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

Sarath Pullyottum Kavil, Jean Riotte, Ramananda Chakrabarti, V. V. S. S.

Sarma, B S K Kumar, J. Prunier, Arnaud Dapoigny, Damien Cardinal

► To cite this version:

Sarath Pullyottum Kavil, Jean Riotte, Ramananda Chakrabarti, V. V. S. S. Sarma, B S K Kumar, et al.. Heterogenous Si Isotopic Composition in Coastal Groundwater: Controls on Dissolved Silicon and Groundwater Discharge Along Indian Coastline. Global Biogeochemical Cycles, 2025, 39 (8), 10.1029/2025gb008706. [https://doi.org/10.1029/2025gb008706](https://doi.org/10.1029/2025gb008706)

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Distributed under a Creative Commons Attribution 4.0 International License
# Global Biogeochemical Cycles®

## RESEARCH ARTICLE

10.1029/2025GB008706

**Key Points:**

* The highest groundwater $\delta^{30}\text{Si}$ variability (from -1.1 to 4.5‰) observed in alluvial aquifers on the Indian coast

* No difference in $\delta^{30}\text{Si}$ between different aquifer lithologies, low rainfall regions exhibits heavier $\delta^{30}\text{Si}$ likely due to agriculture return flow

* The Si supply from fresh submarine groundwater discharge to the North Indian Ocean is <1% of riverine input, and is 0.8‰ lighter on average

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

S. Pullyottum Kavil,
sarathp39@gmail.com

**Citation:**

Pullyottum Kavil, S. Riotte, J., Chakrabarti, R., Sarma, V. V. S. S., Kumar, B. S. K., Prunier, J., et al. (2025). Heterogenous Si isotopic composition in coastal groundwater: Controls on dissolved silicon and groundwater discharge along Indian coastline. *Global Biogeochemical Cycles*, 39, e2025GB008706. https://doi.org/10.1029/2025GB008706

Received 8 JUN 2025
Accepted 5 AUG 2025
Corrected 5 SEP 2025

This article was corrected on 5 SEP 2025. See the end of the full text for details.

**Author Contributions:**

**Conceptualization:** Sarath Pullyottum Kavil, Jean Riotte, Ramananda Chakrabarti, V. V. S. S. Sarma, Damien Cardinal

**Data curation:** Sarath Pullyottum Kavil, V. V. S. S. Sarma, J. Prunier

© 2025. The Author(s). This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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

Sarath Pullyottum Kavil¹,² ID, Jean Riotte³,⁴, Ramananda Chakrabarti⁵,⁶ ID, V. V. S. S. Sarma⁷ ID, B. S. K. Kumar⁷, J. Prunier³ ID, Arnaud Dapoigny⁸ ID, and Damien Cardinal¹ ID

¹LOCEAN-IPSL, Sorbonne Université-IRD-CNRS-MNHN, Paris, France, ²Now at: IGV, Stockholm University, Stockholm, Sweden, ³GET-OMP, IRD, Toulouse, France, ⁴Indo-French Cell for Water Sciences, ICWAR, IRD, Indian Institute of Science, Bangalore, India, ⁵Centre for Earth Sciences, Indian Institute of Science, Bangalore, India, ⁶Interdisciplinary Centre for Water Research, Indian Institute of Science, Bangalore, India, ⁷CSIR-National Institute of Oceanography, Regional Centre, Visakhapatnam, India, ⁸Laboratoire des Sciences du Climat et de l'Environnement, LSCE-IPSL, CEA-CNRS-UVSQ-Université, Gif-sur-Yvette, France

**Abstract** We report stable silicon isotope ratio ($\delta^{30}\text{Si}$) 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 $\delta^{30}\text{Si}$ values (–1.1‰ to 4.5‰). On average, the $\delta^{30}\text{Si}$ 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 $\delta^{30}\text{Si}$ composition. The coastal groundwater $\delta^{30}\text{Si}$ values do not show any dependence on regional aquifer lithology. However, the permeable coastal alluvial groundwaters exhibit the highest variability in DSi and $\delta^{30}\text{Si}$, likely acquiring signatures of shallow surface/subsurface processes through mixing. A broad negative correlation between $\delta^{30}\text{Si}$ 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 $\delta^{30}\text{Si}$ 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.

## 1. Introduction

Continental silicate weathering regulates the atmospheric $CO_2$ consumption over geological time scales and releases dissolved silicon (DSi) in rivers, and subsequently to oceans, where it is incorporated in marine silificers, dominated by diatoms in the present day (Berner et al., 1983; Maldonado et al., 2019). Marine diatoms are responsible for almost 40% of marine primary productivity, driving the deep-sea export of organic carbon through biological carbon pumps (Field et al., 1998; Tréguer et al., 2018). Submarine groundwater discharge (SGD) is considered to be the second largest source of DSi to the ocean, contributing up to 25% of the total silicon flux (Rahman et al., 2019; Tréguer et al., 2021). SGD is ubiquitous in most shorelines and consists of fresh terrestrial groundwater (fresh SGD) and recycled saline groundwater (saline SGD) (Burnett et al., 2006; Chakrabarti et al., 2018; Moore et al., 2010). The fresh SGD originates from shallow and/or deep coastal aquifers, inheriting signatures of terrestrial weathering and anthropogenic inputs. The fresh SGD is primarily driven by a positive hydraulic gradient from the continent (Taniguchi et al., 2002). The saline SGD results from the recirculation of seawater into coastal sediments and aquifers, which can incorporate signatures of water-rock interactions in subsurface regions (Moore et al., 2010). Globally, the SGD DSi flux to the ocean is estimated to be 3.8 Tmol/yr, out of which almost 80% occurs as saline SGD (Cho et al., 2018; Rahman et al., 2019). However, groundwater discharge is often patchy and diffuses spatially, and the discharge timescales vary from hours to millennia (Bratton, 2010; Moore et al., 2010). Thus, the measurement of magnitude and associated chemical fluxes of SGD is a challenging problem and requires extensive sampling of coastal and offshore groundwater reserves. In addition to DSi, SGD is now known to be a significant source of nutrients such as nitrate and phosphorus, impacting coastal biogeochemistry (Santos et al., 2021).
**Formal analysis:** Sarath Pullyottum
Kavil, Jean Riotte, J. Prunier,

Arnaud Dapoigny, Damien Cardinal

**Funding acquisition:**

Ramananda Chakrabarti, V. V. S. S. Sarma

**Investigation:** Sarath Pullyottum Kavil,
Jean Riotte, Ramananda Chakrabarti,
B. S. K. Kumar, J. Prunier,

Damien Cardinal

**Methodology:** Sarath Pullyottum Kavil,
Jean Riotte, Ramananda Chakrabarti,
V. V. S. S. Sarma, B. S. K. Kumar,
J. Prunier, Arnaud Dapoigny,

Damien Cardinal

**Project administration:** Jean Riotte,
Ramananda Chakrabarti,

V. V. S. S. Sarma, Damien Cardinal

**Resources:** Ramananda Chakrabarti,
V. V. S. S. Sarma, B. S. K. Kumar,
J. Prunier

**Supervision:** Ramananda Chakrabarti,
V. V. S. S. Sarma

**Validation:** Sarath Pullyottum Kavil,
Jean Riotte, Arnaud Dapoigny,
Damien Cardinal

**Visualization:** Sarath Pullyottum Kavil

**Writing – original draft:**

Sarath Pullyottum Kavil

**Writing – review & editing:**

Sarath Pullyottum Kavil, Jean Riotte,
Ramananda Chakrabarti,
V. V. S. S. Sarma, B. S. K. Kumar,

Damien Cardinal

In terrestrial environments, groundwater forms an intermediate between soil and river water, dwarfing all other hydrological systems in terms of the total volume of freshwater storage (Gleeson et al., 2015). The DSi in groundwater inherits its signature from surface and subsurface water-soil-plant interactions and can further undergo weathering reactions in the aquifers before discharging into rivers and oceans (Worthington et al., 2016). During weathering of primary silicate minerals, the silicon is partitioned between secondary phases and porewater solution as dissolved silicon. The solubilized silicon can further undergo biotic uptake by plants and siliceous organisms and adsorption onto Fe oxy-hydroxide phases (Opfergelt & Delmelle, 2012). Both the abiotic and biotic processes of Si uptake are associated with significant isotopic fractionations, favoring the preferential incorporation of the lighter ²⁸Si into the solid phase, making the residual solution heavier in silicon isotopic composition (Frings et al., 2016; Opfergelt & Delmelle, 2012). The Ge/Si ratio is routinely coupled with δ³⁰Si measurements to distinguish between secondary clay formation and plant uptake, where Ge is known to be preferentially incorporated and/or adsorbed into secondary minerals (high Ge/Si), while plant phytoliths exhibit low Ge/Si (Delvigne et al., 2009; Derry et al., 2005; Kurtz et al., 2002; Mortlock & Frohlich, 1987). Thus, combining the Ge/Si ratio with δ³⁰Si values can be beneficial to understanding the extent of biogenic versus weathering control on Si cycling on Earth's surface (Frings, Schubring, et al., 2021). The groundwater δ³⁰Si values for globally distributed samples measured so far spans from -1.5 to +2.6‰; high δ³⁰Si values are generally attributed to incongruent silicate weathering (upper continental crust, UCC, δ³⁰Si of -0.25 ± 0.16‰, Savage et al., 2013), while lighter isotopic composition is attributed to the dissolution of secondary phases, such as clays and silcretes (e.g., Georg, West, et al., 2009, Georg, Zhu, et al., 2009; Pogge von Strandmann et al., 2014). The average δ³⁰Si value of global groundwater (0.8 ± 0.8‰, n = 117) is lower than that of reported average of global river water (1.25 ± 0.7‰, n = 557, Ehlert, Reckhardt, et al., 2016; Ehlert, Doering, et al., 2016; Frings et al., 2016, Martin et al., 2021, Kavil et al., 2024). The distinct isotopic composition and large Si fluxes as SGD make groundwater an important reservoir of Si, which can potentially alter the magnitude and composition of Si fluxes to the ocean in glacial-interglacial timescales (Tréguer et al., 2021). Despite large heterogeneity in isotopic composition, complexity in controlling processes and direct impact on riverine and oceanic elemental budget, groundwater Si cycling largely remains overlooked.

