Use of bank filtration systems in the sub-tropical region of the lower Brahmaputra valley

River bank filtration (RBF) is a natural method of obtaining surface water from a river or lake via the sub-surface for domestic use. It has been intensively used worldwide to augment water supply systems for sustainability and can be operated under various conditions. Its energy requirements are comparatively lower than those of conventional water treatment systems. Field investigations were carried out at various locations in Kokrajhar district of Assam, India to assess groundwater quality and aquifer characteristics. The results suggest that major water quality parameters were within the Indian drinking water standards. The iron concentration exceeded the permissible maximum in more than 50% of samples from hand pumps, its concentration ranging between 0.33 and 3.50 mg/L. The pH was mostly in the range 5.4 to 7.4, suggesting that the water is slightly acidic. Aquifer and riverbed material collected along the banks of the Gaurang River were sieved, and classified as coarse silt to fine gravel, with gravel and sand predominant. The hydraulic conductivity, determined from the grain size distributions, were between 5 x 10(-3) and 1.4 x 10(-2) m/s, suggesting good aquifer permeability. The maximum safe well yield was estimated at 2,000 to 7,500 L/min, and the mean travel time at less than a week during the monsoon and more than 3 years under non-monsoon conditions. The aim of this paper is to demonstrate the RBF method for treating river water naturally under wet climatic conditions. The lower Brahmaputra basin is a unique physiographic setting with a powerful monsoon regime and a fragile geological base. The approach was to prepare a comparative study of water quality and aquifer characteristics with the present site and few active RBF sites.