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Ground water and Surface water samples from different districts of West Bengal were collected at block levels both for pre-monsoon and post-monsoon seasons.Different water quality parameters were determined both at the spot and laboratory. For a ground water sample in the Purbasthali-I block the pH and dissolved oxygen were found to be 7 and 7.9 ppm respectively.For a surface water sample in the Kalna block the above parameters varied as 6 and 7.7 respectively. Again the parameters Conductivity(2mS),TDS (2ppt) and As(ppm) for a pre-monsoon ground water sample in the Burdwan district showed 0.833,0.571 and nil. Again when the total hardness(CaCO3)(mg/L) and Chloride(mg/L) for a post-monsoon surface water sample of Burdwan district showed a value of 227.876 and 693.4 whereas for the Birbhum district for a post-monsoon data of a ground water sample showed presence of 71.125 and 959.2 respectively. Based on the parameters evaluated as above, water was judged in terms of their applications as drinking, domestic, irrigation , bathing or survival of aquatic life. In case of Purbasthali block, Burdwan district, water from a deeper tube well was found to be suitable as potable water compared to a shallow one as far as arsenic contamination is concerned. All the data thus generated have been shown in the various tables.

The main aim of this paper is to study the different water quality parameters of several South Bengal districts of West Bengal.

The Ganga River being the major river in India is concerned with the domestic, agricultural and industrially discharged from various sources. The LULC (Land Used and Land Cover) study carried out on this river involving collecting water samples show that expansion of built-up and agricultural lands is causing a reduction in true cover and water bodies. As Kolkata city continues to expand, the water quality is a matter of concern in the river and surrounding areas. The findings indicate that across the buffer zone LULC changes to a significant amount. A growing trend in the built-up area covering the Hooghly river is attributed to an increase in population. In addition, an expansion of agricultural areas at the expense of water body, leading to concern about the impact of Chemical fertilizer on the  water quality.

The study highlights the need for suatainable land use practices to preserve the rivers eco-system and maintain water quality.

The study has demonstrated the impact of changes in land use and land cover(LULC) and population growth on water quality. Localities in the vicinity of Dakshineswar, Shibpur and Garden Reach are particularly vulnerable to water quality detorioration due to LULC changes and increase in population density. The results revealed that areas experiencing significant LULC changes and population growth, exhibit poorer water quality.

The significance of a chemical analysis depends to a large extent on the sampling programme. An ideal sample should be one which is both valid and representative. These conditions are met by collection of samples through a process of random selection. This ensures that the composition of the sample is identical to that of the water body from which it is collected and the sample shares the same physico-chemical characteristic with the sampled water at the time and site of sampling.

The relevant factors for any sampling program are

(a)   Frequency of sample collection

(b)   Total number of samples

(c)    Size of each sample

(d)   Sites of sample collection

(e)   Method of sample collection

(f)    Data to be collected with each sample,  and

(g)   Transportation and care of samples prior to analysis.

For analysis of natural and waste water, two principal types of sampling procedures are employed:

1.     Spot or grab samples are discrete portions of water samples taken at a given time. A series of grab samples, collected from different depths at a given site, reflect variations in constituents over a period of time. The total number of  grab samples should satisfy the requirements of the sampling programme.

2.     Composite samples are essentially weighted series of grab samples, the volume of each being proportional to the rate of flow of the water stream at the time and site of sample collection. Samples may be composited over anytime period, such as 4,8 or 24 hours, depending on the purpose of analysis. Such composite samples are useful for determining the average condition which, when correlated with flow, can be used for computing the material balance of a stream of water body over a period of time.

It may be stated, in general, that it is more meaningful to analyse a large number of separate samples taken at different times and different locations than to compile and analyse a single representative sample.

Separate samples must be collected for chemical and biological analysis since the sampling and preservation techniques are quite different. For accurate analysis, it is desirable to allow a short-time interval between sampling and analysis. As a matter of fact, temperature, pH and dissolved gases (D.O) must be determined in the field and as quickly as possible after sampling.[1]

Water Quality Parameters and Standards

The parameters for water quality characterization are listed in Table-T-1The permissible limits, as laid down by the United States Public Health drinking water standards (USPH) and Indian Standard Institution (ISI) are listed for comparison. This refers to domestic water supplies for drinking water (Table-T-1). The ISI values, available for only a few parameters, are much higher than those for USPH, obviously for no good reasons.

Table-T-1 Showing Water Quality Parameters and Standards By USPH & ISI

Variation of pH and D.O.

From Tables it is observed that pH of the water samples collected from different blocks of Bardhaman district (both for the pre-monsoon and post-monsoon seasons) are within almost neutral region (6.0 - 8.0)[Table I]. The D.O. (ppm) on the other hand varies over a wide range (2.5 for BWN (GW)6 to 10.6 for BWN (SW)13[Table I]. The higher value of D.O. signifies a good quality water for the Ganga surface water while the tap water of Parulia Bazar, Purbasthali is not upto the satisfactory level [the permissible level of D.O. in drinking water > 5]. The intermediate level of D.O. of other water samples are of moderated qualities. The pH of the water samples of Hooghly district, lower, are within normal level (6.0 - 1.5)[TableIII] . The Damodar river water of this district is again found to be having highest D.O. content both for this pre-monsoon (10.2 ppm) and post-monsoon season (10.0 ppm)[same table]. The other sources of water are having permissible level of D.O. content (6.1 ppm 8.2 ppm). The pH of the Birbhum district with samples are normal. The D.O. content of this district is seen to be alarmingly low (2.7 ppm) for Maureswari-1[Table V] from a densely populated bus stand sample and can be concluded to be of poor quality-water .