The Indian coastline stretches for almost 7,500 km, hosting diverse aquifer systems, including gneisses, basalts, limestones and recent alluvium, with precipitation levels ranging from <500 mm/yr to >2,500 mm/yr (CGWB, 2012). Previous works have reported evidence of total SGD in both the East and West coasts of India using various elemental and isotopic tracers, with estimated flux up to $150 \times 10^9 \text{ m}^3/\text{yr}$, almost 30% of annual discharge of Indian monsoonal rivers (Chakrabarti et al., 2018; Das et al., 2025; Krishna et al., 2016; Rahaman & Singh, 2012; Rengarajan & Sarma, 2015). Almost 57% of the Indian coastal area has groundwater table depth between 0 and 10 m above sea level, exhibiting a high terrestrial hydraulic gradient conducive to fresh SGD (Luijendijk et al., 2020; Manivannan & Elango, 2019). Groundwater in extrusive igneous and granitic lithologies can acquire high DSi through silicate weathering, increasing the net SGD DSi delivery to the coast (Rahman et al., 2019). The SGD Si flux estimated on the East coast of India accounts for almost 17% and 40% of Si delivered through the Godavari and Ganges-Brahmaputra rivers, respectively (Georg, Zhu, et al., 2009; Rengarajan & Sarma, 2015). Despite increasing evidence of substantial SGD water fluxes through Indian coasts, we lack an understanding of the variability and drivers of groundwater DSi composition. The present work aims to expand the global groundwater δ³⁰Si data set by analyzing more than 80 coastal groundwater spread along the Indian subcontinent, encompassing a wide range of aquifer lithologies and precipitation zones. We investigated (a) the variability in DSi and δ³⁰Si signatures of peninsular Indian coastal groundwater, (b) the potential sources and/or processes driving Si cycling in groundwater, and (c) the impact of SGD Si fluxes on the North Indian Ocean.

## 2. Materials and Methods

### 2.1. Study Site Description

Indian coastline covers almost 7,500 km, bounded by the Arabian Sea in the West, the Bay of Bengal in the East and the Indian Ocean in the South (Dimri et al., 2023; Manivannan & Elango, 2019). The aquifer systems in Indian coast is dominated by alluvium in the East, Cretaceous Deccan basalts and banded gneiss complex in the West (CGWB, 2012). Alluvium aquifers form the dominant aquifer type in India (29.8%), primarily consists of quartz, plagioclase feldspars and clays (CGWB, 2012; Shamsudduha et al., 2008). Deccan tholeiite basalts form low permeability hard rock aquifers (16.2%), and their major mineral phases include olivine, plagioclase,
clinopyroxenes and orthopyroxenes (Beane et al., 1986). The gneiss complex lithology (15.1%) primarily consists of quartz, biotite-chlorite and plagioclase, with the minor occurrence of amphiboles and pyroxenes (Braun et al., 2009). Most regions with Precambrian crystalline basement rocks are overlain by Tertiary sediments, laterite, and alluvium (Laluraj et al., 2007). Along the Indian coast, 57% of groundwater levels are between 0 and 10 m above the sea level and 7% of the coastal aquifers are affected by seawater intrusion (Manivannan & Elango, 2019). Major rivers discharging to the Indian coast (including Ganges, Brahmaputra, and Godavari) form extensive alluvium deposits near the coast. The Bay of Bengal receives almost 1,600 km³ of fresh river water influx annually, while the Arabian Sea only receives 300 km³ (Krishna et al., 2016). The rainfall in the Indian subcontinent is dominated by the summer monsoon (75%-80% of total rainfall), occurring during June to September. It is followed by North East monsoon (11% of total rainfall) from October to December (Lakshmi et al., 2019). The West coast of India constitutes the windward side of the Western Ghats mountains and receives maximal precipitation during the summer monsoon, while the northeast monsoon dominates on the East coast (Kumar et al., 1992; Rajeevan et al., 2012). Annual average rainfall on the East coast ranges from 300 to 2,500 mm, and on the West coast from 1,000 to 3,500 mm (CGWB, 2012). More than 50% of the land area in India is under cultivation, primarily composed of rice, wheat, sorghum, maize, sugarcane and barley (FAO, 2022; Gopalakrishnan & Kulkarni, 2007). Coastal states are known to be major producers of rice and sugarcane, with 40%-80% of the agricultural land cultivated using groundwater irrigation (Kumar et al., 2021). The coastal groundwaters are significantly impacted by fertilizer use in India. High concentrations of nitrate, for instance, were reported in Odisha, Andra Pradesh, Tamil Nadu, Karnataka, Goa and Gujarat (Kumar et al., 2021).

## 2.2. Sampling and Field Measurements

Groundwater samples were collected along the Indian coastline during the dry season in April 2017 (pre-monsoon). We sampled 86 groundwater from handpumps, open borewells and pumped borewells, mostly within a few kms from the coast. Details of the sampling locations and type of groundwater are described in Table 1. Groundwater table depths were not available as a large part of the samples consist of pumps and borewells, which are isolated from the surface. Modern borewells are reinforced using PVC as well as the casing pipes and the tubing and thus it is unlikely to be source of any Si isotope fractionation. Groundwater samples from open wells were collected using 5L Niskin bottles, and groundwater pumps were purged thoroughly before sampling to prevent sampling bias and isotopic fractionation from Si adsorption on the metal tubing. The groundwater was immediately filtered using 0.22 µm cellulose acetate filters and stored in pre-acid cleaned containers. The conductivity was measured using a conductivity meter (HI2003, accuracy ± 1%) and pH using a glass electrode (WTW, MultiLine P4). The groundwater samples are classified into different aquifer lithology based on the published aquifer system map from online Water Resources Information System (WRIS, https://indlawris.gov.in/wris) and the regional annual average precipitation data was aquired from AQUASTAT (FAO, 2022).

## 2.3. Elemental Concentration

Major cation concentrations (Na⁺, K⁺, Ca²⁺, Mg²⁺) were measured using ICP-OES iCap (Thermo Fischer Scientific) at GESE (Génie Environnemental Sols et Eaux, Sorbonne Université, Ivry Paris). Major anion concentrations were measured with an Ion Chromatograph (Metrohm COMPACT 861) at the Indo-French Cell for Water Sciences (IFCWS, Indian Institute of Science, Bengaluru, India). The accuracy of measurements was monitored using multiple certified reference materials (AnionWS, ION-96.4, ION 915, SUPER-05 and BIG MOOSE 02). The average precision was monitored using standard references and was better than 10%.

