GROUNDWATER IN KOLKATA - AN OVERVIEW OF SPATIAL DISTRIBUTION PATTERN OF CHEMICAL PARAMETERS
Pradip K Sengupta
INTRODUCTION
Kolkata is one of the most populated metropolitan cities of India. For the last 300 years this city has experienced a huge population growth. Due to huge development and increase in population demand of water for domestic purpose mainly has increased by many folds. Domestic water supply is done mainly from the Hoogli River through the Tala pumping stations and the Garden Reach pumping stations. In spite of this surface water sources a huge amount of water is drawn from the groundwater aquifers below Kolkata. This exploitation is so huge that permanent depletion of water level has occurred in the groundwater of Kolkata. The state government and the Central Groundwater board conduct regular monitoring of water level and water quality of Kolkata. The studies have revealed that water quality in Kolkata has deteriorated and arsenic contamination in ground water has been observed in some parts of Kolkata. Study of arsenic contamination in groundwater in Kolkata was conducted in 2001 by the author. 119 tube wells were studied and arsenic was found in 26 samples.
OBJECTIVE
The Objective of the study was to establish the geomorphology of Kolkata with the help of secondary parameters and to find out if there is any scope for correlating geomorphology with chemical quality of groundwater with special emphasis on arsenic concentration.
GEOLOGICAL SETTING OF THE AREA
In the study area, fluvial processes have resulted in the formation of extensive Holocene flood plains with a dominance of coarser grained sediments, representing the overlapping of a number of sub- deltas). Avulsion of the major streams in the area, which are tributaries or distributaries of the River Ganges, within a time scale of 100 years, has resulted in a thick layer of tens of meters of Recent overbank silts and clays incised by channel sands The coastal region of the south Bengal has a mixture of fine- grained sand and mud deposits with peat layers, which have resulted from eustatic influence. Deposition of the lowest parts of the Bengal Alluvium began at the onset of the Pleistocene glacial maximum, with sea level (i.e., regional base level) at least 100 m below present MSL. Thus, the rivers draining the plain during that time must have scoured through the earlier plains. Around 11,000 to 10,000 years before present (BP), deposition of lower delta mud over low-stand oxidized sand units started, suggesting sea level rise to about 45 m below present MSL. The present GBM delta began to prograde into the Bay of Bengal at this time. From ∼10,000 to ∼7,000 years BP, rapid marine transgression occurred resulting in the deposition of fine sediments. Since ∼7,000 years BP to the present, the general trend of the eustatic sea-level curve shows continuing sea-level rise, although with a decreased gradient. (Mukherjee et al, 2007)
Geologically and geomorphologically Kolkata belongs to the lower deltaic plain of the Ganga-Padma river system.. The surface material is clay and clay loam. This clay extends up to a depth of 10 to 25 m bgl in most of the area. Below this clay bed a fine sand bed is found which extends up to a depth of 30-to 35-metre bgl. Below this level another clay, dark brown to grayish brown in colour occur up to a depth of 60 to 100-metre bgl. From this depth another sand zone occur which comprises of fine, medium and coarse sand and extends up to a depth of 120 to 180 metre bgl. Below this sand zone gravel bed occurs. Tertiary black and sticky clay occurs at the bottom of the sand and gravel zone.
From a general hydrogeologic point of view, these sediments have been categorized as aquifer (sand and gravel) and aquitard (clay). The position of the sandy clay is ambiguous: it can act as either less permeable aquifer or higher-permeability aquitard. Its exact category will vary from locality to locality based on the sand/clay ratio and permeability. Although the less permeable sediments like clay transmit some groundwater, they separate the overlying aquifer(s) from lower aquifer(s) by hydraulic conductivity (K) contrast. In the study area, the extent, thickness and K of these clay or aquitard layers are very important as they govern the three-dimensional flow of groundwater at the regional scale. In this report, the names of the sediment types and hydrogeologic categories will be used interchangeably for the description of both hydrostratigraphy and groundwater flow (Mukherjee & Alan E. Fryar & Paul D. Howell 2007)
The uppermost surface of Kolkata is clay of thickness between 5 and 40 metres. The upper clay contains at places lenses of fine sand and peat, which often act as perched aquifers. But these aquifers yield little water. Water content in this clay is great and a small amount of water may trickle down to the underlying sand zone when the piezometric level of the aquifer is sufficiently low.
The first or uppermost aquifer is about 10 to 20 metre thick on an average. The material is essentially fine sand. Most houses in Bansdroni and Garia area lift water from this zone.
The second aquifer occurs between the depth of 65 metre and 120 metre and this aquifer is about 30 to 50 metre thick. This is the most potential and most exploited aquifer of Kolkata. The present study is confined to 1st and 2nd aquifer only.
In general, the upper aquifer material comprises sub-rounded to sub-angular fine to medium sand with occasional clay lenses. Its heavy mineral assemblage (opaques, altered biotite, garnet, tourmaline, kyanite, zircon) indicates a mixed metamorphic-cum-igneous provenance. The intermediate aquifer constituted of sub-angular to sub-rounded medium sand, sandy clay and clay with fine sands and its heavy mineral assemblage (biotite, garnet, kyanite, opaques) indicates a dominantly metamorphic origin.
