Characterising groundwater-surface water connectivity in the lower Gandak catchment, a barrage regulated biodiversity hotspot in the mid-Gangetic basin

Abstract The alluvial aquifer system of the Indo-Gangetic Basin (IGB) is one world’s most important freshwater resources, sustaining humans and river ecosystems. Understanding groundwater recharge processes connections to meteoric surface water necessary for effective resource management human wider ecological requirements. Parts mid-Gangetic Basin, across eastern Uttar Pradesh Bihar, are characterised by stable long-term levels, high annual rainfall, limited historical use compared parts Northwest India example. In this paper we a combination environmental tracers hydrograph observations characterise sources groundwater-surface interaction using transect approach catchment River Gandak, major barrage-regulated tributary Ganga. Stable isotope results show that dominant source recharge, in shallow (0–40 m bgl) Holocene underlying Pleistocene (>40 m bgl), local rainfall. also supplemented from canal seepage irrigation return flow upper mid catchment. These corroborated evidence detailed hydrographs salinity observations, indicating localised canal, lake connectivity groundwater. middle lower catchment, discharge dominated baseflow during peak dry season when barrage gates closed, which contributes flows endangered dolphins gharial crocodiles. Groundwater residence time indicate active modern elevated arsenic (As), iron (Fe), manganese (Mn) exceeded WHO drinking guidelines minority sites, uranium (U) fluoride (F) concentrations but do not exceed guideline values. varied with higher As, Fe Mn catchments U was typically between 500 1000 ?S/cm, isolated due flood-plain wetlands lakes impacted evaporation. At present, Gandak has relatively rainfall low abstraction, maintains levels thus season. Potential future threats therefore ecology sensitivity changes would likely be driven reductions monsoon practices increased use.