5.1.3. Characteristics of the water-bearing zone
There are found a vast difference between hydrogeological conditions in other areas from the coastal region of Bangladesh. The main aquifer transmissivity on coastal aquifer have found the ranges from 250 m2/day to 10000 m2/day with the average value of 1000 m2/day. Generally, the aquifer storage capacity has increased with depth with the grain size of aquifer materials increase. It has indicated by the authors (Bakhtine & Industry, 1966; Zakaria et al., 2015), the thickness of water-bearing in the different depth of the study area and actual hydrogeology has considered as complex in nature. From Fig. 2(a), hydro stratigraphic (Table 2) sections has been divided into five lithologic part with the correlation of two wells: (a) sandstone and topsoil, (b) clay stone zone, (c) sandstone with clay (d) shale (siltstone and clay) and last (e) sandstone. The main water aquifer zone has found in the Early Miocene age and the formation of Middle Boka Bil (Fig. 3(A)). The lithologic responses of the resistivity log (35.6-42.7) oh-m. The cross of the actual water-bearing zone section of the two different wells has been divided depend on their hydro stratigraphic unit. According to the water-bearing condition, the aquifer geometry of the lithologic characteristics has divided into 4 parts on the top to bottom Fig. 4 (A): (1) water table (2) aquiclude (3) aquitard (4) aquifer. From the lithological analysis of individual logs (resistivity and SP log) of the cross-section has been indicated the aquifer on the Middle Boka Bil (Early Miocene age) on 184m to 244m depth and it has considered the main aquifer zone of safe groundwater.