While the lower aquifer is constituted of sub-rounded to rounded fine to coarse sand with occasional clay bodies, and its heavy mineral assemblage (opaques, altered biotite tourmaline, rutile etc.) indicates an igneous provenance (Steering Committee Arsenic Investigation Project, PHE Dept, Government of West Bengal, 1991). The intermediate aquifer usually shows arsenic contamination. At the greater depth, aquifer arsenic may not be present at the beginning but may become contaminated in the course of time. This is what had been observed during our study.
GEOMORPHOLOGY
It is very difficult to identify the different geomorphological units in Kolkata due to the urbanization and absence of exposure of surface materials in many areas. Satellite data do not reveal the detail geomorphic features apart from the pattern of settlements. Study of old maps (1817) published by the NATMO reveals that the settlement of Kolkata at that time was along the Hoogly River extending upto 2to 3 km towards the east. As old settlements generally develop on the levee of a river-side it is assumed that the Hoogly river levee extends from the hoogly river bank to the east.
Land level data generated from reduced level survey of the monitoring wells have been used to generate a contour map of Kolkata with 0.2 meter interval. This map reveals the surface features of Kolkata. From the contour map and traverse survey along Adiganga the reconstruction geomorphology of Kolkata is attempted. These maps show high land near the Hoogly River and also along the channel of Adiganga. The surface is gently sloping towards the east and south east and formed low lying areas in the east which transgresses into the saline lakes or east Kolkata wetlands.
Another attempt was made to determine the depositional features using spatial distribution pattern mapping of the thickness of surface clay of Kolkata. The map is given in figure.5.This map shows that along the Hoogly River the thickness of clay is maximum whereas along Adiganga channel the thickness is moderate. In the eastern part and in the south western part the thickness is minimum.
During this study ttempt has made to understand whether there is any relation between geomorphology or the sedimentation pattern and the chemical quality of groundwater. The groundwater chloride concentration and isoconductivity map of Kolkata is given in Figure 5 and Figure 6. These maps show some distinctive patterns compatible with both surface contour and clay thickness. Along the Adiganga channel both specific conductivity and chloride concentration is low. On the other hand both chloride concentration and specific conductivity is high near the Hoogly river and also at the eastern low lands.
Using these maps the geomorphology of Kolkata is reconstructed.
Geomorphologically Kolkata may be divided into four major features.
1. Hoogly levee deposit: Extends upto 2-3 km from the Hoogli river bank
2. Adiganga levee deposit: Extends from north to south covering a large area from Khidirpur, Alipur, Tollygunge, Bansdroni, Naktal and Garia
3. Adiganga channel deposit : Covers a small srip along the river in Naktala and Garia
4. East Kolkata Marshland deposit: Covers a large area in the eastern part starting from, Ultadanga, Park Circus, Santoshpur and Garia in the east and extends towards the east
DEPTH TO WATER LEVEL CONDITION
Due to heavy exploitation of groundwater a major change has been occurred in the water level condition of Kolkata. Investigations conducted in Kolkata for the last 20 years reflect alarming depletion of piezometric level. At present the piezometric level is 14 to 16 metres below ground level in the Alipur, Babughat, Ballygunge, Kalighat, Park circus area whereas in the Bansdroni and surrounding areas this level is 9 to 11 metre deep. At Garia and surrounding areas the piezometric level is between 8 and 10 metre below ground level. In the north at Baguihati, Sinthi, and Beleghata area water level in pre monsoon period is 12 to 14 metre below ground level, whereas in dumdum and surrounding area this level is from 10 to 12 metre BGL. Piezometric level is at a depth of 6 to 10 metre below sea level on an average. During the post monsoon period piezometric level rises to the tune of 1 to 1.5 metre in Alipur and about 2 metres in Bansdroni and Garia and in north Kolkata and Dumdum.
GROUNDWATER FLOW
Groundwater movement in areas of flat topography in the Bengal basin may be mostly vertical and lateral flow may be limited to local scale. Sikdar et al. (2001) reported the presence of north–south regional flow near Calcutta in the 1950s, but flow had dramatically changed by the 1980s. Harvey (2002) argued against the persistence of any regional flow system in view of the extensive irrigation pumping currently practiced in the Bengal basin. Surface water–groundwater interaction generally occurs within local flow systems. The River Bhagirathi-Hoogly is a losing stream along most of its length and recharges the shallow aquifers. Deeper groundwater generally has insignificant interaction with the surface water bodies, and the deeper aquifers have restricted recharge (Mukherjee et al. 2007). The premonsoon water table contour map of Kolkata shows concentric flow and the contour pattern has no similarity with the topography or the chemical quality of groundwater.
ARSENIC IN GROUNDWATER
Arsenic contamination in the Bengal Delta is confined to the Holocene Younger Delta Plain and the alluvium that was deposited around 10,000–7,000 years bp, under combined influence of the Holocene sea-level rise and rapid erosion in the Himalaya. Further, contaminated areas are often located close to distribution of abandoned or existing channels, swamps, which are areas of surface water and biomass accumulation. Extensive extraction of groundwater mainly from shallow aquifers cause recharge from nearby surface water bodies. Infiltration of recharge water enriched in dissolved organic matter derived either from recently accumulated biomass and/or from sediment organic matter enhanced reductive dissolution of hydrated iron oxide that are present mainly as sediment grain coatings in the aquifers enhancing release of adsorbed arsenic to groundwater. ( Mukherjee et al 2007)
Study for arsenic contamination in Kolkata was conducted by the author in 2001. 119 water samples were collected from different locations.
