Spatial and temporal variations in dissolved silicate along the Indian coastal groundwaters and their export to adjacent coastal waters

Abstract

Silicate plays a vital role in the coastal regions biogeochemistry. Submarine Groundwater Discharge (SGD) is one of the significant silicate sources to the coastal regions. The silicate concentrations in groundwater (GW) are considerably higher than in neighbouring coastal waters, as a result, SGD increases the concentrations of silicate in the coastal waters and influences the marine ecosystem. To examine the spatial and temporal variability in silicate concentrations and export of silicate to the adjacent coastal waters through SGD, groundwater samples are collected at 90 locations along the east and west coast regions of India during the dry and wet periods. This study revealed that the concentration of dissolved silicate was lower during the dry period than during the wet period. The soil silicate concentration displayed a significant variability and has a good correlation with silicate concentrations in the coastal groundwater of India. Silicate in groundwater showed significant linear relations with bacterial respiration, bicarbonate, and pCO2 and inverse relation with dissolved oxygen saturation suggesting that remineralization and weathering processes are responsible for the silicate in the coastal groundwater of India. The average SGD of freshwater varied between 1.6 × 104 m3/day and 1.75 × 1011 m3/day from the Indian coastal groundwater into adjacent coastal regions. The fluxes of silicate through SGD to the Bay of Bengal was estimated as 1.97 ± 0.85 Tg (1 Tg = 1012 g) and 3.22 ± 1.56 Tg to the eastern Arabian Sea. The estimated total SGD fluxes of silicate to the northern Indian ocean from east and west coastal regions of India is 3.07 ± 1.15 Tg/y and it is almost three times higher than silicate discharged from monsoonal rivers 1.03 ± 0.25 Tg/y. The silicate through SGD from Indian coastal groundwater contributes 7% of silicate to the total silicate export to the world coastal regions through SGD.