Abstract:
Submarine groundwater discharge (SGD) acts as major pathway to transport solute-laden terrestrial-sourced
fresh groundwater, as well as re-circulated marine water to the global oceans. The study area, Bay of Bengal
(BoB), a part of the Indian Ocean, receives one of world’s highest terrestrial riverine fresh water discharge,
sediment and solute flux from the adjacent Himalayan and cratonic South Asia. Thus, together with the monsoon-
dominated tropical climate, it forms one of the most complicated, productive and interactive global hydrological
systems. However, understanding such topical phenomena needs intricate mechanistic understanding,
based on high resolution data, which are barely available from the BoB. Delineation of stable isotopic and
chemical signature of hydrologic-sourced components in the SGD to the BoB would help to identify the intraannual
to diurnal-scale impact of seasonality and tidal cycles, as well as interactions with other surface water
bodies. This study provides one of the first documentation of such high-resolution, temporally-variable, stable
isotope patterns of SGD in coastal systems of the BoB, and possibly of any tropical ocean. During post-monsoon
season, the discharging groundwater was observed to have depleted δ18O (ranges −2.12‰ to −4.19‰) and low
Cl− concentrations (745 to>11,500 ppm) (seepage water), which is closely associated with the groundwater
δ18O composition (−3.18‰ to −4.05‰) and Cl− content (775 to>5900 ppm) range. In pre-monsoon season
depleted δ18O values suggests that regional groundwater contributes up to 45m from high tide line (HTL) (up to
88%), and re-circulated seawater-sourced SGD dominates 45m to 110m (extent of study transect) offshore. In
post-monsoon season, terrestrial-sourced groundwater predominates the SGD composition (up to 99%) till
110 m. Changes in δ18O and Cl− content, in pre-monsoon season indicates enhanced infiltration of seawater in
the seepage face, due to lower terrestrial-sourced freshwater discharge, whereas, in post-monsoon terrestrialsourced,
resident freshwater dominates in the seepage face. The study suggests that SGD are sourced to interactions
between local-regional hydrological systems, and do reflect their compositional variability. It also provides
insight of influencing physico-chemical mechanisms, ranging from seasonal to daily-tidal time-scales. The
outcome of this study thus may provide intricate insights in delineating the coastal hydrologic and biogeochemical
processes, as well as detecting, carbon sinks, nutrient sources and primary productivity in a tropical
ocean.
