Introduction

Alluvial sediments in the Indo-Gangetic Basin (IGB) form one of the world’slargest and most heavily used aquifer systems. There is clear evidence for overabstraction and/or groundwater contamination in some parts of the IGB, but not in others. The hydrogeology and external influences on groundwater across the aquifer are complex and diverse. Of these influences, the role of leakage from irrigation canals is now a fundamental part of groundwater dynamics in the IGB aquifer. In some areas, aquifer recharge contains a high proportion of canal leakage (Joshi et al. 2018), but in others it is much less (Lapworth et al. 2015). The Gandak, in NE India, is a sub catchment of the IGB and provides an excellent laboratory for studying the evolution of groundwater-river-canal dynamics in the centre of the basin. Our study, carried out as part of the joint UK-India funded CHANSE project, is the first detailed investigation of the hydrochemistry, stable isotopes and residence time of groundwater in the Gandak aquifer, to quantify the contribution of recharge to the aquifer from canal leakage, the Gandak River and local monsoon rainfall; and to characterise groundwater flow and mixing behaviour.