Assessment of hydraulic conductivity in the multi-layered aquifer system of the Bengal Basin, Bangladesh, by performing slug tests

Zahid, A., Nowreen, S., Islam, M., Hassan, M., Hossain, M., Abbassi, S

Published in ‘Bulletin of Engineering Geology and the Environment’

Abstract

Various studies have described upon importance of the performance of multilevel slug tests to assess hydraulic conductivity of aquifer sediment. For this study, slug tests were performed at 33 locations in 107 numbers of observation wells under six physiographic divisions in the Bengal Basin, Bangladesh. The objective of this paper is to evaluate the potential of multilevel slug tests to determine depth distribution and vertical variations of hydraulic conductivity in layered aquifer systems of the Bengal Basin that can be utilized for resource assessment. The studied aquifer sediments are dominated by unconsolidated fine and very fine sand with lenses of clay and silty clay aquitards. The hydraulic conductivity values commonly correspond with the lithology and their settlements. The unconsolidated sand in the Active and Tidal Delta contains high hydraulic conductivity values of up to 24.4 m/day. Older sediments in the Teppera Surface and Coastal Plain show lower average hydraulic conductivity. Very low hydraulic conductivities in few wells indicate the heterogeneities as well as poor development of these newly installed wells. Individual layers have relatively uniform grain size with well sorted sands, and their hydraulic conductivity values have a small variation, while sediments from multiple layers have variations in grain size with anisotropic signature and their hydraulic conductivity values, in many cases, vary by orders of magnitude. However, multilevel slug tests can provide considerable information about vertical variations in hydraulic conductivity under the right conditions. Although the slug tests were performed with precaution, unknown uncertainties in the well during the operation may influence the values of hydraulic conductivity. A more accurate values of the conductivity variations can be achieved by placing thin, low-conductivity layers in the sand pack that reduces vertical flow. Selection of drilling procedure and the techniques of measurement and analysis are also important to get good results. Factors like leakage and partial penetration should also be minimal for optimum results.