## 2.4. Dissolved Silicon and Si Isotope Analysis

Silicon concentration (DSi) was measured by spectrophotometer following the Grasshoff et al. (1999) protocol. The accuracy of measurements was assessed using the certified reference material PERADE 09 (NRC-Canada#0314). The mean value obtained from the repeated measurements of this standard (n = 25) was 110 ± 3 µM, that is, identical to the certified value of DSi = 110 ± 7 µM (NRC-Canada#0314). For the separation of silicon for isotopic analysis, we followed a slightly modified two-step pre-concentration method adapted from MAGIC (MAGnesium Co-precipitation technique, Hughes et al., 2011; Karl & Tien, 1992; Reynolds, 2006). To an initial volume of 10 ml of groundwater and surface water samples, we added 0.5 ml of 0.1 M MgCl₂ solution to match the seawater Mg concentration, following which 2% (v/v) of 1 M NaOH was added, mixed well and kept undisturbed overnight. The brucite (Mg(OH)₂) formed at high pH scavenges Si, which gets adsorbed onto
<table><caption>**Table 1**
The Sampling Details, Geochemical and Si Isotopic Composition of the Coastal Groundwater Samples</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Latitude</th><th rowspan="2">Longitude</th><th rowspan="2">Aquifer lithology</th><th rowspan="2">Type</th><th rowspan="2">pH</th><th colspan="2">Conductivity</th><th colspan="2">DSi</th><th colspan="2">δ<sup>30</sup>Si</th><th colspan="2">Ge/Si µmol/mol</th><th colspan="2">Na</th><th colspan="2">K</th><th colspan="2">Mg</th><th colspan="2">Cl</th><th colspan="2">NO3</th><th colspan="2">SO4</th></tr><tr><th>μS/cm</th><th>μM</th><th>%c</th><th>1SD</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th></tr></thead><tbody><tr><td>GJ1</td><td>20.5981</td><td>72.9424</td><td>Basalt</td><td>Hand pump</td><td>7.12</td><td>1,120</td><td>509</td><td>4.08</td><td>0.17</td><td>0.36</td><td>5,661</td><td>32</td><td>2,070</td><td>5,527</td><td>91</td><td>596</td><td>0.21</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GJ2</td><td>21.1242</td><td>72.7861</td><td>Alluvium</td><td>Pump</td><td>7.49</td><td>1,360</td><td>675</td><td>1.09</td><td>0.13</td><td>0.21</td><td>5,893</td><td>50</td><td>6,205</td><td>11,436</td><td>196</td><td>982</td><td>0.21</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GJ3</td><td>22.2507</td><td>73.1852</td><td>Alluvium</td><td>Pump</td><td>7.13</td><td>2,110</td><td>409</td><td>3.99</td><td>0.02</td><td>0.37</td><td>5,790</td><td>438</td><td>3,503</td><td>12,807</td><td>694</td><td>1,804</td><td>0.37</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GJ4</td><td>21.6102</td><td>72.2693</td><td>Alluvium</td><td>Pump</td><td>7.41</td><td>2,900</td><td>693</td><td>1.72</td><td>0.01</td><td>0.30</td><td>6,982</td><td>75</td><td>1,861</td><td>7,149</td><td>1,158</td><td>1,068</td><td>0.30</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GJ5</td><td>21.0591</td><td>71.7591</td><td>Alluvium</td><td>Pump</td><td>6.86</td><td>1,960</td><td>414</td><td>2.52</td><td>0.12</td><td>1.28</td><td>50,778</td><td>1,548</td><td>18,888</td><td>60,067</td><td>85</td><td>17,487</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GJ6</td><td>20.7613</td><td>71.0551</td><td>Limestone</td><td>Pump</td><td>7.23</td><td>8,770</td><td>427</td><td>1.69</td><td>0.06</td><td>0.23</td><td>2,843</td><td>1,146</td><td>746</td><td>2,877</td><td>870</td><td>391</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GJ7</td><td>20.8979</td><td>70.3932</td><td>Limestone</td><td>Borewell</td><td>7.10</td><td>2,210</td><td>714</td><td>0.35</td><td>0.01</td><td>0.10</td><td>2,307</td><td>145</td><td>511</td><td>3,496</td><td>9</td><td>530</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GJ8</td><td>21.6439</td><td>69.6032</td><td>Limestone</td><td>Borewell</td><td>7.04</td><td>1,430</td><td>329</td><td>0.88</td><td>0.10</td><td>0.31</td><td>4,187</td><td>149</td><td>1,031</td><td>4,641</td><td>36</td><td>338</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GJ9</td><td>22.2399</td><td>68.9323</td><td>Alluvium</td><td>Pump</td><td>7.32</td><td>4,750</td><td>443</td><td>0.62</td><td>0.20</td><td>0.13</td><td>30,717</td><td>3,077</td><td>3,518</td><td>32,952</td><td>1,634</td><td>2,310</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GJ10</td><td>22.4717</td><td>70.0786</td><td>Basalt</td><td>Pump</td><td>7.29</td><td>1,040</td><td>738</td><td>0.50</td><td>0.18</td><td>0.36</td><td>4,831</td><td>416</td><td>799</td><td>2,598</td><td>1,222</td><td>412</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>AP1</td><td>18.9408</td><td>84.5648</td><td>Gneiss</td><td>Pump</td><td>7.26</td><td>983</td><td>1,144</td><td>0.59</td><td>0.02</td><td>0.23</td><td>2,843</td><td>1,146</td><td>746</td><td>2,877</td><td>870</td><td>391</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>AP3</td><td>17.3167</td><td>82.5695</td><td>Gneiss</td><td>Pump</td><td>6.47</td><td>730</td><td>516</td><td>1.93</td><td>0.21</td><td>0.10</td><td>2,307</td><td>145</td><td>511</td><td>3,496</td><td>9</td><td>530</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>AP4</td><td>16.9569</td><td>82.2552</td><td>Alluvium</td><td>Pump</td><td>7.02</td><td>1,140</td><td>690</td><td>0.48</td><td>0.08</td><td>0.10</td><td>4,187</td><td>149</td><td>1,031</td><td>4,641</td><td>36</td><td>338</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>AP5</td><td>16.3966</td><td>81.8095</td><td>Alluvium</td><td>Borewell</td><td>6.90</td><td>260</td><td>274</td><td>1.68</td><td>0.15</td><td>0.13</td><td>30,717</td><td>3,077</td><td>3,518</td><td>32,952</td><td>1,634</td><td>2,310</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>AP6</td><td>16.1777</td><td>81.1385</td><td>Alluvium</td><td>Pump</td><td>7.33</td><td>5,310</td><td>150</td><td>0.38</td><td>0.13</td><td>0.12</td><td>64,200</td><td>1,076</td><td>1,545</td><td>53,965</td><td>189</td><td>1,501</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>AP7</td><td>15.7809</td><td>80.3801</td><td>Alluvium</td><td>Borewell</td><td>7.79</td><td>6,920</td><td>381</td><td>0.94</td><td>0.07</td><td>0.70</td><td>29,491</td><td>7,375</td><td>4,783</td><td>43,049</td><td>6,876</td><td>3,271</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>AP8</td><td>15.5046</td><td>80.0365</td><td>Alluvium</td><td>Hand pump</td><td>7.28</td><td>6,010</td><td>578</td><td>0.94</td><td>0.07</td><td>0.24</td><td>29,426</td><td>265</td><td>83</td><td>7,561</td><td>1,407</td><td>1,917</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>AP9</td><td>14.2971</td><td>80.0904</td><td>Alluvium</td><td>Pump</td><td>7.55</td><td>2,100</td><td>837</td><td>3.14</td><td>0.29</td><td>0.24</td><td>29,426</td><td>265</td><td>83</td><td>7,561</td><td>1,407</td><td>1,917</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>AP10</td><td>13.5888</td><td>80.0343</td><td>Laterite</td><td>Pump</td><td>7.07</td><td>1,500</td><td>629</td><td>0.69</td><td>0.01</td><td>0.58</td><td>9,224</td><td>315</td><td>1,236</td><td>6,960</td><td>1,175</td><td>2,184</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GA1</td><td>14.9111</td><td>74.0816</td><td>Granite</td><td>Hand pump</td><td>6.65</td><td>460</td><td>523</td><td>1.1</td><td>0.0</td><td>0.55</td><td>806</td><td>72</td><td>750</td><td>1,027</td><td>54</td><td>112</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GA2</td><td>14.9592</td><td>74.0551</td><td>Granite</td><td>Hand pump</td><td>6.66</td><td>4,480</td><td>246</td><td>1.2</td><td>0.0</td><td>0.52</td><td>29,870</td><td>218</td><td>3,038</td><td>43,312</td><td>59</td><td>1,176</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GA3</td><td>15.0021</td><td>74.0786</td><td>Granite</td><td>Borewell</td><td>5.75</td><td>130</td><td>218</td><td>0.6</td><td>0.2</td><td>0.57</td><td>627</td><td>43</td><td>84</td><td>642</td><td>95</td><td>17</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GA4</td><td>15.0421</td><td>73.9991</td><td>Granite</td><td>Hand pump</td><td>5.37</td><td>150</td><td>266</td><td>1.1</td><td>0.1</td><td>0.62</td><td>645</td><td>76</td><td>135</td><td>754</td><td>155</td><td>64</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GA5</td><td>15.1261</td><td>73.9495</td><td>Gneiss</td><td>Borewell</td><td>5.98</td><td>182</td><td>86</td><td>0.6</td><td>0.1</td><td>0.13</td><td>30,717</td><td>3,077</td><td>3,518</td><td>32,952</td><td>1,634</td><td>2,310</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GA6</td><td>15.2068</td><td>73.9504</td><td>Gneiss</td><td>Hand pump</td><td>6.41</td><td>430</td><td>91</td><td>0.5</td><td>0.1</td><td>0.13</td><td>1,248</td><td>106</td><td>260</td><td>733</td><td>154</td><td>308</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>GA7</td><td>15.7103</td><td>73.7257</td><td>Laterite</td><td>Borewell</td><td>6.37</td><td>1,541</td><td>209</td><td>0.7</td><td>0.2</td><td>0.87</td><td>7,319</td><td>127</td><td>1,860</td><td>14,613</td><td>21</td><td>584</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA1</td><td>12.7884</td><td>74.8593</td><td>Gneiss</td><td>Pump</td><td>5.05</td><td>520</td><td>168</td><td>0.3</td><td>0.1</td><td>0.13</td><td>2,671</td><td>321</td><td>341</td><td>2,962</td><td>1,429</td><td>38</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA2</td><td>12.8748</td><td>74.8292</td><td>Gneiss</td><td>Pump</td><td>5.98</td><td>260</td><td>291</td><td>1.0</td><td>0.1</td><td>0.50</td><td>843</td><td>470</td><td>142</td><td>590</td><td>306</td><td>288</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA3</td><td>13.2228</td><td>74.7845</td><td>Gneiss</td><td>Pump</td><td>5.60</td><td>310</td><td>158</td><td>-0.2</td><td>0.0</td><td>0.40</td><td>664</td><td>107</td><td>240</td><td>464</td><td>941</td><td>234</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA4</td><td>13.3889</td><td>74.7431</td><td>Gneiss</td><td>Pump</td><td>5.33</td><td>150</td><td>233</td><td>1.3</td><td>0.1</td><td>0.94</td><td>556</td><td>30</td><td>61</td><td>592</td><td>129</td><td>14</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA5</td><td>13.6189</td><td>74.6926</td><td>Alluvium</td><td>Pump</td><td>6.71</td><td>510</td><td>147</td><td>0.1</td><td>0.1</td><td>0.94</td><td>1,535</td><td>476</td><td>244</td><td>1,656</td><td>728</td><td>242</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA6</td><td>13.8183</td><td>74.6343</td><td>Gneiss</td><td>Pump</td><td>5.62</td><td>190</td><td>128</td><td>-0.1</td><td>0.1</td><td>0.11</td><td>618</td><td>15</td><td>107</td><td>897</td><td>241</td><td>102</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA7</td><td>14.0271</td><td>74.5291</td><td>Laterite</td><td>Borewell</td><td>4.79</td><td>170</td><td>87</td><td>0.8</td><td>0.2</td><td>0.20</td><td>817</td><td>166</td><td>75</td><td>1,212</td><td>138</td><td>32</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA8</td><td>14.2417</td><td>74.4468</td><td>Laterite</td><td>Pump</td><td>6.10</td><td>190</td><td>779</td><td>0.9</td><td>0.1</td><td>0.12</td><td>356</td><td>79</td><td>516</td><td>256</td><td>2</td><td>23</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA9</td><td>14.4308</td><td>74.4231</td><td>Alluvium</td><td>Pump</td><td>5.83</td><td>99</td><td>171</td><td>1.4</td><td>0.1</td><td>0.88</td><td>375</td><td>54</td><td>91</td><td>343</td><td>9</td><td>35</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KA10</td><td>14.6572</td><td>74.3121</td><td>Laterite</td><td>Pump</td><td>6.65</td><td>970</td><td>190</td><td>1.4</td><td>0.1</td><td>0.11</td><td>3,394</td><td>141</td><td>1,163</td><td>6,326</td><td>421</td><td>560</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KL1</td><td>8.3945</td><td>77.0885</td><td>Gneiss</td><td>Pump</td><td>6.02</td><td>334</td><td>140</td><td>-0.01</td><td>0.05</td><td>0.65</td><td>1,448</td><td>275</td><td>152</td><td>1,701</td><td>383</td><td>239</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KL2</td><td>8.8634</td><td>76.6935</td><td>Gneiss</td><td>Pump</td><td>5.63</td><td>150</td><td>199</td><td>0.11</td><td>0.14</td><td>0.68</td><td>685</td><td>70</td><td>66</td><td>695</td><td>150</td><td>61</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KL3</td><td>9.3183</td><td>76.4217</td><td>Alluvium</td><td>Pump</td><td>5.67</td><td>478</td><td>314</td><td>1.25</td><td>0.05</td><td>0.64</td><td>2,100</td><td>205</td><td>541</td><td>2,277</td><td>20</td><td>579</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KL4</td><td>9.9994</td><td>76.2929</td><td>Alluvium</td><td>Pump</td><td>5.96</td><td>92</td><td>119</td><td>1.04</td><td>0.10</td><td>0.12</td><td>291</td><td>87</td><td>72</td><td>365</td><td>7</td><td>40</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr><tr><td>KL5</td><td>10.4192</td><td>76.1052</td><td>Alluvium</td><td>Borewell</td><td>7.05</td><td>480</td><td>337</td><td>-0.43</td><td>0.04</td><td>0.90</td><td>730</td><td>373</td><td>186</td><td>683</td><td>259</td><td>312</td><td>0.40</td><td>11,820</td><td>40</td><td>313</td><td>1,722</td><td>364</td><td>461</td></tr></tbody></table>
<table><caption>**Table 1**
