Chapter 4 - Radon isotopes as tracers for submarine groundwater discharge and associated nutrient and rare earth elements inputs to the Sankarabarani river basin, India

Abstract

Direct release of fresh/recirculated groundwater to the ocean is of greater significance due to its material transfer pathways along the coastal zones. To assess the significance of submarine groundwater discharge (SGD) as a cause of chemical and dissolved fluxes to the ocean, estimation of submarine groundwater discharge, and associated dissolved nutrients and rare earth elements fluxes to the Bay of Bengal from the Sankarabarani river basin, India has endeavored. A total of 180 groundwater samples were collected from the three different locations for 30days by considering high and low tides. The submarine groundwater discharge through the aquifer ranges from 0.75 to 2.90md−1 calculated by the radon mass balance model. The SGD rate is increasing toward the coast, which is higher in location C followed by locations A and B suggesting the influence of recirculated seawater resulting in brackish SGD. From the nutrient mass balance, DIN’s average net invention was 192.00μmold−1, DIP is 4.00μmold−1, and DSi is 97.00μmold−1 suggesting SGD-derived nutrients were higher and responsible for algal blooms influencing the biodiversity of the study area. From the REE mass balance, the SGD supported REE flux was 25.70mmold−1 for Nd, 5.04mmold−1 for Gd, and 2.92mmold−1 for Yb, respectively. The removal of LREEs in a more significant quantum in comparison with MREE and HREEs. The more excellent removal of LREEs is mainly due to increasing salinity and release of MREEs and HREEs due to its particle reactivity. The SGD-derived fluxes were influenced by tidal fluctuations, hydraulic gradient, the conductivity of the formation, groundwater extraction, and other biogeochemical influences. The sources for nutrients and REEs fluxes seem to be influenced by interaction between the fresh and bay water end members. The other dominant sources are the availability of elements, speciation, presence or absence of colloidal fragments, and various sources and sinks.