Spatial Variability of Salinity in Groundwater in Coastal Southwest Bangladesh

Abstract

Acacia Water and Dhaka University came up with Managed Aquifer Recharge (MAR) in order to relieve the freshwater shortage problem during the dry season in the coastal area of Southwest Bangladesh. The performance of these MAR-systems depends on hydrogeological conditions including salinity of the groundwater. Therefore freshwater lenses in the groundwater are critical for efficient use of MAR-systems. The shallow groundwater of the coastal regions is primarily brackish with lenses of fresher water and the spatial variability of salinity in the groundwater is assumed to be very high. Because of the very high spatial variability of salinity in the groundwater this study focuses on the small-scale variability of salinity in the groundwater because a more detailed picture of the spatial variability was needed to understand the processes that cause this variability. The elevated area in Assasuni has been chosen as study area because there is a gradient in elevation and salinity present. The research objective of this study was to map the spatial variability of the salinity of the groundwater in on and around this elevated area of Assasuni and to find the factors that explain this spatial variability. The research methods were divided into two parts. The first part was the fieldwork to map the spatial salinity distribution. The second part was the analysis of the data in form of a spatial analysis and a groundwater model to find the processes cause the formation of a freshwater lens in the groundwater. The salinity measurements indicate a freshwater lens beneath the elevated area in Assasuni and more saline groundwater in the lower lying surrounding area. The formation of the freshwater lens can be explained by a difference in the salinity of the recharge. Which could be by either less saline flooding because of a difference in elevation or through difference in land use, by fresh ponds in the village on top of the elevated area and by brackish and saline inundation due to rice and shrimp cultivation at the surrounding area. It is unlikely that the variation in salinity of the groundwater is caused by differences in lithology. Another explanation that could explain the current distribution of saline groundwater are historical differences in depositional environment. This research gives some indications on what processes cause the variability of the groundwater in the southwestern coastal area of Bangladesh, but more research in needed to fully understand these processes.