GIS for Aquifer Monitoring and Modeling - From Field Surveys to Simulation Models: A case study of Kaluvelly Pondicherry basin, South IndiaPierre Jorcin GIS Analyst Water Hrvest pierrejorcin@hotmail.com BACKGROUND: In the Kaluvelly Pondicherry basin of Southern India, groundwater resources are now threatened by a fast and heavy depletion. As there are no perennial rivers in the area, the irrigation tanks, fed by the monsoons, are not sufficient to cover the agricultural needs for more than a single crop. This has given rise to the overexploitation of the coastal aquifers. The water quality is being badly affected by salinization leaving people without proper drinking water supply in many places. Moreover, because of high permeability, a fast intrusion of seawater through the fresh-saline water interface is likely to happen and risks needs to be evaluated. However, the geological and morphological context indicates potential high capacity zones where extraction could be concentrated. The primary reports available highlighted the need for a better understanding of the aquifer system and its dynamics. In a context of future growth and sustainable development, a groundwater modelling study is required to support integrated water management plans with all stakeholders. The project presented here aims to study the quantification of the groundwater resource of a multi-layered aquifer system for a predictive evaluation with a GIS model. The hydrogeological study will target an identification of best favourable areas for exploitation. In building such a model, there are 3 main steps –
The first step is to set up a GIS database where specific parameters can be entered and analyzed. To answer the conditions of the project, highly accurate data need to be collected and processed, such as groundwater table and its fluctuations, water extraction, natural recharge potential, etc… Therefore, intensive field surveys will be carried out to collect first hand calibrated data, in order to provide reliable data following the scientific requirements. This method allows a direct control on the data and a validation of up to date information. On the other hand, it demands an important investment in terms of time and human resource. However, the quality of the GIS built from such field surveys gives lots of prospect for developing research projects and activities related to water management. The hydro geological model will be built from the GIS database created by Harvest GIS Unit during previous related projects. A census of data availability and accuracy has been carried out, identifying data needed for building the model. Base maps and thematic maps featuring hydrological parameters are already part of the GIS. The drainage network system has been mapped from various data source, such as cadastral maps, engineering reports, and GPS field surveys. The lakes and irrigation tanks with their feeder and surplus outlet channel were marked in detail. Therefore, the run-off pattern and watershed limits can help to identify the most suitable areas for rain water harvesting. Regarding groundwater data, Harvest for several years has been conducting regular monitoring programs, collecting monthly data on groundwater quality, especially salinity rate. Groundwater level is being measured from selected observation bore wells, according to specific parameters such as the aquifer tapped, the volume of water extracted in the immediate surrounding area, etc…Water level contour maps are generated in GIS to study the fluctuation of the groundwater table and its current evolution in time and season wise, together with studies of the rainfall pattern. In Pondicherry Kaluvelly basin, in the main deeper aquifer, groundwater table is found to be 50 meters below the mean sea level, at 15 kilometers from the sea shore. Exact altitude is an essential parameter to trace the piezometric level of the aquifer. For this reason, the observation wells have been located using a differential GPS with centimeter accuracy in longitude, latitude and altitude. (FIGURE 1 )  For water balance evaluation, an intensive field survey has been carried out for a detailed census of more than 6000 bore wells over an area of 250 sq.km, with a handle GPS of 15 meters accuracy. All technical characteristics have been entered in GIS as attribute data, targeting mainly an estimate of the water extracted daily by pumping. The ground surface elevation is known from an existing Digital Elevation Model (DEM) built up thanks to a grid of altitude benchmark data collected from various survey programs using a differential GPS, with centimeters accuracy in X, Y and Z. The DEM will be the reference for the realization of the geological model, to measure the absolute depth of the aquifers in reference to mean sea level.
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