The Analysis of Groundwater Potential Zones

The relationships among the factors, i.e., lineament, slope, groundwater depth, rainfall/runoff, urban storm water logging, land use/land cover, soil texture and drainage density, were weighted according to their response for groundwater occurrence. High to low weight showed larger to smaller impact on groundwater potential respectively. Integration of these factors with their potential weights was computed through weighted overlay analysis in GIS environment to determine groundwater potential zones. Surface runoff and infiltration rate greatly depend on Slope or gradient which is also important factor of suitability of groundwater recharge i.e. higher slope would produce a higher runoff and less recharge. Slope in the study area ranged from 1 to 37%, as per IMSD Guidelines (NRSA, 1995) categorized into 5 classes i.e. 0-1.00% (nearly level), 1.01-4.00% (very gently sloping), 4.01-8.00% (gently sloping), 8.01-16.00% (steep), and >16% (moderately steep).

Slope class having higher value is assigned lower rank due to relatively high runoff while the class having lower value is assigned higher rank due to flat surface (Jhariya et al. 2016).Another input to evaluate the recharge property can be realized by detailed morphometric analysis of the drainage network. The density of the drainage network influences the groundwater recharge and movement as well as the occurrence of lineaments, faults, fractures, major or minor joints, and also provides path ways for groundwater movement and is hydraulically very important as it is an important indicator of water percolation rate (Kumar et al. 2007)Edet et al. 1998; Shaban et al. 2006). Denser the drainage network results usually the less recharge rate. The extraction and analysis of the drainage network was prepared from topographic maps, field data and satellite images. Drainage densities were calculated in each of the grid square following Murthy (Murthy 2000):Drainage density= LWS/AWS (3)Where, LWS = total length of streams in watershed and AWS = area of the watershed. Thus, the obtained drainage density map reveals density value ranging from 0 to 51.5 km/km2, reclassified into five categories i.e. < 2.00km/km2 as very low, 2.01-10.00km/km2 as low, 10.01-20.00 km/km2 as moderate, 20.01-30.00 km/km2 as high and 30.01-51.5 km/km2 as very high drainage density

From recharge point of view, more weightage was assigned to very low drainage density regions, whereas, low drainage densities indicate high permeable surface stream frequency of the area compare to the high drainage density i.e., impermeable ground surface/rock formation. With respect to the groundwater occurrences, the higher drainage density is related to less infiltration of water to the ground and produce higher runoff. The ground water table was collected from Chittagong Water Supply and Sewerage Authority (CWASA), the static water level was obtained by subtracting the elevation of water depth of different tube well in 2016 and the groundwater depth map was prepared. Then the obtained depth of water level ranged between 18.28m to 134.11m and the study area has been categorized into 5 classes i.e. 18.28-28.34m as very shallow groundwater table, 28.35-45.23m as shallow groundwater table, 45.24 -64.00m as moderate groundwater table, 64.01-99.09 m as deep groundwater table and > 99.10m as very deep groundwater table (Fig. 3 c). Wells with deeper water levels during dry season indicates at considerable water extraction this is a favorable site for recharge. Thus, high score value was given for deeper water level (Duraiswami et al. 2009).

Lineaments influences the movement and storage of groundwater, to analyze the lineament with different spectral bands RS and GIS technique were applied. Lineament-length density is obtained by the total length of all recorded lineaments were divided by the area under study following Greenbaum (Greenbaum 1985). Lineament are extracted and then getting lineament density map by ArcGIS 10.1.Groundwater potential is high near lineament intersection zones. The lineament density in the study area as shown in Fig. 3 d is categorized into 5 classes i.e. < 0.430 kmKm2 as very low density, 0.431-0.844 kmkm2 as low density, 0.845-1.30 kmkm2 as moderate density, 1.31-1.90 kmkm2 as high density and >1.91 kmkm2 as very high density. Runoff of the study area was estimated precipitation records for the Patenga station using Hydrologic Engineering Center–Hydrologic Modeling System (HEC-HMS) v4.0 model and the relevant input data for all 41 wards were processed using Geospatial Hydrologic Modeling Extension (HEC-GeoHMS) v 5.0. The details on the model setup can be found in Akter and Ahmed (Akter & Ahmed 2015). The estimated spatial distribution of annual runoff ranging from 3.34-3.58 m for per year in the study area. The depth is categorized into five classes .

During wet period, this city experiences urban storm water logging as well as some parts usually inundated due to tidal effect. The details on water logged locations and the amounts were obtained from Akter et al. (2017) along with field data. For weighted average a minimum value (0.01m) was used for less vulnerable areas. The water logging depth varies between 0.01 to 1.828 m and the study area is categorized into five classes i.e. Land use/ land cover maps were prepared from RS data using supervised classification in ERDAS IMAGINE software with field verification. The whole area were categorized into barren land, buildup area, vegetation and water body . Water bodies are continuous and excellent source of recharge of groundwater. Therefore, water bodies were assigned highest weight for groundwater potential. The agricultural fields with sufficient vegetation cover promote the infiltration rate and prevent excess runoff and, therefore, are assigned high rank for groundwater prospecting.

The rate of infiltration is directly proportional to the density of vegetation cover, i.e. if the surface is covered by dense forest, the infiltration will be more and the runoff will be less. Therefore barren lands are assigned medium weightage. Built-up land are assigned very low weightage as the infiltration rate is very low. An average value of 5 meter depth soil texture was taken from the numerous boreholes of the specific ward. The soil texture of study area reveals ten soil texture categories, clayey soil is dominating most of the areas. Soil ranking was done based on their infiltration rate, i.e., sandy soil with high infiltration rate assigned higher priority compare to less priority for the clayey. The primary objective of any artificial recharge project is to harvest as much runoff water as available. Thus, the higher the runoff water, the greater the water availability for recharge. Accordingly, areas having higher surface runoff are given higher scores.