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ROOF TOP RAIN WATER HARVESTING FOR ARTIFICIAL RECHARGE TO THE DEEPER AQUIFER IN BISHNABGHATA-PATULI AREA (KOLKATA)

Kolkata is one of the most densely populated cities of the world. With the increase in population and industrial activity, demand of water is increasing every day in Kolkata and it has caused increase in use of surface water from Hugli river and also that of ground water from the aquifer, which is under confined condition. Though the city’s average annual rainfall is 1600mm but tremendous ground water development has caused decline in ground water level.
To augment the water resources of the deeper aquifer, Central Ground Water Board, ER, Kolkata has taken a pilot project for artificial recharge to the ground water of the deeper aquifer through roof top rainwater harvesting. The site is located at Baishnabghata-Patuli township, which is in the southern corner of Kolkata city (Fig- 1). In Kolkata it is the first scheme of roof top rainwater harvesting for artificial recharge to ground water of the confined aquifer.

HYDROGEOLOGY OF KOLKATA AREA :

Kolkata is underlain by Quaternary sediments consisting of a sequence of alternation of clay, silty clay, sand and sand mixed with occasional clay. Clay horizon ranging in thickness from 30-50m occurs at he top of the sedimentary sequence. This clay is underlain by a granular zone made up of sand of different sizes with occasional gravel and this forms the aquifer. The principal productive aquifer generally occurs within the depth range of40-100m in the north, 50-160m in the central and southern part and 180-300m in the southwestern part of the city. A minor aquifer consisting of fine to medium has also been found to exist in some parts within the depth range of 20-40m. The value of transmissivity varies from 1700-5200m2/day.
Depth to ground water level ranges from 3.34m to 16.32m bgl in pre monsoon period and from 1.57m to 15.71m bgl in post monsoon period. Study reveals that water level in the confined aquifer in the city area has declined around 3-4m in the last 14 years (Fig-2&3).The chloride concentration in ground water is above 500ppm in the northern and western part of the city, 250-500ppm in the area between Belgachia and Park street and within 250ppm in Garia - Jadavpur area.

HYDROGEOLOGY OF THE PROJECT AREA (BAISHNABGHATA-PATULI (AREA) :

The Project area is underlain by a thick layer of impervious clay upto 27m bgl. from the surface and it is underlain by thin layer of sand of 3m thickness. This sand layer is followed by thick layer of clay down to a depth of 49m bgl. The productive aquifer consisting of fine to medium grained sand occurs below this clay layer down to the explored depth of 152.45 m bgl. Depth to water level varies from 14.68m bgl to 16.32m bgl in pre monsoon period and from 13.41m bgl to 15.71m bgl in post monsoon period.
The study reveals that the presence of thick clay layer near the surface in Kolkata city area act as a barrier to the natural ground water recharge to the underlying aquifers.

Design of the artificial recharge structure in Baishnabghat-Patuli area.

Baishnabghat-Patuli area
 
 
Head
Description
Location: In the campus of ALL INDIA SOIL AND LAND USE SURVEY, GOVT. OF INDIA, KOLKATA.
Roof area: 724 sq.m.
Average annual rainfall: 1600mm.
Quantity of rain water available: 1158.4 m3
Quantity of rain water available for recharge: 926.72 m3 (80%)
Aquifer material: Fine to medium sand.
No. of recharge well: 2
Depth of recharge well: 119.38mbgl.
Diameter of recharge well: 150mm.
Zone tapped: 102.61-114.77mbgl.
Slot size: 1.0mm.
Clay sealing: GL to 85.0mbgl.
Gravel shrouding: 85mbgl to 119.50mbgl.
Static water level: 13.0mbgl.
Discharge: 284 lpm.
Drawdown: i1.1m (After 220 minutes of pumping).
Nos. of recharge trench: 1
Length: 11.57m
Width: 1.65m.
Depth : 3m.
Filter media: sand, gravel and boulder.
Control unit: 2 Nos. control units are situated near the recharge trench.
 











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