INTRODUCTION
Royal Calcutta Golf Club is a 40 hectare green area in south Kolkatta which has a good number of ponds that collects rainwater. But during storm rains the ponds overflow and a huge amount of water is drained out. Members of the Golf Club had proposed to utilise the excess rainwater for artificial recharge.
The Jadavpur Centre for Study of Earth Science had been entrusted with the job of extending technical support for this project.
LOCATION AND GEOGRAPHY
The area is located at the midst of the urban metropolis of Kolkata. The land surface is made undulated for the purpose of the golf game. There are more than 25 ponds of various sizes in the area. Some of the ponds are interconnected by surface drains. Each pond or cluster of ponds has its own catchment area. The average rainfall of Kolkata is about 1400 mm.
GEOLOGY
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.
HYDROGEOLOGY
In tollygunge and surrounding areas subsurfage geological information is availble upto a depth of 120 metres below ground level. Lithological log charts of tube wells at golf green, Roybahadur road, Vidyasagar, Bijoygarh and Niranjan pally have been studied.
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.
From the figure I and II it is very clear that the first or near surface aquitard occur upto a depth between 20 to 40 metres below GL at the RCGC area.
Due to heavy exploitation of groundwater a major change has been occurred in the water lev el 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. 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.
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. (Sengupta 2007)
The study of logcharts and interpolated water level data reveals that the upper aquitard (clay and silty clay) occurs within the depth range of 25 metres and 30 metres. Below that a fine sand aqui fer exists. The thickness of this aquifer is assumed to be about 12 metres. The aquitard has very low hydraulic conductivity and the sand layer has hydraulic conductivitybetween 1m/d to 100 m/d . Depth of peizometric surface is about 16 metre bgl during the pre monsoon period. Seasonal fluctuation of water level is very. This explains that there is very little possibility of natural vertical recharge in that area.
Artificial recharge through pond water recharge structure is a good plan to augment groundwater condition of Kolkata. Since RCGC has a good number of ponds a great amount of rainwater may be harvested through these ponds. The land level survey carried out by the M/S CE testing Co. Pvt. L
td shows the catchment area of individual ponds. It is also found that most of the ponds are interconnected for draining out of excess storm water and a large quantity of water is drained out through a pond east of Green 2. From preliminary observation it appears that rainfall covering an area about 15 hectars may be accumulated at that pond. If 40 % rainfall is recharged the volume of total recharge may be about 84000 cubic metres. The environment of RCGC is free from fertilisers, pesticides and other chemical contaminants.
Since the aquifers of Kolkata are essentially confined and the first aquifer below RCGC ground is available at a depth below 25 metres it is advised that the recharge should be made through a recharge shaft.. Before recharging the pond water should be passed through slow sand filter beds. In this case a three chamber filter bed is recommended.
