Sources and Relative Timing of Groundwater Recharge in Northwest India Using Environmental Isotopes

Abstract:

The Indo-Gangetic basin in northwest India is a major hotspot of groundwater depletion over the last four decades, with significant consequences for future agricultural productivity. However, little is known about groundwater dynamics in this region. We use environmental isotope analysis of rainfall, groundwater and surface water to characterize zones and sources of groundwater recharge in the Ghaggar basin lying between the Himalayan-fed Yamuna and Sutlej river systems. Around 700 groundwater samples were collected from 182 locations within the Ghaggar basin during pre- and post-monsoon periods of 2013. The δ¹⁸O values of river water vary from -7.3‰ to -5.3‰ and δ²H varies from -50.6‰ to -46.4‰, suggesting source of water from lower altitude. Depleted isotopic values of δ¹⁸O between -12.1‰ and -11.5‰, and δ²H between -79.1‰ and -74.9‰, are observed in canal water sourced from the Sutlej. The δ¹⁸O values of groundwater vary from -12.6‰ to -3.1‰ and δ²H from -84.8‰ to -28.6‰. Groundwater isotopic values fall on the LMWL, indicating local precipitation as the main recharge source, although a few depleted samples falling above the LMWL suggest local canal recharge.

A key point in our data is the marked longitudinal as well as vertical variability in composition of groundwater samples reflecting differences in age and recharge sources. Longitudinal variability in groundwater is reflected from a much larger scatter in both δ¹⁸O and δ²H values for the downstream reaches compared to the upstream reaches, suggesting mixing of various sources of recharge. Also, groundwater in downstream reaches is saline or brackish with EC values of >4000 μS/cm reflecting evaporation of irrigation water. Further, groundwater samples above 60 m bgl show tritium values in range of 1.88-8.0 TU indicating sub-modern to modern recharge whereas those below 60m bgl show a much lower values of 0.3-1.88 TU suggesting groundwater is sub-modern to older than 50 yrs. It indicates that replenishment of groundwater below 60 m bgl is very little. Stable isotope data also show that the shallow aquifers have a larger variability in both δ¹⁸O and δ²H values compared to those for the deeper aquifers. Such inhomogeneity in groundwater structure has important implications for sustainable groundwater management in this region