There is ongoing debate around the status of groundwater in the Indo-Gangetic Basin (IGB) alluvial aquifer, which as a whole is the world’s most heavily exploited aquifer. There is clear evidence for over-abstraction and/or contamination in some parts of the aquifer, but other areas do not show the same intensity of human impact. The hydrogeology and external influences on groundwater across the IGB aquifer are complex and diverse. Influences include increasing groundwater abstraction, changes in land use and other economic behaviour, and the impact of the network of engineered irrigation canals that distribute river water across many catchments. The role of leakage from irrigation canals is now a fundamental part of groundwater dynamics in the IGB aquifer. Recharge to some areas of the aquifer contains a high proportion of canal leakage (Joshi et al. 2018), but to others it is much less (Lapworth et al. 2015).
The Gandak catchment, in the northern part of Bihar state in NE India, provides an excellent laboratory for studying the evolution of groundwater-river-canal dynamics in the centre of the Ganges basin. Evidence shows no overall long term declining groundwater level trend in the Gandak alluvial aquifer (MacDonald et al. 2016, CGWB 2016), such as is seen in some other parts of the IGB aquifer. Our study, carried out as part of the joint UK-India funded CHANSE project, is the first detailed investigation of the Gandak aquifer.
We used isotopic, residence time and geochemical tracers to quantify the contribution of recharge from canal leakage, the Gandak River and local monsoon rainfall; and to characterise groundwater flow and mixing behaviour. We show that canal leakage has been a significant source of aquifer recharge since at least the 1960s, but that this is restricted to shallow depths, above ~30 m. Deeper groundwater, below 30 m, shows no evidence of canal or river leakage, but is consistent with long term active recharge from local monsoon rainfall. Groundwater in the shallow zone shows more chemical and isotopic variability than in the deeper zone. This is different from other studied parts of the IGB aquifer, where deep groundwater has been shown to be significantly impacted by over-abstraction and groundwater pollution. These differences are partly linked to hydrogeological conditions and partly to abstraction behaviour, including borehole depth and typical borehole pumping rates by farmers. Most groundwater abstraction is from <30m, and although abstraction has increased significantly over past decades, it does not currently exceed long term recharge rates, as evidenced by available groundwater level data.
CGWB. 2016. http://cgwb.gov.in/Regions/GW-year-Books/GWYB-2015-16/GWYB%20MER%20(Bihar)%202015-16.pdf
Joshi et al. 2018. https://doi.org/10.1016/j.jhydrol.2018.02.056
Lapworth et al. 2015. https://doi.org/10.1002/2015GL065798
MacDonald et al. 2016. https://doi.org/10.1038/ngeo2791