FUNDAMENTALS OF AQUIFER MAPPING

The water Resource Ministry of Government of India has released a notification and a fact sheet on National Aquifer Mapping Programme (NAQUIM). The Central Groundwater board and the State Groundwater Departments will act as the main functionaries in this programme. It is also decided by the government that national level aquifer mapping will progress substantially through the 11^th five year plan period. Considering the fact that those government departments alone cannot accomplish the job it has been decided that stake holders and NGOs would be involved in this endeavor.  This national level programme also has components like nation wise training and human resource development.

But, what is an aquifer map and what it is for?

The concept of aquifer mapping is not new. Many developed countries like Australia and USA have already completed aquifer maps in certain areas like Murray Darling Basin and Texas. They have already published some of the reports and maps. An aquifer map is not a single map, on the other hand it is a group of several maps that would describe the aquifer system and management criteria and options in an area.

In NAQIM programme of India it has been proposed that aquifer maps would include (1) Identification of Aquifer Management Units (AMU) (2) Evaluation of AMU, (3) Data preparation and generation of different categories (4) Prioritization of AMU, (5) Preparation of aquifer management plans and aqifer management and information system, (6)  Participatory groundwater management.

1.      IDENTIFICATION OF AQUIFER MANAGEMENT UNITS:

Aquifer management units are something different from a Hydrogeological unit. In a hydrogeological map generally geological units are described by its hydrogeological characteristics. But an AMU contains much more than that. It includes the 3 dimensional matrix of all information like extent of the aquifer, depth, thickness, geology, hydrology, land use on it, recharge and discharge related information and its political and social boundaries.

2.      EVALUATION OF OF AMU:

This is the most difficult task. Aquifer evaluations are conducted for various purposes including water supply development as well as groundwater contamination and remediation situations. They can range from fairly intense evaluations involving test drilling, geophysical investigations, installation of monitoring or observation wells, test pumping, physical and chemical water quality analyses, analytical and digital modeling to rapid evaluations based on existing data.

3.      DATA PREPARATION AND GENERATION OF DIFFERENT CATEGORIES:

The groundwater departments and states are generating groundwater data on regular basis. But those data are generally categorized on the basis of revenue boundaries and not on the basis of aquifer boundaries. So under NAQIM aguifer related data should be compiled and generated. A suggestive list of required data is given below.

   - Evaluation of existing data

   - Test drilling

   - Installation of monitoring or observations wells

   - Geoprobing and/or hydro probing

   - Geophysical investigations

   - Geophysical borehole logging

   - Groundwater gauging

   - Groundwater sampling and analyses

   - Test pumping

   - Slug testing

   - Analytical and digital groundwater modeling

   - Solute transport modeling

   - Groundwater monitoring

4.      PRIORITISATION OF AMU:

AMUs are basically aquifers subdivided into management units. These units are prioritized according to the demand and usage depending on the evaluated parameters of the aquifer. Before prioritization a management framework is required. The framework may include:

• Establish the baseline groundwater conditions and range of natural variability in the aquifer to facilitate enhanced knowledge and detection of change.

• Provide a consistent approach to understanding potential effects from all development activities on the surrounding environment.

• Facilitate projections of change based on future scenarios, such as expanding development or climate variability and change.

• Support and supplement the current pollution prevention and risk management principles as part of groundwater quality and quantity management.

 Prioritisation index:

Depending on the hydrogeology, quality of water and development status a prioritization index is to be developed by the groundwater authorities.

AMUs are initially ranked according to an index (equation 1) based on normalised current groundwater extraction, the fraction of groundwater allocation currently extracted , the fraction of sustainable yield currently extracted , a potential growth index and an index of the predicted future impact of groundwater extraction on surface water flow.

Apriority evaluation index has been attempted in Murray-Darling Basin Sustainable Yields Project (http://www.clw.csiro.au/publications/waterforahealthycountry/mdbsy/technical/Q-GMU-Prioritisation.pdf) Several other algorithms can be developed to prioritise aquifers in our country. In a GIS platform both raster and vector based analysis may lead to an efficient decision support system.

5.      MANAGEMENT PLAN

This initiative will seek to protect the long term water supply capacity of the aquifer by controlling average annual aquifer use and balancing it with average annual recharge.

                                i.            Aquifer Protection Plan: This initiative will seek to assess the quality of the water in the aquifer and then to protect it by encouraging activities that enhance water quality and by discouraging activities that degrade it.  The key components are

                              ii.            Maintain Status Quo where water level is alarmingly depleted.

                            iii.            Reduce withdrawal of water: Water withdrawal may be reduced by introducing innovative farming, recycling of water and reducing consumption. The management plan will address aquifer specific recommendation.

                            iv.            Enhance Natural Recharge: To enhance natural recharge proper steps should be taken. This may include afforestation, gully plugging, construction of sub surface dykes etc. This initiative will seek to maximize aquifer recharge with good quality water by encouraging beneficial land and water management practices and by investigating the potential to use stream flow.

                              v.            Monitoring plan: This initiative will seek to maintain a current and comprehensive scientific database on the aquifer by collecting, compiling and evaluating data. The current aquifer monitoring activities could be reviewed to coordinate and redesign them, focusing on the comprehensive protection of the Aquifer.

                            vi.            Manage Pollution Risks: Activities carried out in pits and dumping areas within the primary recharge area present a variety of risks to aquifer water quality. The risks associated with these activities could be assessed and, where required, action taken to manage them as part of an aquifer protection plan. Other risks like geogenic contamination like As and Fl should be properly addressed in the management plan to reduce public health hazards.

                                      vii. Reduce Incoming Salt Water: The regional salt water flow to the aquifer could possibly be intercepted by installing a set of management structures or through innovative engineering activities.

6.      AQUIFER EDUCATION PLAN

The potential long term success of this management plan depends on the understanding and cooperation of a diverse group of people.  It is believed that a better informed group makes better decisions.  Only common understanding will engender the cooperation needed to formulate,  implement and maintain a successful long term stewardship plan for the Aquifer. Additional study is required to achieve a better understanding.  Extension education is required to make knowledge gained available to all area residents.

REPORT OF HYDROGEOLOGICAL INVESTIGATION ON THE PROPOSED ARTIFICIAL RECHARGE FROM PONDS AT ROYAL CALCUTTA GOLF CLUB

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 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. (Sengupta 2007)

HYDROGEOLOGICAL SETTING AT RCGC

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.

POSSIBILITY OF ARTIFICIAL RECHARGE

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.

RECHARGE STRUCTURE

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.

Figure shows the schematic diagram of the pond water recharge system of RCGC.