Quantification of source contributions to the water budgets of the Ganga (Hooghly) River estuary, India

Abstract

This study is based on comprehensive data on salinity and δ18O values of the Ganga (Hooghly) River estuary waters sampled during six seasons of contrasting water discharge over two years. In addition, the data for the groundwater collected from areas in proximity to the estuarine transect are also discussed. The δ18O values and salinity depict strong positive correlations (r2: 0.96–0.99) suggestive of efficient mixing in the estuary. The intercepts of the regression lines in the plots of δ18O vs. salinity suggest that the average δ18O values of freshwater were the lowest (−8.1 to −8.4‰) in the post-monsoon periods whereas the highest values (−6.2 to −6.6‰) were recorded in the pre-monsoon periods. The results of mass balance calculations carried out using the δ18O values of the meltwater and non-melt water (groundwater and rainwater) indicate that meltwater accounts for ~10–25% of the freshwater budget of the Hooghly River in the pre-monsoon and monsoon periods during two years of the study. In the plot of δ18O vs. salinity, several samples plot above the theoretical line of conservative mixing of river water and the seawater; indicating that there is internal source(s) of water within the estuary. The contributions of water from three sources (river water, seawater and groundwater) to the estuary were estimated based on the inverse modeling. The model results of higher fractions of seawater at Gangasagar in the monsoon period compared to the post-monsoon period are consistent with the more intense tidal incursion in the summer months that is driven by the prevalent wind circulation and the surface currents in the northern coastal Bay of Bengal. In the upper estuary, the groundwater constitutes ≤10% of the total water flux whereas, in the middle to the lower estuary, the groundwater fractions are typically 20–30%. The observations that the maximum contribution from the meteoric recharge to the Hooghly water flux at Gangasagar (5–16%) is lower than the estimates of the groundwater contributions and that the salinity of shallow groundwater is as high as 4.21‰; together indicate that there is a component of recirculated saline water in the submarine groundwater discharge to the Hooghly River estuary. Our results of salinity-δ18O covariation over several seasons have implications for the reconstruction of paleosalinity in the estuarine and coastal regions using δ18O values of carbonate shells.