chapter 7 water quality index - Shodhganga

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Water quality index (WQI) provides information about water quality in a single value. ... standards for the drinking water, recommended by the Indian Council of.
240

CHAPTER 7 WATER QUALITY INDEX

7.1

GENERAL Accurate information on the quality of water is inevitable to form a

public policy and to implement the water quality improvement programmes. Water quality index (WQI) provides information about water quality in a single value. WQI is commonly used for the detection and evaluation of water pollution and may be defined as a reflection of composite influence of different quality parameters on the overall quality of water (Horton, 1965). WQI indices are broadly classified into two types, they are physico-chemical and biological indices. The physico-chemical indices are based on the values of various physico-chemical parameters in a water sample, while biological indices are derived from the biological information. Here attempt has been made to calculate the water quality index of the study area based on hydrochemical data. 7.2

WQI CALCULATION Calculation of WQI was carried out in this work by Horton’s

method. The WQI is calculated by using the expression given in Equation (7.1). WQI = Where,

qn Wn /

Wn

qn = Quality rating of n th water quality parameter. Wn= Unit weight of n th water quality parameter.

(7.1)

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7.2.1

Quality rating (qn) The quality rating (qn) is calculated using the expression given in

Equation (7.2). qn =

[ ( Vn – Vid) / ( Sn- Vid) ] x 100

(7.2)

Where, Vn = Estimated value of nth water quality parameter at a given sample location. Vid = Ideal value for n th parameter in pure water. (Vid for pH = 7 and 0 for all other parameters) Sn = Standard permissible value of n th water quality parameter. 7.2.2

Unit weight The unit weight (Wn) is calculated using the expression given in

Equation (7.3). Wn = k / Sn

(7.3)

Where, Sn = Standard permissible value of n th water quality parameter. k = Constant of proportionality and it is calculated by using the expression given in Equation (7.4). k = [1/ (

1/ Sn=1,2,..n) ]

(7.4).

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7.3

WQI AND STATUS The ranges of WQI, the corresponding status of water quality and

their possible use are summarized in Table 7.1. Table 7.1 WQI and corresponding water quality status S.No 1 2

WQI 0 – 25 25 – 50

3 4 5 6

51 -75 76 – 100 101 -150 Above 150

7.4

Status Excellent Good

Possible usages Drinking, Irrigation and Industrial Domestic, Irrigation and Industrial

Fair Poor Very Poor Unfit for Drinking

Irrigation and Industrial Irrigation Restricted use for Irrigation Proper treatment required before use.

STANDARD VALUES AND UNIT WEIGHTS OF WATER QUALITY PARAMETERS OF THE STUDY AREA The water quality parameters are selected based on its direct

involvement in deteriorating water quality for human consumption. The standards for the drinking water, recommended by the Indian Council of Medical Research (ICMR) and Indian Standards Institution (ISI) are considered for the computation of quality rating (qn) and unit weights (Wn). For the purpose of calculation of WQI for the study area, eight water quality parameters have been selected. They are TDS, pH, TA, TH, NO3, Cl, Fe and SO4. The values of these parameters are found high above the permissible limits in some of the samples of the study area. The higher values of these parameters would increase WQI value. The standard values of water quality parameters and their corresponding ideal values and unit weights are given in Table 7.2.

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Table 7.2

Standard values of water quality parameters and their corresponding ideal values and unit weights

S. No

Parameters

Sn

Recommending Ideal Value

k

Unit

Agency for Sn

( V id)

value

weight

1

TDS

500

IS

0

0.286

0.000572

2

pH

8.5

IS

7

0.286

0.033647

3

TA

120

ICMR

0

0.286

0.002383

4

TH

300

ICMR

0

0.286

0.000953

5

NO3

45

IS

0

0.286

0.006356

6

Cl

250

IS

0

0.286

0.001144

7

Fe

0.3

IS

0

0.286

0.953333

8

SO4

200

IS

0

0.286

0.00143

7.5

WQI OF THE STUDY AREA The WQI values of the study area for premonsoon and

postmonsoon samples are calculated separately. WQI has been calculated based on eight selected hydro chemical parameters given in Table 7.2 for the all the seventy three samples. 7.5.1

WQI of premonsoon groundwater samples The WQI values of the premonsoon samples are summarized in

Table 7.3. 5 samples of the study area are very poor to unfit for drinking and the groundwater from these locations require proper water treatment before use. 68 samples of the study area are excellent and good in quality during premonsoon season.

