Spatio-temporal variability in soil and water salinity in the south-central coast of Bangladesh

•Salinity varied with time, space, topography and land use activities.•Salinity decreased with increasing distance from the coastline of the Bay of Bengal.•Polder restricts direct salinization of coastal soils.•Landsat imagery based salinity indexes can reliably predict soil salinity.•A large coastal area turned into saline in the last few decades.