| dc.description.abstract | An estuary is a partially enclosed, coastal water body where freshwater from rivers and streams mixes with saline water from the ocean. It is a dynamic system that is characterized by large changes in water chemistry (e.g., salinity) as well as water volume (tidal effects). Additionally, high nutrient availability makes estuaries productive ecosystems and the access to natural resources and water makes these regions densely populated. This thesis focuses on river water-groundwater-seawater interactions in the Ganga estuary in India (Hooghly estuary) and attempts to quantify the seasonally resolved groundwater discharge in this estuary. River water samples from the Hooghly estuary were collected during four seasons (monsoon and non-monsoon over two years between 2019-2021) over 200 km inland from the mouth of the river at Bakkhali (Bay of Bengal), covering both the estuary and the tidal river sections. Surface river water over varying salinities (0.1-25 psu) (n = 84) as well as depth-dependent river water samples (n = 17) from selected locations were collected. Additionally, suspended sediments from selected locations (n = 20) were analysed along with groundwater samples (n = 21) collected from both banks of the river. In-house measurements of cation concentrations using ICP-OES/ICPMS, anion concentrations using an ion-chromatograph and radiogenic Sr isotopes (87Sr/86Sr) using a thermal ionization mass spectrometer (TIMS) were performed on these samples.
This study characterized the boundary between the tidal river section (no salinity changes with tidal action) and the estuary of the Hooghly River geochemically. The salinity change across the estuary over different seasons was modelled using an existing relationship of soil moisture equation with modified variables. Selected cations (Na+, K+, Ca2+, Mg2+, Sr2+) and anions (Cl-, SO42-) in all the surface water samples from the mid-stream, irrespective of sampling season, demonstrated conservative mixing between the fresh water and the surface Bay of Bengal end member. However, the surface river water 87Sr/86Sr demonstrated a non-conservative behaviour. The depth-dependent samples helped us in classifying the Hooghly estuary into Type 1 (well-mixed) based on established classification criteria. The 87Sr/86Sr of the depth-dependent samples also demonstrated a non-conservative behaviour in both the pre-monsoon and monsoon seasons indicating additional inputs of Sr into the estuary. To investigate the additional source(s) contributing the Sr with unique 87Sr/86Sr, selected sediment samples and groundwater samples were analysed. The 87Sr/86Sr of the groundwater showed limited variability and indicates enhanced dissolution of detrital carbonates in the west bank aquifer of the Hooghly estuary. The anion data is indicative of possible anthropogenic contamination. On he multi-endmember mixing plot of 87Sr/86Sr and Sr, the groundwater composition is best explained by mixing between the sediments and the river water, which indicates a strong interaction between the river and the aquifer. The major, trace, including rare earth elements (REE), concentration data for the suspended sediments show limited variations across salinity, depth, and season. The REE pattern demonstrated an average upper continental crust (UCC) like signature with a prominent Gd peak suggesting a possible anthropogenic input. A key observation is the significant difference between the 87Sr/86Sr of the sediments and the dissolved load indicating that there is a lack of equilibration between the dissolved and the suspended phases. The sediment 87Sr/86Sr is highly radiogenic and reflects the signatures of the Himalayan source rocks. The suspended sediments can be traced to their parent High Himalayan Crystalline series rock with additional input from the lesser Himalayas and meta-carbonates.
The geochemical and isotopic compositions of surface river water, depth river water, groundwater, and sediments all indicate input of groundwater into the Hooghly estuary. Using mass balance considerations, the percent contribution of the groundwater to the estuary across seasons was estimated to be between 3 and 40%. To verify the results from the geochemical estimates, a numerical model using MODFLOW-SEAWAT was constructed based on the geometry of the Hooghly River cross sections, and using previously observed groundwater table conditions, tidal variations. and aquifer properties. The model was able to capture the diurnal nature of the bank seepage in a tidally dominated environment and demonstrated the invariability of the bank seepage with respect to tide induced salinity change. The model-estimates of groundwater seepage into the estuary broadly overlap but are on the higher side of the geochemical estimates.
Overall, this study characterized the Hooghly (Ganga) estuary geochemically by analysing spatially distributed, including depth-dependent, river water samples across different seasons, over a duration of two years. Geochemical and radiogenic Sr isotopic data of the water samples and suspended sediments show lack of isotopic equilibration between the dissolved and suspended loads, while a non-conservative mixing in the Sr isotope space indicates seasonally varying contribution (3-40%) of groundwater to the estuary which was also verified using numerical modelling. Groundwater compositions reflect dissolution of detrital aquifer carbonates as well as anthropogenic sources while suspended sediments reflect the compositions of Himalayan provenances. | en_US |