*Continued*</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Latitude</th><th rowspan="2">Longitude</th><th rowspan="2">Aquifer lithology</th><th rowspan="2">Type</th><th rowspan="2">pH</th><th colspan="2">Conductivity</th><th colspan="2">DSi</th><th colspan="2">δ<sup>30</sup>Si</th><th colspan="2">Ge/Si</th><th colspan="2">Na</th><th colspan="2">K</th><th colspan="2">Mg</th><th colspan="2">Cl</th><th colspan="2">NO3</th><th colspan="2">SO4</th></tr><tr><th>μS/cm</th><th>μM</th><th>%c</th><th>1SD</th><th>μmol/mol</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th></tr></thead><tbody><tr><td>KL6</td><td>10.9248</td><td>75.9139</td><td>Alluvium</td><td>Pump</td><td>6.15</td><td>150</td><td>78</td><td>0.57</td><td>0.07</td><td></td><td></td><td></td><td>1.75</td><td>478</td><td>70</td><td>64</td><td>404</td><td>29</td><td>250</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>KL7</td><td>11.4696</td><td>75.6548</td><td>Alluvium</td><td>Borewell</td><td>5.88</td><td>140</td><td>136</td><td>-0.10</td><td>0.10</td><td></td><td></td><td></td><td>3.82</td><td>236</td><td>90</td><td>50</td><td>411</td><td>28</td><td>82</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>KL8</td><td>11.8847</td><td>75.3725</td><td>Laterite</td><td>Pump</td><td>6.33</td><td>180</td><td>170</td><td>1.29</td><td>0.07</td><td></td><td></td><td></td><td>1.15</td><td>665</td><td>146</td><td>147</td><td>802</td><td>423</td><td>155</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>KL9</td><td>12.2428</td><td>75.1449</td><td>Alluvium</td><td>Pump</td><td>6.42</td><td>260</td><td>180</td><td>0.38</td><td>0.24</td><td></td><td></td><td></td><td>1.09</td><td>908</td><td>143</td><td>175</td><td>834</td><td>104</td><td>287</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>KL10</td><td>12.6821</td><td>74.9032</td><td>Alluvium</td><td>Pump</td><td>5.63</td><td>140</td><td>204</td><td>0.42</td><td>0.28</td><td></td><td></td><td></td><td>0.80</td><td>610</td><td>51</td><td>74</td><td>655</td><td>1</td><td>37</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH1</td><td>15.7243</td><td>73.6882</td><td>Schist</td><td>Hand pump</td><td>6.32</td><td>200</td><td>94</td><td>0.68</td><td>0.02</td><td></td><td></td><td></td><td>2.03</td><td>376</td><td>66</td><td>80</td><td>518</td><td>14</td><td>50</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH2</td><td>16.1077</td><td>73.4725</td><td>Quartzite</td><td>Pump</td><td>5.60</td><td>1,457</td><td>527</td><td>0.18</td><td>0.04</td><td></td><td></td><td></td><td>2.16</td><td>259</td><td>39</td><td>581</td><td>308</td><td>22</td><td>9,713</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH3</td><td>16.4731</td><td>73.3943</td><td>Basalt</td><td>Pump</td><td>4.87</td><td>200</td><td>445</td><td>0.63</td><td>0.17</td><td></td><td></td><td></td><td>0.99</td><td>309</td><td>17</td><td>103</td><td>409</td><td>36</td><td>32</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH4</td><td>16.9938</td><td>73.317</td><td>Laterite</td><td>Pump</td><td>6.22</td><td>280</td><td>100</td><td>0.52</td><td>0.09</td><td></td><td></td><td></td><td>4.10</td><td>706</td><td>85</td><td>155</td><td>747</td><td>203</td><td>261</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH5</td><td>17.5015</td><td>73.1855</td><td>Laterite</td><td>Pump</td><td>5.96</td><td>160</td><td>236</td><td>1.00</td><td>0.06</td><td></td><td></td><td></td><td>0.76</td><td>372</td><td>0</td><td>143</td><td>372</td><td>4</td><td>39</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH6</td><td>17.9758</td><td>73.0918</td><td>Basalt</td><td>Borewell</td><td>6.27</td><td>230</td><td>813</td><td>0.66</td><td>0.02</td><td></td><td></td><td></td><td>0.45</td><td>732</td><td>166</td><td>311</td><td>553</td><td>227</td><td>51</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH7</td><td>18.6471</td><td>72.8783</td><td>Basalt</td><td>Hand pump</td><td>7.33</td><td>2,270</td><td>520</td><td>0.99</td><td>0.00</td><td></td><td></td><td></td><td>0.28</td><td>21,078</td><td>253</td><td>1,759</td><td>17,624</td><td>14</td><td>1,828</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH8</td><td>19.3753</td><td>72.8275</td><td>Alluvium</td><td>Pump</td><td>7.14</td><td>1,250</td><td>884</td><td>0.86</td><td>0.05</td><td></td><td></td><td></td><td>0.17</td><td>7,363</td><td>210</td><td>1,278</td><td>4,439</td><td>74</td><td>582</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH9</td><td>19.9562</td><td>72.8231</td><td>Basalt</td><td>Pump</td><td>6.82</td><td>410</td><td>1,246</td><td>1.26</td><td>0.11</td><td></td><td></td><td></td><td>0.58</td><td>690</td><td>41</td><td>852</td><td>388</td><td>7</td><td>99</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MH10</td><td>20.1356</td><td>72.8065</td><td>Basalt</td><td>Pump</td><td>6.79</td><td>560</td><td>1,070</td><td>1.05</td><td>0.06</td><td></td><td></td><td></td><td>0.79</td><td>1,053</td><td>0</td><td>769</td><td>892</td><td>32</td><td>188</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD1</td><td>21.6701</td><td>87.2859</td><td>Alluvium</td><td>Borewell</td><td>7.26</td><td>840</td><td>983</td><td>-0.15</td><td>0.09</td><td></td><td></td><td></td><td>1.02</td><td>3,386</td><td>96</td><td>758</td><td>259</td><td>31</td><td>135</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD2</td><td>21.3413</td><td>86.7631</td><td>Alluvium</td><td>Borewell</td><td>7.03</td><td>570</td><td>752</td><td>-0.04</td><td>0.19</td><td></td><td></td><td></td><td>2.68</td><td>1,657</td><td>31</td><td>872</td><td>743</td><td>64</td><td>57</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD3</td><td>20.8119</td><td>86.7609</td><td>Alluvium</td><td>Pump</td><td>7.25</td><td>1,030</td><td>565</td><td>-1.07</td><td>0.10</td><td></td><td></td><td></td><td>3.25</td><td>5,726</td><td>155</td><td>913</td><td>3,947</td><td>60</td><td>316</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD4</td><td>20.5014</td><td>86.3277</td><td>Alluvium</td><td>Pump</td><td>7.16</td><td>715</td><td>407</td><td>-0.55</td><td>0.08</td><td></td><td></td><td></td><td>0.82</td><td>4,190</td><td>242</td><td>570</td><td>2,831</td><td>4</td><td>78</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD5</td><td>20.0989</td><td>86.1886</td><td>Alluvium</td><td>Pump</td><td>6.17</td><td>650</td><td>687</td><td>0.47</td><td>0.14</td><td></td><td></td><td></td><td>1.46</td><td>2,966</td><td>271</td><td>575</td><td>2,347</td><td>6</td><td>218</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD6</td><td>19.8422</td><td>85.9032</td><td>Alluvium</td><td>Borewell</td><td>6.85</td><td>830</td><td>460</td><td>0.40</td><td>0.20</td><td></td><td></td><td></td><td>2.07</td><td>2,750</td><td>202</td><td>903</td><td>2,504</td><td>10</td><td>135</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD7</td><td>19.9016</td><td>85.3656</td><td>Laterite</td><td>Pump</td><td>6.29</td><td>240</td><td>963</td><td>0.12</td><td>0.01</td><td></td><td></td><td></td><td>2.31</td><td>770</td><td>139</td><td>171</td><td>364</td><td>60</td><td>44</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD8</td><td>19.5972</td><td>85.1211</td><td>Alluvium</td><td>Pump</td><td>6.37</td><td>665</td><td>1,354</td><td>0.54</td><td>0.10</td><td></td><td></td><td></td><td>0.83</td><td>1,434</td><td>654</td><td>608</td><td>2,935</td><td>446</td><td>212</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD9</td><td>19.3503</td><td>84.9882</td><td>Alluvium</td><td>Borewell</td><td>7.07</td><td>876</td><td>350</td><td>0.82</td><td>0.11</td><td></td><td></td><td></td><td>0.76</td><td>4,398</td><td>44</td><td>1,310</td><td>3,783</td><td>3</td><td>1,058</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>OD10</td><td>19.1611</td><td>84.7112</td><td>Alluvium</td><td>Hand pump</td><td>6.89</td><td>5,830</td><td>680</td><td>0.72</td><td>0.27</td><td></td><td></td><td></td><td>1.19</td><td>20,426</td><td>12,808</td><td>7,858</td><td>37,207</td><td>2,564</td><td>4,963</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN1</td><td>12.9399</td><td>80.2401</td><td>Alluvium</td><td>Pump</td><td>7.16</td><td>5,420</td><td>440</td><td>0.74</td><td>0.2</td><td></td><td></td><td></td><td>1.12</td><td>34,904</td><td>892</td><td>5,186</td><td>40,760</td><td>65</td><td>4,926</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN2</td><td>12.3821</td><td>80.0894</td><td>Alluvium</td><td>Pump</td><td>6.31</td><td>280</td><td>319</td><td>1.32</td><td>0.1</td><td></td><td></td><td></td><td>0.73</td><td>793</td><td>61</td><td>401</td><td>1,082</td><td>245</td><td>157</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN3</td><td>11.7479</td><td>79.7687</td><td>Alluvium</td><td>Pump</td><td>6.85</td><td>2,090</td><td>775</td><td>1.88</td><td>0.1</td><td></td><td></td><td></td><td>0.13</td><td>9,890</td><td>832</td><td>2,143</td><td>9,685</td><td>8</td><td>1,937</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN4</td><td>11.0311</td><td>79.8503</td><td>Alluvium</td><td>Borewell</td><td>7.25</td><td>2,280</td><td>292</td><td>1.50</td><td>0.2</td><td></td><td></td><td></td><td>0.88</td><td>14,810</td><td>1,073</td><td>1,740</td><td>14,114</td><td>28</td><td>1,927</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN5</td><td>10.3765</td><td>79.8497</td><td>Alluvium</td><td>Pump</td><td>7.15</td><td>2,750</td><td>670</td><td>1.19</td><td>0.0</td><td></td><td></td><td></td><td>0.54</td><td>10,826</td><td>2,765</td><td>2,499</td><td>11,656</td><td>5,196</td><td>2,049</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN6</td><td>10.0388</td><td>79.2333</td><td>Alluvium</td><td>Pump</td><td>7.09</td><td>810</td><td>382</td><td>0.65</td><td>0.0</td><td></td><td></td><td></td><td>1.26</td><td>3,629</td><td>194</td><td>623</td><td>3,694</td><td>775</td><td>425</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN7</td><td>9.3316</td><td>78.9741</td><td>Alluvium</td><td>Hand pump</td><td>7.20</td><td>1,370</td><td>509</td><td>0.70</td><td>0.1</td><td></td><td></td><td></td><td>0.57</td><td>7,161</td><td>213</td><td>865</td><td>6,412</td><td>584</td><td>520</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN8</td><td>9.2353</td><td>78.7875</td><td>Alluvium</td><td>Pump</td><td>7.36</td><td>5,820</td><td>590</td><td>4.29</td><td>0.1</td><td></td><td></td><td></td><td>0.08</td><td>40,161</td><td>4,712</td><td>3,510</td><td>41,994</td><td>3,599</td><td>4,000</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN9</td><td>8.7571</td><td>78.1566</td><td>Alluvium</td><td>Borewell</td><td>7.10</td><td>1,100</td><td>837</td><td>3.26</td><td>0.0</td><td></td><td></td><td></td><td>2,903</td><td>1,931</td><td>1,995</td><td>2,503</td><td>4</td><td>1,080</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TN10</td><td>8.0885</td><td>77.5493</td><td>Alluvium</td><td>Hand pump</td><td>7.08</td><td>1,760</td><td>1,072</td><td>0.94</td><td>0.0</td><td></td><td></td><td></td><td>0.40</td><td>8,684</td><td>795</td><td>1,095</td><td>11,872</td><td>2,632</td><td>882</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>WB1</td><td>22.1889</td><td>88.7132</td><td>Alluvium</td><td>Pump</td><td>7.30</td><td>1,042</td><td>422</td><td>-0.66</td><td>0.17</td><td></td><td></td><td></td><td>0.00</td><td>8,752</td><td>155</td><td>498</td><td>2,087</td><td>119</td><td>305</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>WB2</td><td>22.2271</td><td>88.4461</td><td>Alluvium</td><td>Pump</td><td>7.31</td><td>1,100</td><td>473</td><td>-0.51</td><td>0.05</td><td></td><td></td><td></td><td>2.36</td><td>6,603</td><td>167</td><td>1,262</td><td>2,828</td><td>42</td><td>784</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>WB3</td><td>22.2181</td><td>88.0949</td><td>Alluvium</td><td>Pump</td><td>7.21</td><td>1,250</td><td>738</td><td>-0.49</td><td>0.06</td><td></td><td></td><td></td><td>0.66</td><td>5,943</td><td>102</td><td>854</td><td>3,885</td><td>24</td><td>544</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>WB4</td><td>22.4632</td><td>87.9733</td><td>Alluvium</td><td>Pump</td><td>7.03</td><td>1,350</td><td>830</td><td>-0.37</td><td>0.02</td><td></td><td></td><td></td><td>1.28</td><td>9,000</td><td>156</td><td>677</td><td>5,635</td><td>7</td><td>243</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>WB5</td><td>22.0428</td><td>88.0657</td><td>Alluvium</td><td>Borewell</td><td>7.49</td><td>1,600</td><td>602</td><td>-0.35</td><td>0.05</td><td></td><td></td><td></td><td>1.72</td><td>9,318</td><td>83</td><td>1,053</td><td>9,841</td><td>14</td><td>351</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>WB6</td><td>22.0129</td><td>87.8173</td><td>Alluvium</td><td>Pump</td><td>7.12</td><td>1,500</td><td>711</td><td>-0.21</td><td>0.15</td><td></td><td></td><td></td><td>1.68</td><td>6,256</td><td>131</td><td>1,628</td><td>7,573</td><td>41</td><td>482</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>**Table 1**
*Continued*</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Latitude</th><th rowspan="2">Longitude</th><th rowspan="2">Aquifer lithology</th><th rowspan="2">Type</th><th rowspan="2">pH</th><th colspan="2">Conductivity</th><th colspan="2">DSi</th><th colspan="2">δ<sup>30</sup>Si</th><th colspan="2">Ge/Si</th><th colspan="2">Na</th><th colspan="2">K</th><th colspan="2">Mg</th><th colspan="2">Cl</th><th colspan="2">NO3</th><th colspan="2">SO4</th></tr><tr><th>μS/cm</th><th>μM</th><th>%c</th><th>1SD</th><th>μmol/mol</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th><th>μM</th></tr></thead><tbody><tr><td>WB7</td><td>21.8712</td><td>87.7571</td><td>Alluvium</td><td>Pump</td><td>7.03</td><td>1,647</td><td>639</td><td>-0.31</td><td>0.14</td><td></td><td></td><td></td><td>2.81</td><td>6,777</td><td>162</td><td>2,810</td><td>12,548</td><td>18</td><td>423</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>WB8</td><td>21.7852</td><td>87.7441</td><td>Alluvium</td><td>Pump</td><td>7.35</td><td>3,680</td><td>290</td><td>4.48</td><td>0.02</td><td></td><td></td><td></td><td>0.18</td><td>26,622</td><td>1,203</td><td>3,093</td><td>20,186</td><td>129</td><td>3,993</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>WB9</td><td>21.7205</td><td>87.6637</td><td>Alluvium</td><td>Pump</td><td>6.91</td><td>610</td><td>1,085</td><td>-0.84</td><td>0.10</td><td></td><td></td><td></td><td>0.31</td><td>3,005</td><td>114</td><td>1,077</td><td>2,247</td><td>7</td><td>101</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>WB10</td><td>21.6234</td><td>87.5206</td><td>Laterite</td><td>Borewell</td><td>7.26</td><td>923</td><td>365</td><td>1.59</td><td>0.02</td><td></td><td></td><td></td><td>0.61</td><td>4,364</td><td>338</td><td>2,044</td><td>2,931</td><td>164</td><td>1,182</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

