Abstract:
This study interprets the observed spatio-temporal variation in the oxygen and hydrogen isotopic composition (δ18O, δD) of shallow unconfined groundwater of Central India (Madhya Pradesh) in terms of changing moisture sources for the Indian Summer Monsoon rainfall, orography and hydrogeological settings. Shallow groundwater samples collected from 282 locations for both pre-monsoon (April) and post-monsoon (November) seasons were analysed for δ18O and δD. The isotopic composition of groundwater varies spatially and seasonally from pre-monsoon to post-monsoon. A clear seasonal variation in isotopic composition with different spatial characters is ascribed to different recharge sources, mixing and evaporation. Backward wind trajectory analysis confirms that the observed isotopic variation from east to west is due to different moisture sources contributing to rainfall in these regions. The isotopic composition of groundwater in most (58% of the geographical area) of the study area have lower values in the post-monsoon season compared to the pre-monsoon season suggesting seasonal recharge of groundwater by isotopically depleted Indian Summer Monsoon (ISM) rain, while 15% of the study area shows the opposite trend caused by different recharge source (irrigation return flow, allochthonous water, and evaporated surface water). Isotopic mass balance in two adjoining aquifers (Sedimentary hard rock and Volcanic rock) reveals that post-monsoon groundwater is an admixture of 39% ISM and 61% of pre-monsoon residual water in sedimentary hard rock, while in the volcanic system, it is 32% ISM and 68% pre-monsoon residual which suggest a higher rate of groundwater recharge in sedimentary hard aquifers compared to the volcanic system. This study provides new insights into the recharge characteristics in a data-poor region of central India covered by different hydrogeological units.