Lake bank filtration in landslide debris: irregular hydrology with effective filtration

A lake bank filtration (LBF) scheme comprising of nine tubewells on the bank of the lake Naini in Nainital, India exists in landslide debris while most bank filtration sites globally are in alluvial aquifers. The water quality and stable isotopes (δ¹⁸O and δ²H) have been studied to assess the proportions of bank filtrate drawn by the wells. Results show that of the nine wells, two wells perennially abstract mainly bank filtrate, three abstract predominantly bank filtrate during non-monsoon but groundwater during monsoon, and four wells largely abstract groundwater perennially. Bank filtrate proportion in a well is not dependent on its distance from the lake. Also, more than one groundwater stream appears to be contributing to the well field. Such anomalous hydrology is likely due to hydrogeological heterogeneity in the landslide debris or drainage from fractures and faults in the underlying geology. The study shows that an LBF well in a landslide deposit can sustainably deliver water of drinking quality at a short distance of ~5 m and travel time of ~2 to 3 days from the lake.