Delineation of spring recharge zones using environmental isotopes to support climate-resilient interventions in two mountainous catchments in Far-Western Nepal

Abstract

Though springs are the primary source of water for communities in the mid-hills of Nepal, an in-depth scientific understanding of spring systems is missing, preventing the design of effective climate-resilient interventions for long-term sustainability of springs. This study marks the first attempt to combine environmental isotopes analysis with hydrometric and hydrogeological measurements to identify dominant recharge zones for springs in two mountainous catchments-Banlek and Shikarpur-in Far-Western Nepal. In total, 422 water samples collected from rainfall, springs and streams between March 2016 and March 2017 were analyzed for their isotopic composition (δ 18 O and δD). Isotopic composition of rainwater shows seasonality, suggesting that different sources of water vapor cause rains in monsoon and in dry season. Rainfall responses of individual springs were used to identify connections to unconfined and deeper groundwater strata. The isotopic composition of springs in the two catchments ranges from −9.55 to −8.06‰ for δ 18 O and −67.58 to −53.51‰ for δD. The isotopic signature of the spring sources falls close to the local meteoric water line for the corresponding season, indicating strong rainfall contribution to springs. Altitudinal isotopic gradients suggest mean recharge elevation of 2,600-2,700 m asl for springs in Shikarpur, which lies beyond the surface-water catchment, and a recharge elevation of 1,000-1,100 m asl for Banlek, which partially extends beyond the surface-water catchment. The demarcated recharge zones will be used by government agencies to implement recharge interventions to increase the resiliency and reliability of springs in Far-Western Nepal.

Identifying Groundwater and River Water Interconnections Using Hydrochemistry, Stable Isotopes, and Statistical Methods in Hanumante River, Kathmandu Valley, Central Nepal

“…Wet season river samples are plotted near the GMWL and have a similar slope and intercept (R 2 = 0.86) as the GMWL (Figure 4a), indicating recent meteoric water as a major source for river discharge [4,8,34,58]. Ca-HCO 3 water type, defined from piper diagram and lighter isotopic composition of rainfall during wet season [62,64], also suggests a similar water source. However, there was no evidence of evaporation in a previous study in the Kathmandu Valley [58,60]; dry season river samples plotted below the GMWL with lower slope (slope = 5.46 with R 2 = 0.79; Figure 4b) as compared with the GMWL may indicate a possibility of evaporation [26,65].…”

“…Groundwater samples plot below the GMWL, with a similar slope (R 2 > 0.9) in both wet and dry seasons (Figure 4b), indicating recent meteoric water as a major recharge source for these dug wells [12]. Although meteoric water is a major source for dug wells, spatial variation is noticeable in groundwater since isotopic composition of precipitation is dependent on rainfall amount, elevation, and source of water vapor of rainfall [62,64,66].…”

“…Fewer studies regarding isotopic analysis of meteoric water have been carried out on a local scale [60][61][62]. The local meteoric water line (LMWL), established by Gajurel et al [60] and Giri [61], has a similar slope and intercept value as the global meteoric water line (GMWL) reported by Craig [45].…”

Unpacking future climate extremes and their sectoral implications in western Nepal

“…The highlands in Karnali are relatively water-poor regions in Nepal (Panthi et al 2018). The indigenous communities rely on natural springs as their primary source of water for drinking and irrigation and are highly vulnerable to changes in precipitation (Matheswaran et al 2019). The southern lowlands are prone to disasters, such as floods and droughts, and the northern highlands experience landslides and flash floods.…”

Unpacking Future Climate Extremes and their Sectoral Implications in Western Nepal

“…The highlands in Karnali are considered to be relatively water-poor regions in Nepal (Panthi et al 2018). The indigenous communities rely on natural springs as their primary source of water for drinking and irrigation and are highly vulnerable to changes in precipitation (Matheswaran et al 2019). The southern lowlands are prone to disasters, such as oods and droughts, and the northern highlands experience landslides and ash oods.…”