Groundwater δ18O and δD from central Indian Peninsula: influence of the Arabian Sea and the Bay of Bengal branches of the summer monsoon

Results of oxygen and hydrogen isotopic investigations of shallow groundwater and river water samples from the Central Indian Peninsula are reported. The study was undertaken to isotopically characterise the near surface water sources and identify the influence of the two branches of the Indian Summer Monsoon (June-September) in this region. The western part of the study area receives vapour flux largely from the Arabian Sea branch, whereas the eastern part receives significant vapour flux from the Bay of Bengal branch. No significant rainfall occurs in the study area during northeast winter monsoon period of October-December or the remaining part of the year. Therefore, the isotopic character of shallow groundwater from this region could be used to study the relative influence of the vapour sources from the Arabian Sea and the Bay of Bengal. The study shows that: (i) The northeast part, receiving significant rainfall from the Bay of Bengal branch, has a distinctly depleted isotopic composition (δ18O≈−5±2‰) compared to areas receiving dominant rain from the Arabian Sea branch (δ18O≈−1±1‰). (ii) In the west coastal belt, the average δ18O in groundwater is close to the average δ18O of precipitation at Bombay (−1.4±1.1‰). But, groundwater samples in the entire Deccan Plateau region show signatures of evaporative enrichment in the form of (a) low slope (∼6.5) of δ18O-δD regression line; (b) low d-excess (0±5‰) and (c) slightly enriched δ18O values (−1±1‰) compared to those expected from the limited precipitation data from stations in Maner basin approximately in the middle of the study area. (iii) Depleted ground waters along the central Indian east coast could arise due to continental rainout of the Arabian Sea vapour source as the moisture-laden monsoon winds crossover to the Bay of Bengal. Alternatively, it is necessary to postulate that the surface waters of the Bay of Bengal contributing to inland vapour flux are significantly more depleted during the monsoon period than the known depletion of ∼1‰. It is also possible that both factors are operative.