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Groundwater Potentiality in and around Jharia Coalfield using Geographic Information System
Slope
Slope is the change of elevation of a surface and it is expressed as a percentage of rise over run. A value of 0% represents a slope of 0? while a value of 100% represents a slope of 45?. The relationship between percentage and degrees as represented on this case is non-linear. In the present study, slope map is generated from the DEM. Slope plays a very significant role in determining infiltration vs. runoff. Infiltration is inversely related to slope i.e. more gentle the slope is, infiltration would be more and runoff would be less and vice-versa. The slope map of the study area is presented in the Fig. 3, which shows an overall gentle slope towards east-southeast.
Drainage
Drainage pattern of an area is very important in terms of its groundwater potentiality. It is the source of surface water and is affected by structural, lithological and geomorphological control of an area (Schumm, 1956). The drainage pattern in the present study area is dendritic in nature. This may be due to more or less homogeneous lithology and structural controls. Damodar river is the main control of drainage system along the Jharia coalfield. It is a fourth order stream to which a number of third to first order streams, viz. Jamunia, Khudia, Katri, Ekra, Tisra, Chatkari etc. join. Damodar river flows along the southern periphery of the coalfield and is guided by the Great Boundary Fault. The main flow direction is from west to east. The drainage map of the study area is shown in the Fig. 4.
Lineament
In Jharia coalfield, faults are the dominant linear feature. The Gondwana sediments of Jharia basin are largely disturbed by a large number of various types of fault system. In the southern part of the basin, Southern Boundary Fault is present with a trend approximately along WNW- ESE direction. Besides Southern Boundary Fault to the south, a number of interbasinal faults also exist (E-W trending faults, NW-SE and NNW-SSE trending faults, NE-SW trending faults, low angle faults). The lineament data have been prepared by various lineaments from the geological map of Jharia coalfield that have been digitized as a line layer. The fault frequency indicates the infiltration of water into the sub-surface. A high frequency of faults provides an indication of higher infiltration and vice versa. The lineament map along with the litho-stratigraphic map is shown in the Fig. 2.
GIS Modelling
The study area mainly depends on the rainfall during the monsoon in order to meet the requirements of domestic as well as agricultural purposes, the scarcity of which could lead to acute water crisis resulting in severe drought conditions. The study area is a part of the Permo-carboniferous Gondwana basin. It comprises of lower Gondwana sedimentary formations overlying the Precambrian metamorphics. In the study area, groundwater condition in various litho-stratigraphic units can be described under three broad divisions, viz. weathered formation, fractured formation and abandoned water logged area.
To determine the status of the groundwater table, water level data have been collected from different dug wells of Jharia Coalfield. Water level data from dug wells are taken during pre-monsoon period (May, 2004) and post-monsoon period (November, 2004). Figure 5 shows the location of the dugwells from where water level data at 46 networking stations have been collected. The pre-monsoon and post-monsoon water table elevation maps generated from the dataset are shown in Figs. 6a and 6b.
Overlay analysis
In the present study, the choice among a set of zones for evaluation of groundwater potentiality has been based upon multiple criteria such as drainage pattern, control of lithological units, steepness of slope, lineament frequency and water table elevation. The process is known as Multi-Criteria Evaluation (Sarkar et al., 2001). For a multi-criteria geospatial modelling, firstly a template has been created by identifying the quadtrees used in the analysis. If a basemap has been assigned to the study area, this function would confine the analysis to the data falling within Class 1 as defined by the basemap and the number of input quadtrees that can be selected is reduced to one less than the total number. A default weight is calculated by dividing 100 by the number of quadtrees used in the overlay and is assigned to each quadtree in the analysis. A default of 0 for each class score is assigned to each quadtree class. Each class is labelled with the short legend title taken from the input quadtree. Different categories of derived thematic maps have been assigned scores in a numeric scale of 0 to 5 depending upon their suitability to hold groundwater. An aggregation of these product values leads to the final weight map.
Mathematically, this can be defined as:
GW = f (Dr, Lin, Lith, Sl, WTE)
where, GW is groundwater, Dr is drainage, Lin is lineament, Lith is lithology, Sl is slope and WTE is water table elevation.
The groundwater potential map value, thus derived is given in the equation:
GWP = Σ Wi CVi with Σ Wi = 1.
where, GWP is the groundwater potential map value, Wi is the probability value of each thematic map, and CVi is the individual capability value.
GWP can be expressed as the summation of:
(0.294* CVLin) + (0.118* CVLith) + (0.235* CVDr) + (0.235* CVWTE) + (0.118* CVSl).
The resultant final weight map indicates the potentiality of groundwater occurrence in the Jharia Coalfield (Fig. 7). To derive this final weighted map, a probability weighted approach (Sarkar et al., 2001) has been adopted that allows a linear combination of probability weight of each thematic map (W) with the individual capability value (CV). Using Baysian statistics, the capability values are calculated from their assigned scores in a numeric scale. These capability values are then multiplied with respective weight of each thematic map (Table 1). This map has then been classified into four categories of potentiality, namely, Excellent, Very Good, Good, Poor and Very Poor.
Results and Discussion
In the Archean metamorphics of the Jharia coalfield, groundwater occurs in semi-confined to confined aquifer condition. Groundwater occurs under unconfined condition in the top weathered mantle of the variegated Barren Measures and the Barakar and Raniganj sandstones except for Talchir shales. This is because the original rocks being nonporous and nonpermeable, even weathering of top layer does not become conducive to groundwater movement, example being Shastrinagar inhabited area where boring of tube well has been unsuccessful. It is under semi-confined condition in the deeper fractures zones that have imparted secondary porosity and permeability in these rocks. The Gondwana sandstone, in general, is known to constitute good aquifers at many places. However, the yield potential of the areas adjoining active mines in the coal belt is poor. With continued dewatering of the mine pits, the neighbouring wells register gradually lowers water levels long before the advent of summer and many of them ultimately get dried up. Thus, the active mines often act as groundwater ‘sinks’. Apart from this, the boring of deep tube wells at many places decreases the water level considerably and causes lowering of water level in the neighbouring wells.
The groundwater potentiality map derived by the multi-criteria evaluation technique reveals distribution of various potential zones of groundwater in and around the Jharia coalfield. The northwestern and southwestern part of the basin shows poor to very poor groundwater potentiality while along the flood plain, river terraces, it shows good to excellent groundwater potentiality. In the southeastern part of the coalfield, groundwater potentiality is good though in reality many areas in the southeastern part show poor groundwater reserve. This may be due to the active mining activities in that part which have disturbed the groundwater reserve in that area. Areas in the northeastern part of the coalfield show good to poor groundwater potentiality. In the eastern side, some areas near the river shows excellent groundwater potentiality.
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