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Table 7.3 WQI values of premonsoon samples Sample ID

WQI

Sample ID

WQI

Sample ID

WQI

Sample ID

WQI

1

36

21

4

41

3

61

7

2

66

22

36

42

68

62

1

3

36

23

3

43

3

63

3

4

4

24

4

44

3

64

4

5

3

25

162

45

5

65

4

6

36

26

6

46

2

66

4

7

68

27

4

47

34

67

36

8

5

28

5

48

37

68

2

9

4

29

36

49

4

69

5

10

4

30

99

50

67

70

3

11

3

31

3

51

3

71

4

12

3

32

4

52

6

72

4

13

7

33

3

53

3

73

2

14

3

34

4

54

192

15

4

35

5

55

3

16

5

36

5

56

2

17

4

37

4

57

162

18

100

38

2

58

2

19

5

39

4

59

3

20

5

40

2

60

2

7.5.2

WQI of postmonsoon groundwater samples The WQI values of the postmonsoon samples are summarized in

Table 7.4. 8 samples of the study area are very poor to unfit for drinking and the groundwater from these locations require proper water treatment before use. 65 samples of the study area are excellent too good in quality during postmonsoon season.

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Table 7.4 WQI values of postmonsoon samples Sample WQI Sample ID WQI ID 1 3 21 4 2 162 22 36 3 5 23 3 4 3 24 3 5 4 25 100 6 3 26 38 7 163 27 4 8 4 28 68 9 4 29 68 10 5 30 69 11 3 31 34 12 67 32 4 13 38 33 34 14 4 34 36 15 4 35 576 16 5 36 68 17 5 37 35 18 5 38 3 19 4 39 4 20 4 40 3

7.6

Sample ID 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

WQI Sample ID WQI 3 35 4 194 99 34 3 38 3 98 67 36 67 416 35 67 448 3 34 34

61 62 63 64 65 66 67 68 69 70 71 72 73

34 37 35 3 36 5 36 37 4 6 3 4 37

RESULTS AND DISCUSSION WQI is commonly used for the detection and evaluation of water

pollution and it may be defined as the reflection of composite influence of different quality parameters on the overall quality of water. The WQI of the study area during premonsoon and postmonsoon seasons are analyzed here. The overall water qualities during premonsoon and postmonsoon seasons with representing samples are presented in Table 7.5.

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Table 7.5 WQI of the premonsoon samples and postmonsoon samples S.No

1

WQI

0-25

Representing premonsoon samples

Status

Excellent

S4, S5, S8, S9, S10, S11, S12, S13, S14, S15, S16, S17, S19, S20, S21, S23, S24, S26, S27, S28, S31, S32, S33, S34, S35, S36, S37, S38, S39, S40, S41, S43, S44, S45, S46, S49, S51,S52, S53, S55, S56, S58, S59, S60, S61, S62, S63, S64, S65, S66, S68, S69, S70, S71, S72, S73

Representing postmonsoon samples

S1, S3, S4, S5, S6, S8, S9, S10, S11, S14, S15, S16, S17, S18,S19, S20, S21, S23, S24, S27, S32, S38, S39, S40, S41, S43, S47, S49, S58, S63, S65, S68, S69, S70, S71, S73

2

26-50

Good

S1, S6, S22, S29, S47, S48, S67

3

51-75

Fair

S2, S7, S50

4 5

76-100 101-150

18,30,42 Nil

6

157

Poor Very poor Unfit for drinking

S13, S22, S26, S31, S33, S34,S42, S46, S48, S52, S55, S59, S60, S61, S62, S64, S66, S67, S72 S12, S28, S29, S30, S36, S37, S51, S53, S56 S25, S45, S50 Nil

S25, S54, S57

S2, S7, S35, S44, S54, S57

The WQI of premonsoon samples state that 78.08% of the total groundwater samples are excellent, 9.59 % are good, 4.13 % are fair, 4.13 % are poor and 4.13% are unfit for drinking. And the WQI of postmonsoon samples state that 49.31 % of the total groundwater samples are excellent, 26.03% are good, 12.33 % are fair, 4.11 % are poor and 8.22% are unfit for drinking. The groundwater quality is better during premonsoon season than during postmonsoon season.