Bhujal News Quarterly Journal, April-Sept, 2009
Dasdia ‘mouza’ of Haringhata-I, Block Nadia District of West Bengal is bounded by longitudes 88°35′23.16″ - 88°36′13.18″ and latitudes 22°57′7.85″ - 22°56′47.2″. Bamanbaria (J. L. No. 39) bound the village in the north, Dutta Para (J. L. No. 67) in the south and Jamuna river in the east and west (Fig. 1). The total area of the ‘mouza’ is 1.72 sq km.
In the absence of any other sustainable water sources, groundwater is being usedextensively for drinking, domestic, and agricultural purposes in Dasdia ‘mouza’. With increasing population, the demand for water has increased manifold. This has led tocontinuous fall in the water table over the years. Apart from this, the groundwater is alsocontaminated with arsenic. As reported from local people three villagers have died recently due to arsenic contamination and many people are showing signs of arsenic contamination on their body. Therefore, this ‘mouza’ needs special attention from the point of view of groundwater. Thus a detailed study was carried out during the period 8th January, 2006 to 18th January, 2006 to recommend suitable methods for (i) supplying arsenic-free water to the local people and, (ii) artificial recharge to groundwater for arsenic dilution and preventfurther recession of the water table.
Based on the ‘mouza’ map (scale 16” = 1 mile), the plot numbers were selected formeasuring water level in selected network stations within the said plot. The plotnumbers were selected in such a way that their spatial distribution covered the entire‘mouza’. Fifty eight network stations were selected for measuring the water level,reduced level and collecting water samples for chemical analysis of arsenic and iron.The water samples collected each day were sent to the laboratory for chemicalanalysis of arsenic and iron within twenty four hours. The data were used to preparevarious thematic maps using GIS (ILWIS 3.3 Academic Version). To select suitablesites for artificial recharge of groundwater and supply of arsenic free water usingcross and overlay operations of ILWIS 3.3 Academic Version was used.
The land use pattern of this area has undergone a pronounced change over years.The extensive plains lying adjacent to river Jamuna are agricultural land wherecultivation is carried out through the year using either groundwater or surface water(river and pond). The northern and central parts of Dasdia ‘mouza’ are residentiallands. On the other hand the eastern, western and southern parts are used foragriculture purpose throughout the year and groundwater is being tapped forirrigating the land through motorized shallow tubewells. The soil of the area consistsmainly of sands, silts and clays deposited by the Jamuna river. The soil contains 50 -56% sand, 20-40% clay, silt, and 4% organic matter. The pH value of this soil isgenerally 6.5. The climate is predominantly influenced by the northeast andsouthwest monsoons. The region enjoys a tropical rainy climate with a distinct dryseason in December. The average annual rainfall of the area is around 2000 mm.January is the coldest month when temperature rarely goes below 10oC. Summer is hot and oppressive. May is the hottest month, when average temperature rises to30oC. The area does not exhibit any marked topographical features. The landscape is mostly flat, the elevation varying between 7.26 and 9.14 m above mean sea level (Fig. 2). The area is drained by river Jamuna in both the east and west. There are numerous surface water bodies (ponds), spread over the area, which forms local topographic depressions. Most of these water bodies dry up during summer. The large ponds in the area are generally perennial in nature.
The study area covers a very small part of the extensive Indo-Gangetic alluvial plainof Quaternary age, which is a part of the Bengal Basin. The Quaternary geology of thearea consists of younger fluvio-deltaic plains. A fining upward sequence of thesediments indicates a fluvio-deltaic environment of deposition. Yellow to yellowishbrowncolour sediments occurring within 35m - 52m depth indicate a fluvio-deltaicenvironment and deposition under oxidizing condition. Dark brown to grey colouredsand from 15m-35m depth was again found which indicate a reducing environment.The provenance of Bengal Delta is closely related with the tectonic history of thisregion. The geological sections through the Bengal Delta, of which Dasdia forms asmall part, indicate the presence of various cycles of depositional environment. Thebasal deposits are marked by gravel and coarse sand followed upward by mediumsand, fine sand, silt and clay.
The geological succession from the Mesozoic to Recent of Nadia district as suggestedby Biswas (1959 and 1963) is presented in Table 1.
