A GIS BASED CHARACTERIZATION OF FLOODPLAIN MORPHOLOGY AND RELATION TO ARSENIC SAFE AQUIFERS IN SOUTHEASTERN BANGLADESH

High levels of As in the Holocene alluvial aquifers of the Bengla Delta Plain, the largest fluvio-deltaic basin of the world shows significant relationship with geology and hydrogeology of the basin. The present study is focused on the identification of the landforms of the two different parts of complex Meghna basin which includes the floodplains of North and South Matlab, Bangladesh through the visual interpretation of Google image and their relation with the distribution of As in the aquifers. The interpretation reveals that the visually identified geomorphic features of North and South Matlab are characteristically different which can be linked to varied subsurface geology and thereby varied nature of the distribution of As in their aquifers. This finding can be easily related to targeting As-safe aquifers which are different in the North and South Matlab. In the North Matlab, the older natural levees are the potential zones of safe water supply and SASMIT targeted the Intermediate Depth Aquifers (IDA) in the range of 120 m depth for As-safe tubewell installation. North Matlab is a wide and older floodplain characterized by very distinct north-south trending linear features of natural levees along with abandoned channels, point bars, back swamps and meander swales. More than 95% of the wells installed in IDA produced water with As concentration below Bangladesh standard of 50μg/L. On the contrary, South Matlab is an active and younger floodplain characterized by newly formed natural levees of irregular pattern. The overall geomorphic expression of South is quite different from that of the North. In the South Matlab, As-safe aquifers were found in both shallow red sediments in the range of 45-80 m depth and in the Intermediate Depth Aquifers of 120 m depth range. From SASMIT investigations and installations, it was evident that in the geologic setup of the North Matlab, IDA could be the target for As-safe tubewells but in the South Matlab, it can be both IDA and the shallow red sediments. This study reveals that the approach of safe tubewell installation could be different in these two different geologic setups linked with two distinct geomorphic expressions and this hypothesis can be tested as a new strategy for safe tubewell installation in the floodplain areas.