Isotopic Characterization of groundwater in semi-arid Western India: Insights into Hydrogeological Processes

The study area located in the semi-arid western India is the tenth most populous Indian state with an estimated population of 60.3 million (~5% of Indian population) and population density of 308 persons/km². About 42.6% of the population lives in the urban area and 57.4% in rural areas. Surface water paucity, seasonal availability and a varying amount of rainfall in this region make groundwater an important and most preferred source to fulfil the demand for drinking water, agricultural, industrial and sustain an important native terrestrial ecosystem. The over-dependency on groundwater leads to various problems related to quality and quantity of groundwater like a rapid decline of the water table, mining of static groundwater, seawater intrusion and groundwater contamination by geogenic and anthropogenic sources. Sustainable water management across the state must be underpinned by the clear understanding of groundwater recharge characteristics, relationship between recharge sources and groundwater, factors controlling the interaction between surface water and groundwater, deletion of the areas not receiving a freshwater influx. Shallow groundwater samples were collected during IWIN nation programme (2008 to 2013)  for the year 2009 to measure the stable isotopes of oxygen and hydrogen in groundwater from 205 locations during post-monsoon (November ) season and 207 locations in pre-monsoon season ( May-June) to understand the factors governing Spatio-temporal variation in isotopic composition and obtain insights about the spatially variable recharge characteristics and possible controlling factors. The oxygen and hydrogen isotopic values and their spatio-temporal variations in the study area demonstrate that (1) d¹⁸O depletion in post-monsoon GW compare to pre-monsoon infer the seasonal recharge of GW (2) d¹⁸O of post-monsoon GW ( -2.3‰) lie in between the d¹⁸O of pre-monsoon GW ( -1.9‰) and d¹⁸O of southwest monsoon rainfall (-4.1‰) indicates post-monsoon GW is the mixture of these two components (3) Seasonal variation in d¹⁸O deduce that 56% ( ~109773 km2) of the total land area (~ 196773 km2) shows seasonal GW recharge while 32% (62727 km2) is not receiving any freshwater influx (4) 36% of post-monsoon samples while 40% of the pre-monsoon samples have negative d-excess values and shows a decreasing trend with d¹⁸O infer evaporation of sub-surface water prior to recharge (5) Alluvial aquifer of north Gujarat has depleted d¹⁸O compare to adjacent high elevated hard rock aquifer indicates irrigation return flow of deep static water.