GEOMORPHIC AND HYDROGEOLOGIC CHARACTERS OF THE TISTA ALLUVIAL FAN AREA, NORTHWEST BANGLADESH

The Tista alluvial fan of northwest Bengal basin is bounded by the Himalayan Frontal Thrust towards the north, the Pleistocene Barind Tract to the south, the Indian craton to the far west, and the Brahmaputra River and the Precambrian Shillong Plateau toward the east. The fan has developed by sediments carried by rivers, originating from the Himalayas, and draining the Siwalik foredeep. Alluvial sediments grade from coarser at the north to finer at the south, corresponding to elevation changes ranging from 99 m to 45 m from north to south. The gradient of the fan varies between 0.82 m/km at the north to 0.44 m/km at the south. Groundwater study from the Tista fan reveals significant information on water chemistry of Bangladesh that is predominantly covered by floodplain and deltaic deposits.

Three aquifers and three aquitards comprise the top 100 meters of alluvial sediments of the fan. The general direction of groundwater flow is from north to south. Groundwater system in the Tista fan is in hydrodynamic equilibrium, and groundwater levels in aquifers fluctuate seasonally between 2 m to 7 m in a cyclic manner. The surface water system of the study area is almost entirely fed by groundwater during the dry season.

Specific electrical conductivity of groundwater has the highest level (1240 µS/cm) at the southeastern part of the study area, indicating deterioration of local water quality due to increased salinity. Maximum concentration of the dissolved ions (1035.3 mg/L) has been found mostly around the urbanized and densely populated areas, suggesting that the dissolved ions are locally clustered, and anthropogenic in origin. Groundwater types of the study area are mainly of Ca-Mg-HCO3 and NaCl. Arsenic concentration in analyzed groundwater samples at the Tista alluvial fan is below detection limits, and the water quality is safe for both drinking and irrigation purposes.