precipitated brucite. After centrifugation and recovery of the precipitate, the supernatant from the first step was again subjected to a second addition of 1% NaOH, and the additional precipitate was recovered by centrifugation after 1–2 hr. Both brucite precipitates were dissolved with 1M HCl and merged. The DSi concentration of the supernatant was analyzed by spectrophotometer and found negligible (on average <2% of initial Si), confirming that all the Si was adsorbed onto the brucite precipitate.

Separation of Si from other cations was carried out using cation exchange resin (BioRad DOWEX 50W-X12, 200–400 mesh, in H⁺ form) following the procedure of Georg et al. (2006a). After the purification process, an aliquot was analyzed for determining the concentration of major elements (Na, Mg, Ca, K, DSi) by ICP-MS (Agilent 7500a) to ensure that the Si/X (Where X = Na, K, Ca or Mg) weight ratio was always >50 to minimize the matrix effects in the plasma. The organic matrix was removed by treatment with 50 µL of 30% H₂O₂ since it has been shown to potentially induce a matrix bias during MC-ICP-MS analyses (Hughes et al., 2011). The remaining potential matrix effect from Cl⁻, NO₃⁻ and SO₄²⁻ was corrected by the artificial doping of anions (Merck, Suprapur grade H₂SO₄, HNO₃ and HCl) in all samples and standards (Hughes et al., 2011). The measurements were performed on MC-ICP-MS (Thermo Neptune+, LSCE-IPSL, Gif-sur-Yvette) in dry plasma mode with Mg doping, as described by Closset et al. (2016). Samples were injected into the plasma with an ApexΩ HF desolvating nebulization system connected with a PFA nebulizer (100 µL/min uptake rate). The δ³⁰Si values were expressed as the permil deviation of the ³⁰Si/²⁸Si ratio from the quartz standard NBS28 (RM8546), expressed as

$$ \delta^{30}\text{Si}_{\text{sample}} = \left[ \left( \frac{^{30}\text{Si}/^{28}\text{Si}}{\text{sample}} \right) - \left( \frac{^{30}\text{Si}/^{28}\text{Si}}{\text{NBS28}} \right) \right] * 1000 \quad (1) $$

Blank levels were below 1% of the main signal and were subtracted from each sample and standard analysis. The analysis was carried out in medium-resolution mode (m/Δm ~ 6,000) to optimize the separation of the ³⁰Si peak and ¹⁴N¹⁶O interference and was performed on the interference-free left side of the flat peak (Abraham et al., 2008). The δ²⁹Si and δ³⁰Si values were compared to the mass-dependent fractionation line, and samples falling outside of its analytical error were excluded from the final data set (Figure S1 in Supporting Information S1). Typical analytical conditions are provided in Table S1 of Supporting Information S1. Laboratory protocol has been intercalibrated for seawater as part of the GEOTRACES intercalibration exercise generated over the entire procedure (Grasse et al., 2017). The analytical precision and accuracy were monitored by the long-term measurements of a secondary reference material, Diatomite, whose average δ³⁰Si value of 1.19 ± 0.18‰ (2SD, n = 100) was in agreement with the intercalibration of 1.26 ± 0.2‰ (Reynolds et al., 2007). All the samples measured in the present study are chemically replicated at least once and 12 samples were chemically triplicated with an average 2SD of ±0.19 ‰.

## 2.5. Germanium Analysis

The Ge concentrations in groundwater samples were measured by isotope dilution using a ⁷³Ge spike (Neonest AB®) added to approximately 10 ml of samples before a two-step pre-concentration procedure adapted according to the MAGIC co-precipitation technique (Karl & Tien, 1992; Reynolds, 2006). The Ge separation and purification were achieved using the BioRad cation exchange resin DOWEX 50W-X12 (200–400 mesh). The analyses were performed on a high-resolution inductively coupled double-focusing plasma mass spectrometer (Element XR, Thermo Fischer Scientific®) at Géosciences Environnement Toulouse (Toulouse, France). To improve the
sensitivity of the analyses and to reduce the matrix interferences, a CETAC Aridus II desolvating nebulizer has been coupled to the HR-ICP-MS. A bracketing standard was run for every five samples to correct for the instrumental drift during the analysis. All procedure blanks analyzed were on average <0.02 nM (n = 4), and were subtracted from each sample. The signal intensity measured for the ⁷²Ge and ⁷³Ge isotopes in the samples were respectively 40 and 50 times higher compared to the limit detection of the instrument. The Ge concentration in selected supernatant solutions (n = 14) after MAGIC co-precipitation were below detection limit of the instrument, ensuring complete Ge recovery. The accuracy of Ge measurement were determined by repeated analyses of the reference material SLRS-6 (NRC-Canada), giving Ge concentration of 0.088 ± 0.016 nM (2SD, n = 5). This was in accordance with published values measured by HR-ICP-MS (Yeghicheyan et al., 2019, [Ge] = 0.094 ± 0.010 nM, n = 15) and with unpublished values analyzed by isotope dilution using a ⁷⁰Ge spike at GET (0.090 ± 0.002 nM, n = 4).