Formations belonging to Quaternary systems are the principal repository of thegroundwater in the area under study. Although the lithological characters of theseformations are fairly well known from the logs of the bore holes drilled in the area,the demarcation of the boundary between the Pleistocene and the Recent Series invertical sequences has not been possible due to paucity of faunal and lithologicalevidences.
A generalized lithological succession as revealed from the study of lithologicalsamples brought out during drilling of a shallow drinking water tubewell in the‘mouza’ during the study period is given in Table 2.
In Dasdia ‘‘mouza’’ groundwater occurs in a thick zone of saturation within thealluvial sediments and is unconfined in nature. Field investigation revealed thepresence of a clay bed mixed with silt and fine sand at the top of the lithologicalcolumn. This clay and silt bed is discontinuous in nature and is punctuated by a finesand bed. This indicates that the groundwater in the area is in unconfined condition.Sand bodies, mainly medium to coarse texture, are the principal repository ofgroundwater.
In general three promising saturated granular zones of considerable thickness arepresent within the area. One is from 15 m to 30 m (consisting of fine to mediumsands) the second is at a depth of 36 m to 60 m and the third one below 100mconsisting of medium to coarse grained sand with gravels.
In general tubewells are being sunk to tap two different depth zones, one tapping theaquifer at the depth range of 15 to 60 m bgl and the other tapping the aquifer atdepths greater than 100 m. Amongst the monitored wells, fifty one tubewells tap theshallow aquifer and only seven tubewells tap the aquifer at greater depths. The waterlevel in both these depth ranges are similar, possibly indicating a hydrauliccontinuity between them and hence may be considered as a single aquifer system.
The transmissivity of the aquifer in this region varies from 3497 to 13951 m2/day (Deshmukh et al., 1973). The tubewells constructed within 150m depth are capable of yielding 200 m3/hr. The huge thickness of the alluvium with good aquifer material and effective recharge prospect makes the entire area one of the most developed well fields in the district. The whole region is suitable for construction of deep and shallow tube wells.
The depth to water table varies from 4.21m to 8.02 m below ground level (b.g.l.) withan average of 6.5m b.g.l. The spatial distribution map of the depth to water table isgiven in Figure 3 and is self-explanatory. The area occupied by various depths towater table class is given in Table3. It is observed from the table that thegroundwater rest at a depth of 6-7 m below ground level in 67 % of the total area.
The water table elevation above the mean sea level (msl) varies between -0.27 m to 4.4 m. The negative sign indicates that the water table is below the mean sea level. To determine the direction of groundwater flow and gradient of the water table a contour map depicting theelevation of the water table with respect to msl has been prepared (Fig. 4).
A perusal of the map reveals that there are three groundwater mounds in the eastern part of‘mouza’ flanking the Jamuna river. The northernmost one is defined by the 4m contour. The groundwater flows in all directions from this mound towards the two troughs located to the northwest and east-southeast of the mound. The northwestern trough is defined by 1.0 m contour while the eastsoutheast one is defined by the 1.5 m contour. The easterly flow of groundwater indicates that the Jamuna river is effluent in nature, receiving water from thegroundwater body. The second groundwater mound is located south of this moundand is defined by the 3.5 m contour. From this mound groundwater again flows in alldirections. A part of the easterly flow is towards the Jamuana river and hence in thispart the Jamuna river is also effluent in nature. Towards the southwest of thismound, in the extreme southern part of the ‘mouza’, there is a large groundwater trough. The deepest part of this core lies about 0.27 m below msl. Groundwater flowsfrom all directions towards this trough. The third groundwater mound lies in thesoutheastern part of the ‘mouza’ defined by the 2.0 m contour. The groundwater flowis again radial. South of plot no. 1066 along the Jamuna river the groundwater is ata higher elevation (around 1.5m a.m.s.l) and towards the west the elevationdecreases. This indicates that the groundwater flows away from the river and hencethis part of the Jamuna river has an influent character losing water to the aquifer.
Using the water table elevation data, trend surface analysis has been carried out using ILWIS 3.3 Academic Version to separate the local fluctuations in water levels from the major flow system by fitting a mathematical surface to the water table elevation data represented on a map. Fitting of trend surface is essentially a statistical technique involving non-orthogonal polynomials. The principle of least squares has been used to compute the surface of best possible fit. The computed surface is such that the sum of squares of the distances between the observed elevations of the water table and the corresponding point on the mathematically fitted surface is the least.
