Heterogenous Si Isotopic Composition in Coastal Groundwater: Controls on Dissolved Silicon and Groundwater Discharge Along Indian Coastline

Abstract

We report stable silicon isotope ratio (δ30Si) of over 80 groundwater samples collected along the Indian coast, spanning a wide range of aquifer lithologies (alluvial, basalt, metamorphic, laterite and limestone), climate (semi-arid to tropical wet) and land use settings. Indian coastal groundwater exhibits large spatial variability in dissolved silicon (DSi) (80–1350 μM) and δ30Si values (−1.1‰ to 4.5‰). On average, the δ30Si value of the Indian coastal groundwater (0.8 ± 1.1‰, 1SD, n = 85) is comparable to published groundwater globally (0.8 ± 0.8‰, n = 117), and significantly lower than Indian riverine δ30Si composition. The coastal groundwater δ30Si values do not show any dependence on regional aquifer lithology. However, the permeable coastal alluvial groundwaters exhibit the highest variability in DSi and δ30Si, likely acquiring signatures of shallow surface/subsurface processes through mixing. A broad negative correlation between δ30Si values and the Ge/Si ratio is best explained by the partitioning of Si into secondary minerals phases within the weathering zone. The majority of coastal groundwater follows a steady-state model evolution, indicating a dynamic equilibrium between Si supply and the formation of secondary phases. In regions of low annual rainfall, groundwater irrigation can lead to infiltration of return flow water to aquifer systems, leading to their heavy δ30Si values. The fresh submarine groundwater discharge along the Indian coast is estimated to be 2.1 GmolSi yr−1, which is less than 1% of the riverine Si flux to the North Indian Ocean and 0.3% of the global fresh groundwater Si flux.