## 2.6. Modeling Si Isotopic Evolution in Groundwater

Once solubilized by the silicate mineral weathering reactions, the DSi is partitioned into secondary clays, taken up by plants and/or adsorbed onto Fe oxy-hydroxides. Depending on the nature and the intensity of the partitioning processes, the isotopic signature of DSi will evolve between two theoretical frameworks: (a) a Rayleigh model, where precipitated solids, adsorbed phase, or taken-up fraction do not further exchange with the remaining solution (Equation 2), (b) a steady-state open flow through batch reactor model (here onwards steady-state model) where supply of fresh Si into the system is balanced by precipitation and/or uptake and/or adsorption (Equation 3) (Bouchez et al., 2013; Frings et al., 2015).

$$ \text{Rayleigh : } \delta^{30}\text{Si}_{\text{GW}} = \delta^{30}\text{Si}_{\text{o}} + ^{30}\epsilon * \ln(f_{\text{Si}}) \quad (2) $$

$$ \text{Steady state : } \delta^{30}\text{Si}_{\text{GW}} = \delta^{30}\text{Si}_{\text{o}} - ^{30}\epsilon * (1 - f_{\text{Si}}) \quad (3) $$

The δ³⁰SiGW value denotes the measured δ³⁰Si values of groundwater, and δ³⁰Si₀ denotes the initial δ³⁰Si, which is the upper continental crust (UCC) composition of -0.25 ± 0.1‰ (Savage et al., 2013). The published δ³⁰Si values of granitic gneiss (-0.3 ± 0.1‰, Riotte, Meunier, et al., 2018) and Deccan basalts (-0.1 ± 0.1‰ Wille et al., 2018) in Indian subcontinent confirms the narrow δ³⁰Si variability in bedrock composition. The quantity δ³⁰ε, also known as isotope fractionation, is defined as ³⁰ε = α-1 and expressed in per mil (‰) (Coplen, 2011). Here α is the isotopic fractionation factor associated with secondary mineral precipitation, adsorption or plant uptake. The fSi in Equations 2 and 3 indicates the fraction of Si remaining in the dissolved phase, estimated by normalizing it with a conservative element (in our case Na) in solution and bedrock and estimated as:

$$ f_{\text{Si}} = (\text{DSi/Na}^*)_{\text{groundwater}}/(\text{Si/Na})_{\text{UCC}} \quad (4) $$

We assume the stoichiometric release of Si and Na from the bedrock as a result of silicate weathering and preferential loss of Si from solution by incorporation into secondary phases (Georg et al., 2006b; Hughes et al., 2013; Wedepohl, 1995), while Na is only marginally incorporated and remains in solution. The fSi values reflect the proportion of Si incorporated into the secondary phases. A value of 0 implies complete removal, whereas a value of 1 indicates that Si remains entirely in solution. The “*” denotes the atmospheric/seawater correction to discard any contribution from atmospheric deposits and seawater mixing using Cl concentration as described in Gaillardet et al. (1999);

$$ [\text{Na}^*] = [\text{Na}]_{\text{measured}} - (\text{Na/Cl})_{\text{rain}} * [\text{Cl}]_{\text{measured}} \quad (5) $$

here (Na/Cl)rain is the elemental seawater ratio and [Na]measured and [Cl]measured are measured concentration in groundwater (Mondal et al., 2011). Further interpretation relies on two assumptions: (a) isotopic fractionation during mineral dissolution is considered transient and negligible as weathering progresses over time, (b) the UCC elemental and δ³⁰Si signature is considered as a reasonable approximation for the signature of primary silicates (Frings, Oelze, et al., 2021; Geilert et al., 2014; Ziegler et al., 2005). However, the calculated fSi values are likely to be an overestimate, given that quartz correction has not been applied for the UCC composition.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** The schematic map of the Indian subcontinent shows the sampling location of 86 coastal groundwater samples and the dissolved silicon (DSi, µM), δ³⁰Si (‰), and Ge/Si (µmol/mol) ranges.</figcaption></figure>

## 3. Results

### 3.1. Coastal Groundwater Chemical Composition

The sampling details of elemental and isotopic composition are given in Table 1. The electrical conductivity (EC) of coastal groundwater samples ranged from 92 to 8,770 µS/cm, and an average pH was 6.7 ± 0.7 (Figure S2 in Supporting Information S1). High conductivity can be indicative of seawater intrusion, high evapotranspiration and/or excessive contribution from fertilizers. We identified 10 coastal groundwaters in India with EC > 3,000 µS/cm and Cl concentration >20 mM that can be considered as brackish groundwater (Jiao & Post, 2019). The fraction of seawater in groundwater was estimated using two endmember mixing based on Cl concentration: (a) coastal Bay of Bengal seawater as saline endmember (Jiao & Post, 2019; Mondal et al., 2011) and (b) average fresh groundwater from the present study as fresh endmember. The estimated fraction of seawater in coastal groundwater ranges from 3% to 10%. Based on the measured elemental concentration, Na was identified as the dominant cation in coastal groundwater for most samples, followed by Mg, K and Ca. The atmospheric correction was not applicable in few groundwater as a result of significant Cl contribution from fertilizers, such as potash (KCl), which would significantly overcorrect the Na concentration (Buvaneshwari et al., 2020). The concentration of anions in groundwater follows the order $[Cl^-] > [SO_4^{2-}] > [NO_3^-]$ (Table 1).

### 3.2. Dissolved Silicon and Silicon Isotopes

The dissolved silicon (DSi) concentration in coastal groundwater samples exhibits large spatial variability, ranging from 78 to 1,354 µM, with an average of 484 ± 304 µM (±1SD, *n* = 86). The groundwater DSi values were comparable to reported global coastal fresh groundwater DSi values from extrusive igneous (604 ± 192 µM) and complex lithologies (288 ± 245 µM) (Rahman et al., 2019). The δ³⁰Si values of coastal groundwater ranged from -1.1 to 4.5‰, with an average composition of 0.8 ± 1.1‰ (*n* = 85, ±1SD), close to the reported global δ³⁰Si value of groundwater (0.8 ± 0.8‰, *n* = 117; Frings et al., 2016, Ehlert, Reckhardt, et al., 2016; Ehlert, Doering, et al., 2016; Martin et al., 2021, Kavil et al., 2024) and more heterogenous (Figure 2). The Indian coastal groundwater also exhibits higher DSi compared to Indian rivers (176 ± 101 µM, ±1SD, *n* = 168) and lower δ³⁰Si values (1.5 ± 0.6‰, ±1SD, *n* = 152) (Ganges: Fontorbe et al., 2013; Frings et al., 2015; Cauvery and Netravathi: Sarath et al., 2022). Overall, a low groundwater DSi was observed in the West coast of India (365 ± 280 µM, ±1SD, *n* = 47) compared to the East (627 ± 271 µM, ±1SD, *n* = 39) (Figure 1). However, there was no significant difference in δ³⁰Si between the east and west coast. The heaviest and lightest δ³⁰Si values were observed in alluvial aquifers located on the North East coast of India. We do not observe any consistent relationship between the DSi and δ³⁰Si values in Indian coastal groundwater and within similar lithology or climatic zones (Figure 2).
<figure><img src="image_4.png" /></figure>

**Figure 2.** Plot of δ³⁰Si versus DSi (µM) in coastal groundwater of India sorted by aquifer lithology. The compiled average (±1SD) of DSi and δ³⁰Si from Indian river water (Fontorbe et al., 2013; Frings et al., 2015, 2016; Sarath et al., 2022) and the surface Bay of Bengal seawater (Singh et al., 2015) are given as blue and purple diamond respectively. The brown line and shaded area represent the upper continental crust δ³⁰Si composition and ±1SD published by Savage et al. (2013) and the black line represents the global groundwater average reported so far (0.8 ± 0.8‰, see Section 3.2 for references).

### 3.3. Germanium Concentration and Ge/Si Ratio

The germanium concentration in coastal groundwater ranged from 0.04 to 2.9 nM, with an average of 0.57 ± 0.6 nM (n = 80, 1SD). The measured Ge values fall in the higher range of reported Ge concentration in groundwater globally, ranging from 0.03 to 0.5 nM (Aguirre et al., 2017; Baronas et al., 2020). The Ge/Si ratio in Indian coastal groundwater ranged from 0.1 to 4.1 µmol/mol, with an average of 1.3 ± 1.0 µmol/mol, higher than the current published average of 0.4 ± 0.4 nM (n = 50, Aguirre et al., 2017; Baronas et al., 2020). We observe no consistent relationship between Ge/Si and regional lithology and mean annual rainfall. The lowest Ge/Si values were observed in alluvial groundwaters from the South East coast and highest in the lateritic groundwater on the East coast.

## 4. Discussion

Unlike river systems, which integrate water-soil-plant interactions at the scale of the entire river basin, aquifers are spatially limited, reflecting weathering and biogeochemical processes within the local critical zone. A systematic groundwater sampling along the Indian coast, spanning diverse pedoclimatic contexts within the peninsula allows us to discuss the potential broad environmental controls on coastal groundwater δ³⁰Si (e.g., lithology, climate; see Section 4.1) and also the sources and processes controlling the Si isotopic composition in the groundwater (weathering, adsorption, plant uptake; Section 4.2). Finally, we quantify Si fluxes and isotopic impact of fresh and saline SGD along the Indian coast (Section 4.3).

### 4.1. Broad Environmental Controls on Groundwater δ³⁰Si

Over longer timescales, the lithology and climate are two major factors determining the silicate weathering rates of a region (Maher, 2010; White & Blum, 1995). We do not observe any significant difference in δ³⁰Si of coastal groundwater from the major aquifer lithologies on the Indian coast (Figure 3). This indicates that the aquifer lithology has no impact on the partitioning of silicon during the water-rock-plant interactions within the critical zone. The high DSi in groundwater from basaltic aquifers in west coast can be explained by intrinsically high reactivity of basaltic mafic minerals (Dessert et al., 2003), but limited groundwater samples are available from basaltic and granitic lithologies compared to alluvium (Table 1). Another interesting observation from Figure 3 is the highly heterogenous DSi (531 ± 284 µM) and δ³⁰Si (0.84 ± 1.3‰, n = 48) signatures in alluvial aquifers, which includes all groundwater lower than the UCC composition as well as most of the highest δ³⁰Si signatures. The findings also complement the previously observed 2‰ range in groundwater δ³⁰Si measurements within sedimentary aquifer types, such as sandstone, limestone and alluvium from various regions around the globe (Georg, West, et al., 2009, Georg, Zhu, et al., 2009; Martin et al., 2021). The alluvium aquifer systems cover almost 30% of India, and the Quaternary alluvium can extend up to hundreds of meters deep along the coast particularly in the Ganga-Brahmaputra delta (CGWB, 2012). The alluvial sediments primarily consist of detrital quartz, feldspar grains and clay minerals, including kaolinite and illite (Mukherjee et al., 2009). The unconsolidated alluvial aquifers exhibit high groundwater recharge rates, primarily due to high permeability and transmissivity compared to fractured-crystalline aquifers and intense groundwater abstraction and return flow cycles related to agriculture (Bhanja et al., 2019). This, along with high horizontal hydraulic conductivity (10–800 m/day) can result in mixing with surface water and runoff components through highly porous and permeable sands and gravel (Bhanja et al., 2019; Larkin & Sharp, 1992). The mixing of Si reservoirs with distinct δ³⁰Si values, each driven by biogeochemical processes within the weathering zone (for e.g. plant uptake or clay dissolution), is required to explain such large variability in δ³⁰Si values of alluvial groundwater (Georg, West, et al., 2009; Georg, Zhu, et al., 2009; Kavil et al., 2024).

