Dissolved inorganic carbon (DIC) and its δ13C in the Ganga (Hooghly) River estuary, India: Evidence of DIC generation via organic carbon degradation and carbonate dissolution

In this study, we present comprehensive data on dissolved Ca, dissolved inorganic carbon (DIC) and its carbon isotope composition (delta C-13(DIC)) of (i) the Ganga (Hooghly) River estuary water sampled during six seasons of contrasting water discharge over 2 years (2012 and 2013), (ii) shallow groundwater from areas adjacent to the estuary and (iii) industrial effluent water and urban wastewater draining into the estuary. Mass balance calculations indicate that processes other than the conservative mixing of seawater and river water are needed to explain the measured DIC and delta C-13(DIC). Results of mixing calculations in conjunction with the estimated undersaturated levels of dissolved O-2 suggest that biological respiration and organic carbon degradation dominate over biological production in the estuary. An important outcome of this study is that a significant amount of DIC and dissolved Ca is produced within the estuary at salinity >= 10, particularly during the monsoon period. Based on consideration of mass balance and a strong positive correlation observed between the excess DIC and excess Ca, we contend that the dominant source of DIC generated within the estuary is carbonate dissolution that is inferred to be operating in conjunction with degradation of organic carbon. Calculations show that groundwater cannot account for the observed excess Ca in the high salinity zone. Estimated DIC contributions from anthropogenic activity are minor, and they constitute ca. 2-3% of the river water DIC concentrations. The estimated annual DIC flux from the estuary to the Bay of Bengal is ca. (3-4) x 10(12) g, of which ca. 40-50% is generated within the estuary. The monsoon periods account for the majority (ca. 70%) of the annual DIC generation in the estuary. The annual DIC flux from the Hooghly estuary accounts for ca. 1% of the global river DIC flux to the oceans. This is disproportionately higher than the water contribution from the Hooghly River to the oceans, which accounts for ca. 0.2% of the global river water flux. The results of this study suggest that estuaries in regions affected by tropical monsoon can be important in terms of their production of significant amounts of DIC and its delivery to the world's oceans. (C) 2015 Elsevier Ltd. All rights reserved.