Water Quality Indices (WQI) - Water Quality Management: Assessment and Interpretation - page 28

The NSFWQI is a subjective method and opinions of practicing professionals
have affected the result of classi cation because the rating curves and intermediate

weighting, the two parts of this technique, arise from experts

opinions. This
method cannot describe temporary variation of surface water quality. On the other
hand, this is a simple and easy method for analyzing water quality. All the
parameters of water quality, which are employed in this method, are usually
available in most monitoring programs. Most non technical people can Conve-
niently understand the result of classi

'

cation. The NSFWQI relates to some other
water quality index methods such as Horton, Prati and Dinius (Ott 1978 ) having a
less ambiguous region for aggregation of sub-indices.

3.4 British Colombia Water Quality Index as (BCWQI)
Method

The BCWQI method as an additive index in 1999 by the ministry of environment,
land and parks of Canada was created to assess water quality. In this method, water
quality parameters are calibrated with a certain limit and the amount of exceeding
was determined. This limit can include recommended guidelines for maintaining
the operational capability of the water in a certain design or each standard in which
amount of the different uses of the water is. Therefore, so the use of standards in
every area, region or country is one of the advantages of this method and the quality
classi cation based on all measured parameters existed in each standard is possible.

Equation ( 3.2 ) is used to calculate the

final index.

" #

r

ð F

F 3
3 Þ

2

1

þ F

2

þð = : ð 3 : 2 Þ

BCWQI ¼ 1 435

where F 1 = the percentage of parameters that have exceeded a certain limit,
F 2 = the number of times generally that exceed certain limits, as a percentage of the
total number of impressions and F 3 = the maximum exceeding of a certain limit
(standard limit). The percentages of exceeding are de

ned as follows:

Percentage of exceeding = [(maximum allowed limit − measured value) /measured

value]*100
Number 1.453 is chosen to ensure maximum number of BCWQI method reaches
to number 100. Sampling frequency and increase the number of stations are the
important notes that enhance the accuracy. About the disadvantage of this method it
can be stated that this index is not able to describe the trend of water quality until are
not exceeding from standard limits. Also, due to using the maximum exceed (F3),
not specifying how many impressions are located above the maximum standard.
Table 3.3 presents the interpretation of the pollution based on the BCWQI method.
In a speci c use, high values of a parameter may be desirable, while in the other

use this amount is not acceptable. Accordingly, some experts believe the