The groundwater recharge dynamics in India largely depend on the balance between annual monsoonal precipitation and pumping for irrigated agriculture and domestic needs, which has also led to rapid groundwater depletion in the Indian Peninsula (Asoka et al., 2017; Rodell et al., 2009). Interestingly, the δ³⁰Si and DSi
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Box plot showing DSi and of δ³⁰Si of Indian coastal groundwater according to the type of lithology, the annual average precipitation and seawater intrusion in groundwater. The aquifer lithology classification is based on WRIS, India and precipitation data from AQUASTAT, FAO and saline intrusion based on Cl concentration and conductivity. The bracket represents the categories with significant differences, and significance is based on ANOVA and Games-Howell post hoc test and is the level of significance is denoted by * (p-value < 0.05), ** (p-value < 0.01), *** (p-value < 0.001) and (n.s. Denotes no significance).</figcaption></figure>

composition of coastal groundwater in regions with low annual average rainfall (<1,200 mm yr⁻¹) was significantly higher than the higher rainfall settings (Figure 3). The rainfall intensity of the region can directly impact the δ³⁰Si composition of groundwater by controlling the weathering rates and nature of secondary phases formed within the weathering zone (Opfergelt et al., 2012). Here, we limit our discussion to the long-term impact of regional rainfall on groundwater silicon cycling and not seasonal variability. The high annual rainfall leads to intense chemical weathering and formation of kaolinites and iron-aluminum oxy-hydroxides, which are predominant in the soil profiles on the West Coast, while the low rainfall regions of the East Coast are dominated by smectite-type clays (Deepthy & Balakrishnan, 2005). The published evidence suggests that the δ³⁰Si value of kaolinite-type clays is more fractionated in Si isotopes with respect to parent bedrock than in smectite-type clays (Cornelis et al., 2019; Opfergelt et al., 2012). Assuming a similar proportion of Si immobilized as clays, the residual solution (such as soil solutions and groundwater) from soil profiles with high rainfall (i.e., kaolinite dominant soil profiles) should exhibit a heavier δ³⁰Si composition. However, we observe the contrary in coastal groundwater, and thus, differences in weathering intensity and clay formation cannot explain the observed heavy isotopic composition in groundwater from low rainfall settings. In addition to direct control of the formation of secondary phases, rainfall can also indirectly affect the water use dynamics of the region. The coastal plains are cultivation hotspots for high-water demand crops such as rice, maize and sugarcane, which can take up dissolved silicon from irrigated water (FAO, 2022; Haynes, 2017; Sarath et al., 2022). Thus, indirect groundwater recharge as agricultural return flow into the aquifers can alter the Si cycling in groundwater in areas under irrigated agriculture (Dewandel et al., 2008; Sarath et al., 2022). In low rainfall and arid climate settings, the irrigation return flow can significantly increase the groundwater DSi and EC as a result of evapotranspiration, and NO₃⁻ is typically derived from fertilizers (Figure S3 in Supporting Information S1). A detailed discussion of the impact of plant uptake in groundwater δ³⁰Si is given in Section 4.2.2.
The Si in coastal aquifers can also originate from seawater intrusion, where the local seawater from the Bay of Bengal exhibits a low DSi (<60 µM) and high δ³⁰Si values (2.1 ± 0.5‰) (Singh et al., 2015). The identified brackish groundwater along the Indian coast spanned alluvial, granitic and limestone aquifer lithologies, with varying degrees of seawater mixing (3%–10%) and shows no relationship with DSi (Figure 3, Figure S4 in Supporting Information S1). The brackish groundwater exhibits variable DSi (from 150 to 680 µM) and δ³⁰Si values (0.4–4.5‰), and a non-conservative mixing trend with significantly high δ³⁰Si composition (Figure S4 in Supporting Information S1). The non-conservative nature of mixing makes it difficult to constrain a saline SGD endmember based on brackish groundwater, and to use DSi as a tracer for SGD in Indian coast (Oehler et al., 2019). The aquifer sediments in alluvial plains can host reactive silicates as well as recent diatomaceous earth, which can alter the DSi signatures in mixing zones (De & Bhattacharyya, 2009; Oehler et al., 2019). However, high δ³⁰Si values in brackish groundwaters TN8 and WB8 are unlikely to result from biogenic silica (diatoms) dissolution, as δ³⁰Si values of these Si sources are typically lower than the seawater composition (Ehlert, Doering, et al., 2016; Savage et al., 2013). Thus, the influence of seawater intrusion on the Si isotopic composition of coastal groundwater is considered to be negligible. Overall, our results highlight the large variability in DSi and δ³⁰Si composition of alluvial groundwater, with a significantly heavy δ³⁰Si composition in coastal groundwater with low annual rainfall. The lithology and climate are not the primary drivers of silicon isotopic signatures in Indian coastal groundwaters. In the next section, we will look more closely into specific sources and processes that can explain the observed δ³⁰Si composition in coastal groundwater.

## 4.2. Si Sources and Biogeochemical Processes in Coastal Groundwater

### 4.2.1. Incongruent Silicate Weathering Signatures in Groundwater

The primary source of dissolved silicon in groundwater is the weathering of silicate minerals, which can be partly immobilized in soil and saprolite through biogeochemical processes before reaching the saturated zone (Georg, West, et al., 2009; Pogge von Strandmann et al., 2014). Indeed, most of the groundwater samples exhibit heavier δ³⁰Si than the UCC signature of -0.25 ± 0.16‰ and broadly correlates negatively with the Ge/Si ratio (Figure 4, R² = 0.24, *p* value < 0.01), evidencing incongruent nature of silicate weathering with the formation of secondary phases (Baronas et al., 2020; Savage et al., 2013). The extent of partitioning into secondary phases, estimated as fSi (Equation 4), indicates that 80% to >99% of Si mobilized by silicate weathering was incorporated into secondary phases. The resulting δ³⁰Si composition of the DSi fraction reaching coastal groundwater systems can be modeled based on the extent of incorporation and/or adsorption onto secondary mineral phases (clays and/or oxyhydroxides) and, plant uptake of DSi in shallow soil and saprolite layers (Equations 2 and 3). As observed from Figure 4, the majority of Indian coastal groundwater follows a steady-state open flow through the model framework, with isotope fractionation (³⁰E) between -0.5‰ and -2‰. Within the weathering zone, the formation of clay minerals generally follows the transformation of Al-hydroxides to high surface area amorphous precursors through Si adsorption and subsequent conversion to crystalline clays over time (Oelze et al., 2014). The clay minerals inherit lighter ²⁸Si isotope as a result of a fast unidirectional kinetic adsorption process during the early stages of weathering, followed by a slow re-equilibration, depending on the surface area of precipitating solid and extension of water-rock interaction (Fernandez et al., 2019; Oelze et al., 2014, 2015). The field observations indicate that the ³⁰E of clay neoformation depends on the mineralogy, with less silicified 2:1 clays such as smectites and vermiculites, exhibiting a smaller ³⁰E (-0.4‰ to -1.1‰) compared to more silicified kaolinite type clays (-1‰ to -3‰) (Frings, Oelze, et al., 2021; Georg, West, et al., 2009; Opfergelt et al., 2012). Thus, depending on the differences in the dominant clay mineral in the region (³⁰E) and the extent of Si incorporation (fSi), we can explain the δ³⁰Si range between -0.25 and 2‰, observed in coastal aquifers. In the following subsections, we address alternate mechanisms that can explain the observed heavy (δ³⁰Si > 2‰) and light (δ³⁰Si < -0.25‰) silicon isotopic composition in coastal groundwater.

### 4.2.2. Plant Uptake and Adsorption Processes

An interesting subset of coastal groundwaters exhibits heavy δ³⁰Si values (>2‰) with a fSi ≤ 0.01 and Ge/Si < 0.5, pointing to a DSi and Ge-depleted groundwater, fitting more to a Rayleigh model framework. One potential process that could explain the heavy δ³⁰Si signatures and low Ge/Si ratio is adsorption onto pedogenic Fe oxy-hydroxides during percolation through weathering profile, especially in groundwater underlying thick lateritic soil profiles (Figure 3). The relatively fast adsorption kinetics of Si onto Fe oxy-hydroxides in laboratory experiments and also soil porewaters generally follows a Rayleigh model, with a reported ³⁰E<sub>oxyhydroxide-solution</sub>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a) The plot between δ³⁰Si and fraction of silicon remaining in solution ($f_{Si}$) for all coastal groundwater and river water in Indian subcontinent. The Rayleigh (dashed line) and steady state (solid line) model based evolution of δ³⁰Si in the solution starting from a upper continental crust is depicted for plant uptake ($^{30}\epsilon = -0.3\%$ to $-1.1\%$, Frick et al., 2020 and references therein), adsorption ($^{30}\epsilon = -1.1\%$ to $-1.6\%$, Delstanche et al., 2009), smectite and kaolinite precipitation ($^{30}\epsilon = -0.4\%$ to $-2.0\%$ respectively, Frings, Oelze, et al., 2021; Opfergelt et al., 2012). The published soil porewater from South India: Riotte, Meunier et al., 2018; Riotte, Sandhya et al., 2018 and river water: Fontorbe et al., 2013, Frings et al., 2015, Sarath et al., 2022. (b) δ³⁰Si and Ge/Si plot of coastal groundwater. The gray region encompasses the variability in primary minerals Ge/Si and δ³⁰Si composition and identified various endmembers from literature (Cornelis et al., 2010; Frings et al., 2021a, 2021b; Lugolobi et al., 2010; Opfergelt et al., 2012; Riotte, Meunier, et al., 2018).</figcaption></figure>

between $-1.1\%$ and $-1.6\%$ for ferrihydrite and goethite, respectively (Delstanche et al., 2009; Riotte, Meunier, et al., 2018). In addition, Ge adsorption as surface complexes onto Fe oxy-hydroxides can also lead to Ge depletion in groundwater (Pokrovski et al., 2006). However, none of the groundwater underlying laterite and ferruginous concretions exhibits δ³⁰Si > 2‰, only two coastal groundwater fits the published $^{30}\epsilon$ range of adsorption following a Rayleigh fractionation model (Figures 3 and 4). Furthermore, the evidence from field settings indicates that clay formation has dominant control over the Ge/Si ratio of soil rather than adsorption processes (Scribner et al., 2006). Thus, adsorption onto Fe oxy-hydroxides alone cannot explain the observed heavy Si isotopic composition in coastal groundwater.

