Hydrogeological characterization of saline water aquifer deploying multiple well logs at Khulna, a coastal region of Bangladesh

Abstract

Khulna city lies at the southwestern coastal region of Bangladesh where salinity of groundwater is very high. In fulfilling the growing demand of water supply at Khulna city corporation, this study conducted geophysical well logging at three boreholes to explore suitable and sustainable groundwater aquifers. The natural gamma ray log, self-potential log, and short normal resistivity and long normal resistivity logs were deployed affixing in a single probe. The results showed that the subsurface lithology is composed of clay, silty clay, sandy clay, fine sand, medium sand, medium to fine sand, and fine to medium sand. Thickness of the units varies both laterally and vertically like top clay/silty clay unit is 9.0m thick at boreholes KHL01 and KHL08 while 52m at borehole KHL07. Additionally, two permeable horizons were delineated termed as aquifer-1 and aquifer-2 at shallow and greater depths. Aquifer-1 thickness varies from 21.0 to 44.0m and depth ranging from 30.0 to 85.0m. Aquifer-2 delineated between 241.0 and 284.0m with thickness 27.5–30.5m. The average resistivity value of aquifer-1 was higher than that of aquifer-2 at borehole KHL01. On the contrary, at boreholes KHL07 and KHL08, the average resistivity value of aquifer-2 is much higher than that of aquifer-1. The results thus showed that the groundwater salinity varies laterally and vertically. In essence, the deep aquifer-2 at south side of Khulna city could be a safe drinking water source. This study would guide for further research in exploring sustainable safe aquifer at greater depth (>300.0m) for the coastal region of Bangladesh.