Polynomial trend surfaces of degrees1 to 4 have been fitted to the water table elevation data in order to separate the major component of the spatial variation of the variable from the less significant component. The goodness of fit and correlation coefficient increases with increase of degree of the computed surface, but so far as the present data are concerned, the degree-four surface is statistically highly significant and has been utilized for separating the local fluctuations from the major groundwater flow system in the aquifer underlying the study area.
The general equation of the trend surface of the water table is as follows:
Z = 1.440 + 0.002*X + 0.008*Y + 6.708e-007*X2 +3.940e-006*XY + 6.490e-006*Y2 + (-)6.804e-009*X3 +(-)2.523e-008* X2Y + (-)2.323e-008*XY2 + (-)4.575e-008*Y3 +(-)1.476e-012*X4 +(-)2.906e-011* X3Y +(-)6.905e-011*X2Y2 +(-) 1.400e-011*XY3 +(-) 1.507e-011*Y4
where, X and Y are the Cartesian coordinates of the network stations and Z is thecalculated elevation of the water table with reference to the mean sea level
A perusal of the degree-four trend surface map (Fig. 5) reveals that there is a WNWESEtrending groundwater mound covering the northeastern part of the ‘mouza’. Groundwater flows outward from this mound in all directions. A part of the groundwater flowstowards a groundwater trough located in the southern part of the ‘mouza’. The troughtrends roughly parallel to the mound. Further south there is a tendency of the formation ofanother mound from where groundwater flows in to the trough. These mounds and the trough control the flow of groundwater in the aquifer underlying Dasdia ‘mouza’.The local flow components indicated on the water table elevation contour map (Fig. 4)have been eliminated in the trend surface map, thus bringing out very clearly themajor groundwater flow system. This major groundwater flow directions will help usin pinpointing sites for artificial recharge to groundwater in concomitant with otherparameters.
Fifty eight groundwater samples were collected from different network stations foranalysis of arsenic and iron. The summary of the results of the chemical analysis ispresented in Table 4.
A perusal of the results of chemical analysis reveals the following:
• Arsenic concentration of 32 water samples is below the standard.
• Arsenic concentration of only 2 water samples coincides with the standard
• 41 % of the samples have arsenic concentration higher than the standard of0.05 mg/l.
• The highest concentration (0.61 mg/l) of arsenic is observed in plot no. 1323,located in the southern part of the ‘mouza’.
• The lowest concentration (0.022 mg/l) of arsenic is observed in plot no. 1130,located in the southern part of the ‘mouza’.
• Arsenic concentration of the deeper part of the aquifer i.e. > 100 m b.g.l. isalso very high ranging from 0.038 to 0.61 mg/l. Therefore, the deep aquifer isalso not safe to be tapped for drinking purpose.
• Iron concentration of all the samples is higher than the desirable level of 0.3mg/l.
• 12% of the samples have iron concentration greater than the permissible limitof 1 mg/l.
To understand the spatial distribution of arsenic and iron in the groundwater ofDasdia ‘mouza’ two maps (Figs. 6 and 7) showing various concentration zones havebeen prepared. A perusal of the map showing spatial distribution of arsenic (Fig. 6) ingroundwater reveals that the entire eastern part of the ‘mouza’ along the Jamunariver and in the southern part of the ‘mouza’ the groundwater has arsenicconcentration above the permissible limit of 0.05 mg/l. The rest of the area thegroundwater has concentration below the desirable limit except for four smallpockets where the concentration is just above the desirable limit. Two of thesepockets are located in the northern part of the ‘mouza’ and the other two are locatedin the southern part. The total area where arsenic concentration is
The spatial distribution of iron in groundwater of Dasdia ‘mouza’ (Fig. 7) reveals thatthe area has concentration above the desirable limit of BIS 10,500, 1991. An area ofabout 0.13 sq km. in the southeastern part of the ‘mouza’ has iron concentrationabove the permissible limit of 1.0 mg/l (Table 6). The groundwater of plot no.1295located in the south-eastern part of the ‘mouza’, just beside River Jamuna, hasmaximum iron concentration of 1.21 mg/l. On the other hand the minimumconcentration of 0.38 mg/l is observed in plot no.705 located in the south-centralpart of the ‘mouza’.
