GIS for Aquifer Monitoring and Modeling - From Field Surveys to Simulation Models: A case study of Kaluvelly Pondicherry basin, South India


A lot of investment has been put into these monitoring programs. By collecting data directly from the field, the GIS can be designed and calibrated from the beginning of its creation. Data integration is planned according to the need of the geographic analyses, and made to answer only requested criteria. To minimize the investments put in surveys, each campaign is planned at multiple scales, with various levels of precision. Advantage of intensive field surveys is in getting first hand calibrated data ready for specific processing. Accurate data can help to make rapid scientific assessments on various themes, whereas the same study would take more time if done through several steps using second hand information. For a local organization like Harvest, working to implement research and development projects, it helps a lot to empower the structure by building its capacity, and not relying on other sources to validate important information.

Modeling geology by contours mapping and cross sections generation

Pondicherry Kaluvelly basin geology consists of a crystalline bedrock overlaid by a sedimentary series of several layers from Cretaceous and Tertiary, with alluvium from Quaternary. The hydro geological context is a complex multiple layers aquifer system, with an altercation of sandstone, limestone and clay.

The objective is to get a precise knowledge of the aquifer capacity based on the knowledge of the geological formations characteristics, such as their depth, thickness and volume. For this purpose, a 2 dimensions model can be built within the GIS to draw the aquifers geometric structure.

A surface geological map of the project area was available from the Geological Survey of India at 1: 250 000 scale, published in 1984. This map has been checked on the ground to verify its accuracy at the scale of our study, and eventually redefine the limits of the formation outcrop areas. The map has been found to be of excellent quality, with all boundaries matching the GPS points taken on the ground where the soil configuration changes. Because of erosion factor, limits have been slightly modified and retraced in two places. Detailed lithological data available from drilling reports are compiled together to build a database for the whole studied area. Among a wide range of data gathered for the whole sedimentary basin, a sampling set has been selected based on their precision and validity. Data source are various, mainly the Central Ground Water Board deep observation wells, the Tamil Nadu Water and Drainage Board, the Public Work Departments, and the Pondicherry Agro Service and Industries Corporation, all re-interpreted for the sake of the project. 280 logs were located using a 15 meters resolution handle GPS, over an area of 700 sq.km, with a average density of 5 samples per 10 sq.km grid, a maximum density of 35 samples per 10 sq.km grid, and a maximum gap of 5 km between two samples for the few areas of lower density sampling. (FIGURE 2)



The first step is to enter data in the GIS, and process the data to obtain a uniform classification of the formations stratigraphy from the detailed soil and lithological samples. For each and every formation, the depth of the top and bottom are defined and this will be the input value for the model. Values are converted in absolute values in reference to mean sea level, free from the ground surface elevation fluctuations. This is done by importing values from the DEM, as locations of the wells have been taken by GPS. The model will cover information on the geology from the ground surface down to the rock basement at 500 meters depth.

The methodology follows a step by step process, with a validation of results at every stage.

The model consists of surface contour maps of the probable bottom of every geological formation. The maps are traced using interpolation method combined with cross sections drawing and analyses. The methodology applied here makes use of computer processing techniques with GIS tools and standard scientific methods for geology layer mapping. Primary surface contour maps are generated from the set of sampling data and are corrected step by step by tracing cross sections and analyzing their characteristics. Results of the contour maps are checked following a spatial panel distribution and re-modified step by step to fit with the actual geological regional pattern. GIS is the combination of tools, hardware, software and human resources mobilized to process thematic layer data, and can not be limited to automatic computer processing. Cross sections are generated automatically and then re-traced according to the global understanding of the geology. These results are then re-entered in the GIS to create contour maps entirely calibrated by the user. To achieve this task, a new surface contour map is generated using calibrated new samples points at selected location with the required attribute data. Additional samples points are added in order to modify the shape of the surface model based on our interpretation and scientific knowledge. (FIGURES 3 and 4)





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