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Arsenic Mapping for North 24- Pargana District of West Bengal –using GIS and Remote Sensing technology
The intermediate aquifer, constituted of sub-angular to sub-rounded medium sand, sandy clay and clay with fine sands, and its heavy mineral assemblage (biotite, garnet, kyanite, opaques) indicates a dominantly metamorphic origin.
While the lower aquifer is constituted of sub-rounded to rounded fine to coarse sand with occasional clay bodies, and its heavy mineral assemblage (opaques, altered biotite tourmaline, rutile etc.) indicates an igneous provenance (Steering Committee Arsenic Investigation Project, PHE Dept, Government of West Bengal, 1991). The intermediate aquifer usually shows arsenic contamination.
Data Used :
For the current project the following data sets and maps were used.
- The District Statistical Hand Book , 24 Parganas North published by the Bureau of Applied Economics and Statistics, Government of West Bengal.
- The Survey of India Topographical Sheets
- The Natmo Map
- The database in Foxpro
- The LISS image
- The Pan image.
The application software on Geographical Information System used was SPANS. The Image Processing application software used was Geomatica Prime. Both these products are from PCI Geomatics, Canada.
Methodology :
The processing steps involved and followed in this project are given below.
- The NATMO District planning Map Series (1:2,50,000) of North 24 Parganas was scanned at a resolution of 400 dpi in the Tagged information File format and was then geo-registered with the Survey of India Toposheets (1: 50,000). The georeferencing information was collected from the 8 SOI Topographical sheets as mentioned earlier. The process of geocoding required ground control points which was collected uniformly over the whole district and them mathematically warped to fit into the real world co-ordinates using Longitude/Latitude projection and Referenced Ellipsoid with Indian Datum.
- The IRS LISS 1C and Panchromatic data was used for the project. The following images with Path/Row 108/55, 108/56 , 108/57 and 109 / 55 covers the whole district. The individual scenes were geocoded with control points thereby enabling each scene to return the co-ordinates at each position. The LISS and Pan image is having a spatial resolution of 23.5 meters and 5.8 meters respectively.
- All the scenes were mosaic using PCI Solutions. Thus a whole area covering the whole district was obtained. The NATMO map and the image was overlaid and the district boundary was digitized . The boundary of the district was used to mask the satellite image and the image falling within the boundary was restored. The block boundaries were also digitized and overlaid on the image as shown in figure 1.
North 24 Parganas with block boundaries.
figure 1
- The LISS Image and Panchromatic image was fused using HIS Transformation and the resultant image has the spectral resolution of the LISS Image and the spatial resolution of the PAN Image. Hence we get a colored image with the resolution as that of the pan image.
- Thus scanned maps have different scale with respect to SOI maps.A project was created in the SPANS GIS environment. From the geocoded scanned NATMO map, the block boundaries and other layers of information were digitized in the SPANS environment. The different blocks and their names were taken into the database of SPANS. The various fields in the database are Block names, Mouzas as point values, the Arsenic recorded in the wells in the block, the PH values, the iron contents and the Chlorine contents at few observed wells are recorde in the database. All the layers of information are recorded as either point or line or polygons in the graphical mode.
- From the arsenic values as provided from the field data, contour was generated over the whole district using point-to-surface generation programs in SPANS as shown in figure 2. The thematic map indicates the area of high arsenic concentration and low arsenic concentration as observed from the water tested in the laboratory and plotted on the map as points corresponding to each block. The arsenic concentration map is shown below
Arsenic Concentration map
Figure 2
- The depth of Arsenic occurrence as obtained from the field data posted as point information on the map is mapped sing the depth values at which arsenic is reported to have a value above the danger level of .5 mg/lt. This depth of occurrence map is saved as a quad tree for further analysis.
- A site suitability map for safe drinking water was prepared using the modeling tool in SPANS. The modeling used for the purpose is Matrix Overlay. The two parameters used for the purpose of matrix overlay are Arsenic concentration thematic map and the depth contour thematic map of water presence or indirectly the water table map. The low safe zone of arsenic concentration and low water level depth was chosen to be the ideal site for safe drinking water zone. Raster GIS of modeling was used in this regard. The result of matrix modeling is shown in figure 3.
Suitability Zone using Matrix Modeling
Figure 3
- Multi-criterion modeling was performed to prepare a zonation index map showing “Danger” to “Safe” zones. This modeling was used in this area having the type of regional settings as of North 24 Parganas. These modeling can be applied to any area of such settings. The three parameters used for this analysis are Arsenic concentration map, Depth map and Population distribution map of the area. Different weightages and scores were applied on these parameters to find an area of highest danger and safe areas. These results confirm with the earlier studies on the same area.
- Image was classified using Supervised Classification scheme and the land cover and land use was classified using Maximum Likelihood Classifier and obtained 6 different classes of the district. The result of multi-criterion is overlaid on the classified image which is shown in figure 4.
Multi-criterion analysis on classified image
Figure 4
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