WATER RESOURCE DEVELOPMENT AND MANAGEMENT
In order to pinpoint the probable sites for tapping surface water after necessarytreatment for arsenic free drinking water and artificial recharge of groundwater CrossOperation and Overlay methods of ILWIS 3.3 Academic Version has been used. Atfirst the depth to water table map has been crossed with the spatial distribution mapof arsenic. The area with arsenic concentration is
Location A: The area in around this location has very high arsenic concentration(>0.2 mg/l). A wetland is located in Plot no. 1175. The wetland rests on agroundwater trough and hence groundwater will flow into the wetland and will keep it perennial throughout the year. This wetland can be used to supply arsenic freewater. Before using this wetland the following steps should be implemented:
1. The wetland water should be tested to determine the arsenic concentrationarsenic at various places.
2. The wetland should be desilted.
3. The catchment area of the wetland should be kept free of activities such asbathing, washing of clothes and utensils, bathing and drinking of animals,dumping of household and other wastes.
4. The water from the wetland should be pumped into the primary tank wherechlorine and alum should be added for primary treatment. This partiallytreated water should then be passed through a slow sand filter bed and theninto a tertiary chamber where the water may again be treated with chlorineand alum. From this chamber, treated water can be taken for drinkingpurpose through a tap. It is necessary that the villagers pump water into theprimary tank before taking water from the tertiary tank.
Location B: In and around this location Plot no 1049 is a wetland that can also beused to supply arsenic free water. Arsenic concentration in the area east of thiswetland ranges between 0.05 and 0.1 mg/l. The wetland is located on a groundwatermound. Hence this wetland should be desilted or excavated up to a required depth tomaintain water throughout the year or a shaft may be constructed in the wetland totap the base flow. The other steps as mentioned for location A should also be strictly adopted.
Location C: This is the third alternative site located near the school (Plot no. 1120).Just beside the school there are a number of wetlands. One of them may be selectedfor providing arsenic free water. Here also steps 1 - 4 should be strictly adopted toensure safe drinking water.
For artificially recharging the aquifer through roof top rainwater harvesting Plot no.1051 near Location B is the only choice. In this plot, there is a concrete roof of about850 sq ft. The arsenic concentration is between 0.05-0.1 mg/l and the depth to water table is 4-5 m. Rainwater collected on the roof can be recharged into the aquiferthrough a bore well with strainers placed at depth of 50 m. The plot is located on thegroundwater mound and hence the recharged water will move in all directions awayfrom the mound and dilute the arsenic present in the aquifer in and around the areaespecially in the eastern direction. Piezometer may be installed and monitoredcontinuously to understand the effect of the recharged water on the arsenicconcentration.
The authors convey thanks to Prof. Ashoke K. Dutta, Director, IISWBM and Mr.Stephen Gonsalves, Director, Calcutta Urban Services for their encouragementduring this work. The authors also acknowledge the help rendered by Mr. PradipSingharoy during the fieldwork. The authors are thankful to SIMAVI, Netherlands fortheir financial assistance.
• Biswas, B., 1959, Sub-surface geology of West Bengal, India, Proc. Symp. Dev.Petrol. Res. ECAFE, Min. Res. Dev. Sr. No. 10, p. 159-161.
• Biswas, B., 1963, Result of exploration for petroleum in the western part ofBengal Basin, India, Proc. of the Second Symposium on the Development ofPetroleum resources of Asia and Far East, Mineral Resources Development SeriesNo. 18, united Nations, Bangkok, p. 241-250.
• Deshmukh, D. S., Prasad, K.N., Niyogi, B.N., Biswas, A.B., Guha, S.K., Seth,N.N., Sinha, B.P.C., Rao, G.N., Goswami, A.B., Rao, P.N., Narasimhan, T.N., Jha,B.N., Mitra, S.R. and Chatterjee, D., 1973, geology and Groundwater Resourcesof the alluvial areas of west Bengal, bulletin of the Geological survey of India, Sr.B., No. 34, 451p.
P. K. Sikdar, Paulami Sahu, Surajit Chakraborty, Antara Adhikari, Piyali Halder -Department of Environment Management, IISWBM, Kolkata
Tania Majumder - Calcutta Urban Services, Kolkata