Variation of Conductivity and TDS

The conductivity value (2ms) for the Bardhaman district varies from 0.240 for the tube well sample from Bhatar bazar to as high as 0.876 for a tube well sample of Dihat more, Katwa – II[Table II]. In general it is found that in both cases of ground water and surface water the magnitude of conductivity is lowered to some extent from the premonsoon season onto the postmonsoon season[Table II ].This can be attributed to the dilution effect. The TDS values (2ppt) on the other hand is seen to be very low (0.234) for the Ganga river, Kalna Municipality so also only 0.271 for the same river water sample at Katwa [same tables]. The TDS values for the ground water samples are on the higher range, upto 0.678 (2 ppt). This trend is very much distinct for the premonsoon (Table II), as well as for the post monsoon samples. This observed behaviour can again be due to higher extent of dilution of the dissolved solids in surface water in comparison to when that is confined in ground water.

Similar pattern of behaviour is observed in case of conductivity data (2ms) for the district Hooghly. The lowest value here is seen for the Damodar river (Pursurah) sample (0.298) for the pre-monsoon and (0.253) for the post-monsoon sample Table II . The highest magnitude here is observed for HOOG (GW)3 sample, that is for a ground water (TW) sample which is again lowered in case of the corresponding post-monsoon season results. This trend in behaviour is again manifested in the TDS (2ppt) values - only 0.165 for the HOOG (SW), and 0.137 for the pre-monsoon and post-monsoon seasons respectively. The higher values in case of ground water samples is again prominent here [(0.561) and (0.489) respectively]. In case of Birbhum district, however, we could not collect the premonsoon samples. Here also we find the same trend in behaviour for both the parameters of conductivity (2ms) and TDS (2 ppt), viz., 0.124 andO.079 respectively for the Maurakshi river sample, Table V. Here the highest values of these parameters occur for a ground water sample Birm (GW)2 with magnitudes of 0.75 and 0.49 respectively.

This general behaviour may be due to higher extent of dilution of the different ions in case of a river sample than that of a ground water sample.

Occurrence of Arsenic

In case of Bardhaman district we have found arsenic to occur in case of the Ganga river at Katwa burning ghat both for the pre-monsoon (0.125 ppm) and post-monsoon (0.105 ppm) seasons. At Kalna, Table II  however, downstream with respect to Katwa, arsenic was absent for the Ganga river water sample. This amount of arsenic is twice the permissible level of 0.05 ppm (ISI). Among the other ground water samples one location at Purbasthali- II block [BWN (GW)6] was found to contain 0.04 ppm of As in a tap water sample only in the pre-monsoon season. Another ground water (TW) sample BWN (GW)1 from Bhatar bazar bus stand was found to contain As both during the pre-monsoon (0.025 ppm) and post-monsoon (0.02 ppm) seasons. The BWN (GW)9 sample from Purbasthali - I block was found to contain 0.025 ppm of As at a depth of 70 ft. TW whereas for the TW samples from the same location at higher depths (250 ft and 300 ft) had negligible As content. The Purbasthali block was earlier reported to contain As by several workers.[2,7]. The presence of As in the shallow TW in comparison to the deeper ones may be due to the higher extent of dilution at the lower acquifer containing ground water. For the Hooghly district the blocks Khanakul - 1 and Khanakul - II were affected by ground water As contamination as is evident from the two TW samples HOOG (GW)4 and HOOG(GW)5 having As contents of 0.088 and 0.04 ppm respectively for the pre-monsoon season and 0.062 and 0.032 ppm respectively for the post-monsoon season Table V. The other parts of Hoohgly district are safe as far as arsenic occurrence is concerned.

Fig.1; Pictorial representation of occurrence of arsenic in different parts of West Bengal.

 

Variation of total hardness and chloride

The total hardness of different samples were found to vary from 8 to 228 (mg/L) both for the Burdwan and Birbhum districts. The chloride content was found to be within 355 to 1385 (mg/L) for samples collected from different locations both for Burdwan and Birbhum districts with a higher range for the Birbhum district. The only sample from Tilpara barrange of Mourakshi river was found to contain an abnormally high value of chloride i.e., 1,17,203 (mg/L)[ Table VI].[2,5]

Table I: pH and D.O. Data of Bardhaman District ()Premonsoon

GW = Ground Water, SW = Surface Water, TW = Tube Well

Table II: Conductivity, TDS and Arsenic data (Premonsoon) of Bardhaman district

Table III : pH and D.O. data of Hooghly district (Premonsoon)

Table IV : Conductivity, TDS and Arsenic data (Premonsoon) of Hooghly district

Table V : pH, D.O., Conductivity and TDS data of Birbhum district (Postmonsoon)

Table VI: Total hardness and Chloride data(Postmonsoon) of Birbhum district

The pH values of all the water samples suggest them to be good for agriculture and to support aquatic life. The conductivity and TDS data suggest both to be suitable for agricultural purposes, the surface water in particular than the ground water, BIS limit for TDS being 500 - 2000 mg/L. The hardness of the water samples analysed, on the other hand, suggest them to be suitable for using in industries and for domestic purpose. [BSI limit being 300 – 600 mg/L]. According to BIS the chloride limit should be 250 – 1000 mg/L. Most of the analysed samples of Birbhum district contain chloride above the permissible limit. The chloride concentration serves as an indicator of pollution by sewage. It can also corrode concrete by extracting calcium. For potable water a deeper groundwater should be chosen as far as arsenic contamination is concerned. Purbasthali block of Burdwan district and Khanakul block of Hooghly district were found to be arsenic affected.

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