Paleochannels and their potential for artificial groundwater recharge in the western Ganga plains

Over the last few decades, a steep general decline in the groundwater table is being observed in the western Ganga Plains (India), the average rate being about 0.15 m/year. The area comprises of dominantly vast stretches of alluvial plains within which there occur three major paleochannels of the Ganga river, characterized by serpentine-meandering pattern and having an average width of almost 4–6 km and strike length of about 60–80 km. From the point of view of artificial recharge of groundwater, the paleochannels hold a distinct promise. The paleochannel-aquifer geometry has been delineated by integrating satellite sensor and well-litholog data. The first aquifer (≈25–30 m thick) in the alluvial plains is unconfined and consists of fine to medium sand whereas the second aquifer is confined. The paleochannel-aquifer is unconfined and is mainly composed of coarse sandy material along with boulder and pebbles beds and extends to a depth of about 65 m. The aquifer is well inter-connected with the adjacent alluvial aquifers. Analyses of soil samples from boreholes systematically sited on the paleochannel and its either flanks indicate that the value of hydraulic conductivity ranges from 30 to 75.3 m/day for samples falling in the paleochannel, and that between 13.5 and 22.3 m/day for the alluvial plain aquifers. The natural groundwater recharge rate due to precipitation, estimated using tritium tagging technique, is found to be 18.9–28.7% in the paleochannel area, and 6.3–8.9% in the alluvial plains. Data from stable isotopes of groundwater samples from the first unconfined aquifer indicates that the alluvial plains aquifer gets recharged by both rainfall and/or canal water, whereas rainfall is the dominant source for groundwater recharge in the paleochannel-aquifer. Monitoring of groundwater levels for 2 years (2006 and 2007), both during pre- and post-monsoon periods has been systematically carried out and it has been observed that groundwater flows away from the paleochannel in both pre- and post-monsoon periods, indicating that recharging of aquifers in alluvial plains is also through paleochannels. Thus, it may be inferred that such paleochannels can play a very important role in artificial recharge of groundwater.