Delineating seasonal porewater displacement on a tidal flat in the Bay of Bengal by thermal signature: Implications for submarine groundwater discharge

摘要

Submarine groundwater discharge (SGD) to the Bay of Bengal is an important groundwater discharge process to southern Asia as well as to the global oceans. Water-fluxes in these regions are not very well understood. The present study demonstrates the use of sea-bed porewater temperature as a tracer for delineating seasonal SGD from a shallow coastal aquifer to the Bay of Bengal. Porewater temperature mapping along with chemical profile of porewater column were used to delineate groundwater discharge zones for different seasons in a hydrologic year. While, the temperature profiles were used to evaluate total groundwater discharge (both terrestrial and marine), the salinity and total dissolved solids profiles were used to evaluate the component due to terrestrial (freshwater) discharge. Several springs and seeps were mapped to finger-print the freshwater discharge zones and identify the terrestrial sourced-SGD temperature windows. These zones are also identified to be enriched with algal mats, and hence may indicate the pathways of nutrient-solute discharge that enhance the primary productivity in the marine ecosystem. Assuming vertical flow in the seabed, simulated rates of discharged groundwater provided a good match with measured discharge rates. The porewater fluxes were estimated by applying analytical solutions of the heat conduction-advection equations to the observed vertical temperature profiles. High resolution thermal profiles to identify T-SGD signatures and chemical profiles along mapped transects reveal the nuances of 3-D seasonal hydrodynamics of groundwater-seawater interactions. The calculated vertical porewater fluxes through the seabed ranged from negligible to 2.45 x 10(-2) m(3)/m(2)/d(1) within a zone extending from high tide line to 40 m offshore, and represent the vertical porewater velocity and the average vertical porewater velocity is found as 8.2 x 10(-3) m(3)/m(2)/d(1). Approximations suggest that in present-day condition, total average annual SGD to Bay of Bengal is about 1.16 x 10(7) m(3)/y. Seasonal estimates suggest that the averages SGD during pre-monsoon and post-monsoon seasons are ranges 0.6-0.8 x 10(7) m(3)/y for pre-monsoon and 0.8-1.5 x 10(7) m(3)/y for post-monsoon, respectively. The findings suggest that monsoonal activity result in a significant porewater displacement from tidal flat sediments by increased groundwater discharge. (C) 2015 Elsevier B.V. All rights reserved.