted parameter
outside of the actual ambient network site locations is
to impart the greatest influence on the water quality index.
not possible.
2. Method acknowledges that different water quality
3. Cannot determine the water quality for specific
[43,53]
parameters will pose differing significance to overall water
uses nor can it be used to provide definitive
quality at different times and locations.
information about water quality without considering
3. Formula is sensitive to changing conditions and to
all appropriate physical, chemical and biological data.
significant impacts on water quality.
4. Cannot evaluate all health hazards (toxics, bacteria,
metals, etc.).
Weight Arithmetic WQI
1. Incorporate data from multiple water quality parameters
1. WQI may not carry enough information about the
into a mathematical equation that rates the health of water
real quality situation of the water.
body with number.
2. Many uses of water quality data cannot be met
2. Less number of parameters required in comparison to all
with an index.
water quality parameters for particular use.
3. The eclipsing or over-emphasizing of a single bad
3. Useful for communication of overall water quality
[6,54]
parameter value
information to the concerned citizens and policy makers.
4. A single number cannot tell the whole story of
4. Reflects the composite influence of different parameters
water quality; there are many other water quality
i.e. important for the assessment and management of water
parameters that are not included in the index.
quality.
5. WQI based on some very important parameters can
5. Describes the suitability of both surface and groundwater
provide a simple indicator of water quality.
sources for human consumption.
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