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