Geochemistry of Devi River Estuary, East Coast of India: Insights from Distribution of Elements and CO2 Dynamics

Abstract

Estuaries represent a transition zone at the land-sea interface, and it is characterized by strong geochemical gradient due to the mixing of fresh river- and saline ocean- water. It is a highly dynamic system where behavior and fate of elements have been modified drastically due to large scale variation in different physio-chemical properties. Estuaries also play an important role in modulating the global CO2 budget. Thus, this thesis aims to carry out an extensive geochemical investigation over a period of one year (2018-19) in the tropical mangrove dominated Devi river estuary, formed by a major distributary of the Mahanadi River, which is the third largest river of Peninsular India, in order to understand the behavior of elements in surface water and sediment, and its role in air-water CO2 exchange. The study is based on sampling conducted during May 2018, September 2018 and February 2019 that represent summer, monsoon, and winter periods, respectively. Salinity in the Devi river estuary ranges from 1.1 to 33.1 ppt and 0.1 to 29.5 ppt during summer and winter, respectively. Contrarily, salinity decreases in the monsoon period (0.1-13.6 ppt). The pH of the estuarine water ranges from 6.24 to 8.97. The alkalinity and dissolved inorganic carbon range from 23.5 to 232 mg/l and 44 to 382 mg/l, respectively. The major ion chemistry and nutrient distribution show that Na, Cl, SO4, NO3 and PO4 behave conservatively whereas DSi, Ca and Mg exhibit non-conservative behavior. This is attributed to one or more processes involving mixing of river and ocean water, weathering of minerals, biological processes and carbonate mineral precipitation/dissolution. Nutrient stoichiometric ratios – NO3:PO4, DSi:PO4 and DSi:NO3 – vary spatially and seasonally as a result of differences in geochemical processes prevailing in the fresh- and marine-water environment. The distribution of dissolved trace elements (Sr, Ba, Fe, Mn, Mo, V, Cu, Cr, Co, Ni, Zn, U and Pb) in the estuary is influenced by their respective sources, abundances in the fresh and marine end-members, mixing processes and their ability to take part in various geochemical reactions occurring in the transitional zone. All the trace elements behave non-conservatively in the estuary during at least one season within the studied period. Flocculation related removal of Fe, Mn, Mo, V, Pb and U occurs in the low- to mid- salinity zone. A pronounced mid-estuarine maxima is exhibited by Ni and Zn, whereas Cu shows a poor mid-estuarine maxima. Barium shows peak concentrations in the low salinity zone due to significant addition. A high pCO2 (1565 ± 782 μatm) in comparison to that of atmosphere is found in the estuary. The pCO2 is higher during winter (2071 ± 828 μatm) compared to summer (1675±665 μatm) and monsoon (951 ± 316 μatm). Based on the relationship among DO%, pH, pCO2 and chlorophyll, the process of organic matter degradation is inferred to be a major controlling factor for pCO2. Net annual CO2 efflux from the Devi river estuary is estimated to be 40.1±7.5 mol C m-2 yr-1. The highest efflux was observed during summer season (126±69 mmol C m-2 d-1) in spite of higher pCO2 in winter. This is due to higher temperature and wind speed in summer. The Devi river estuary is estimated to contribute about 0.003–0.006% of the total global CO2 emission from estuaries. Bulk chemical composition of the estuarine sediments show an enrichment of Ba, Nb, Pb, Rb, Th and Zr with respect to the upper crust. Different weathering indices along with the A–CN–K and A-CNK-FM ternary plots suggest that sediments are dominantly derived from a felsic source rock and have undergone low to moderate chemical alterations. High LREE/HREE (11.16±3.68), negative Eu anomalies, and (La/Yb)n and (Tb/Yb)n values confirm that sediments are derived from the Eastern Ghat Group of rocks. Upper estuary sediments show negative Eu anomalies which is similar to that of the source. However, positive Eu anomaly is mostly observed in lower estuary. Contrasting Eu anomalies between upper- and lower-estuarine sediments are uncharacteristic of previously studied major global estuaries. Concentration of REEs, Sc, Fe, Mo, V, Zn, Zr, Nb, U, Ti, Na and P in sediments increases up to 20 ppt salinity, and followed by declining trend towards the mouth. This is mostly due to salinity-induced flocculation of colloidal particles. Gradual decline in concentration of Cr, Co, Ni, Cu, Rb, Sr, Sb, Cs, Ba, Pb, Al, Mn, Mg, Ca and K with increase in salinity is attributed to desorption of elements from sediments. The SiO2 content shows increasing trend towards mouth due to incorporation in diatom frustules in the lower estuary. The findings of this study emphasize the importance of intrinsic physicochemical parameters, mainly salinity, pH and redox condition, on different geochemical processes for governing behaviour of different elements and nutrients. Additionally, it highlights the role of small-scale estuaries in significant contribution of CO2 to the atmosphere. It further contributes towards the increasing dataset of pCO2 and CO2 effluxes from estuaries worldwide in order to accurately estimate the role of estuaries in the global carbon budget.