The heavy δ³⁰Si composition in groundwater and deviation from modeled steady state behavior can also result from substantial plant uptake of DSi from the soil solution. Thus, plant uptake, storage and cycling as phytolith imparts significant control over DSi export and isotopic composition of shallow soil water and aquifer systems (Struyf et al., 2010; Vandevenne et al., 2015). Plant uptake preferentially incorporates a lighter ²⁸Si isotope, enriching the residual solution in heavy ³⁰Si (Frick et al., 2020). The silicon uptake by cultivated crops such as rice (4 wt% Si), sugarcane (1.5%) and maize (0.8%), and associated isotopic fractionation has been extensively documented through hydroponic experiments as well as on the field settings (Frick et al., 2020; Riotte, Meunier, et al., 2018; Sun et al., 2016). For instance, Riotte, Meunier, et al. (2018) observed that soil porewaters (0–20 cm deep) in a paddy field in South India exhibit high δ³⁰Si values, up to 3.9‰, due to the preferential uptake of lighter ²⁸Si isotope from soil solutions, in addition to adsorption in the shallow soil layers. The observed heavy δ³⁰Si signatures in coastal groundwater following a Rayleigh model is consistent with kinetic silicon isotopic fractionation during plant uptake, with documented fractionation factors in crops ranging from $-0.3\%$ to $-1.1\%$ (Ding et al., 2005; Frick et al., 2020; Sun et al., 2016). The low Ge/Si ratio (0.23 ± 0.11 µmol/mol) in these groundwaters points to a lack of discrimination or preferential plant uptake of Ge compared to Si, consistent with recent findings from Frings, Schubring, et al. (2021) (Figure 4b). The Ge uptake dynamics depends largely on plant species, and signature recorded in groundwaters can result from a combination of weathering, plant uptake, and remineralization of labile phytolith and non-phytolith Ge pools in soil and saprolite (Frings, Schubring, et al., 2021; Wiche et al., 2018). In regions of low annual rainfall and high water demand, crop irrigation is mainly
sustained through groundwater pumping (Davis et al., 2018). After nearly two decades of drilling millions of borewells, India has the largest groundwater-irrigated cultivated area in the world (almost 44% of the total cultivated area, FAO, 2015). A significant proportion of the irrigated water (up to 50% in rice-cultivated regions) can reach the groundwater systems as irrigation return flow (Dewandel et al., 2008). Thus, in cultivated areas with low rainfall and groundwater irrigation, a substantial part of the return flow, enriched in a ³⁰Si isotope, can infiltrate the underlying groundwater systems (Séraphin et al., 2016; Tulip et al., 2022). These infiltration of return flow to shallow groundwaters, especially in alluvial settings with high hydraulic conductivity and absence of primary silicate minerals, can explain the observed heavy groundwater δ³⁰Si signatures and low Ge/Si (Figures 3 and 4). The interpretation is coherent with observation from large river systems, where exceptionally heavy δ³⁰Si in river water is attributed to contribution from agricultural return flow (Ding et al., 2004; Sarath et al., 2022). However, the extent to which the return flow can impact groundwater δ³⁰Si is subject to additional constraints, such as groundwater storage, lateral connectivity and discharge, water-rock interactions and residence time (Kavil et al., 2024; Maher, 2011).

### 4.2.3. Dissolution of Secondary Si- Phases in the Weathering Zone

In addition to silicate weathering, DSi in coastal groundwater can originate from dissolution of precipitated secondary minerals (e.g., clays, Fe and Mn oxy-hydroxides) as well as poorly crystalline or amorphous silica (ASi) phases that are ubiquitous in soil and sediments (Cornelis et al., 2011, 2014). Previous works have assigned the light (more negative) δ³⁰Si composition of groundwaters to a shift in Si source from primary mineral weathering to dissolution of secondary smectite-type clays and siliceous pedogenic precipitates such as silcretes (Georg, West, et al., 2009, Georg, Zhu, et al., 2009; Pogge von Strandmann et al., 2014). The coastal groundwater samples from the present study with δ³⁰Si composition lower than UCC values (−0.25 ± 0.16‰, Savage et al., 2013) are restricted to the alluvial aquifer of North East coast, mainly composed of Holocene sediments and an isolated sample from South West coast (Figures 3 and 4). The light ²⁸Si source in this aquifer could be sourced from, (a) complete or partial dissolution of illite and smectite type clays (δ³⁰Si ranges from −0.2 to −1.1‰, Bayon et al., 2018; Frings, Oelze, et al., 2021), (b) dissolution of amorphous Si in aquifer sediments (Cornelis et al., 2014; Desbarats et al., 2017), (c) reductive dissolution of Fe-Mn oxy-hydroxides which is widespread in the shallow aquifers of the Bengal basin (Islam et al., 2004; Varner et al., 2023). The clay mineralogy in alluvial sediments in the North East coast is dominated by illite, kaolinite, smectite and interstratified clays, with the transformation of illite and smectite-type clays to kaolinite occurring in weathering profiles (Singh et al., 1998). The partial dissolution of these pedogenic clay minerals and/or poorly crystalline aluminosilicates formed as a result of dissolution and reprecipitation of pedogenic clays is a source of light ²⁸Si to groundwater (Cornelis et al., 2014). Among the coastal groundwaters with low δ³⁰Si composition, a high Ge/Si value can be indicative of clay mineral dissolution, while a low Ge/Si value indicates dissolution of amorphous phases (clay precursors and phytolith) and/or predominant release of lighter Si through desorption from oxy-hydroxides (Figure 4b). Indeed, recent studies suggest that Ge/Si ratio in the amorphous silica phases is predominantly controlled by subsurface fluid residence times, and fast incorporation kinetics of Si can form a depleted δ³⁰Si reservoir with low Ge/Si ratio (Fernandez et al., 2021). In addition, Holocene alluvial aquifers in Bengal plains are known for their low dissolved oxygen (DO, Figure S2 in Supporting Information S1, data from Kumar et al., 2021), where microbially mediated reductive dissolution of Fe-Mn oxy-hydroxides associated with organic matter occurs, making them prone to arsenic contamination (Islam et al., 2004). The dissolution of Fe-Mn oxy-hydroxides (including amorphous and poorly crystalline oxides and hydrous oxides of Fe(III)) can also release the ²⁸Si-rich silicon adsorbed on these phases into the groundwater systems, leading to a low δ³⁰Si composition (Delstanche et al., 2009). Overall, the δ³⁰Si composition in coastal groundwater lower than the underlying bedrock in alluvial aquifers is mainly sourced from complete or partial dissolution of precipitated secondary phases, including crystalline clays and oxy-hydroxides and amorphous aluminosilicates within the regolith.

### 4.3. Submarine Groundwater Silicon Fluxes and Isotopic Impact

Dissolved silicon (DSi) in SGD forms a mixture of two components: fresh, terrestrial groundwater (fresh SGD) and recycled saline groundwater (saline SGD) originating from seawater and recirculated through coastal aquifers (Burnett et al., 2006; Moore et al., 2010). The fresh groundwater samples from Indian coastal aquifers (within a few km from the coast) can be considered as representative fresh SGD end members, and resulting Si fluxes a source of DSi to the coastal ocean, which can be estimated as:
<table><caption>**Table 2**
The Fresh Groundwater Derived Si Fluxes, and Average δ³⁰Si Composition of Fresh Submarine Groundwater Discharge Flux to East Coast (Bay of Bengal) and West Coast (Arabian Sea) of India</caption><thead><tr><th></th><th>Y<sub>fsgd</sub><br>km<sup>3</sup> yr<sup>-1</sup></th><th>[DSi]<sub>fsgd</sub><br>µM</th><th>fSGD<sub>Si</sub><br>Gmol yr<sup>-1</sup></th><th>δ<sup>30</sup>Si<sub>avg</sub><br>%c</th></tr></thead><tbody><tr><td>East coast</td><td>2.3</td><td>666 ± 272</td><td>1.5 ± 1.7<sup>a</sup></td><td>0.6 ± 1.1</td></tr><tr><td>West coast</td><td>1.6</td><td>364 ± 288</td><td>0.6 ± 0.7<sup>a</sup></td><td>0.9 ± 0.9</td></tr><tr><td>Total fSGD<sub>Si</sub></td><td>3.9</td><td></td><td>2.1 ± 1.7<sup>a</sup></td><td><strong>0.6 ± 0.8</strong></td></tr></tbody></table>

Note. The fresh SGD water flux from Luijendijk et al. (2020), the DSi and δ³⁰Si<sub>avg</sub> composition from present work. <sup>a</sup>Assuming a 100% uncertainty in fresh SGD flux estimate.

$$fSGD_{Si} = [DSi]_{fgw} * Y_{fsgd} \quad (6)$$

here the fSGD<sub>Si</sub> is the total Si fluxes in Gmol yr<sup>-1</sup> to Indian coast through fresh SGD, [DSi]<sub>fgw</sub> the fresh groundwater endmember dissolved silicon concentration, and Y<sub>fsgd</sub> is the total coastal fresh SGD water flux. The volumetric flux of fresh SGD along the Eastern and Western Indian coastline from Luijendijk et al. (2020) was used to best represent Y<sub>fsgd</sub>. The average fresh groundwater DSi and weighted δ³⁰Si along the Eastern coast (Bay of Bengal) and Western coast (Arabian Sea) of India is given in Table 2.

Overall, the estimated total fresh SGD DSi flux from the Indian coastline is 2.1 GmolSi yr<sup>-1</sup>, which represents <1% of total riverine Si supply to North Indian Ocean (Mangalaa et al., 2017). The fresh SGD DSi flux along the Indian coastline only accounts for 0.3% of the global fresh SGD Si supply,

and insignificant compared to the global saline SGD Si flux of 3.1 TmolSi yr<sup>-1</sup> estimated by Rahman et al. (2019). The discharge weighted δ³⁰Si values of fresh SGD DSi flux from West and East coast of India was 0.68%c (Table 2), almost 0.8%c lighter than Indian riverine δ³⁰Si composition of 1.5 ± 0.6%c (Fontorbe et al., 2013; Frings et al., 2015; Sarath et al., 2022). Despite the low δ³⁰Si composition in fresh coastal groundwater, the overall change in δ³⁰Si composition of Si input fluxes (rivers and groundwater) to the North Indian Ocean is <0.05%c, which is within our analytical uncertainty.

The saline SGD endmember along the Indian coast is more difficult to constrain, given the paucity of saline groundwater endmember in the present study and the non-conservative nature of saline SGD for both DSi and δ³⁰Si values (Figure S4 in Supporting Information S1). The total SGD water flux estimated using radium mass balance from the Indian coast varies from 12 to 115 km³ yr<sup>-1</sup>, almost two orders of magnitude higher than fresh SGD water flux (Rengarajan & Sarma, 2015; Yadav et al., 2019). Assuming that the total SGD water flux is entirely of saline groundwater origin and an average saline groundwater DSi composition of 422 ± 165 µM (n = 10) from the present study, we can estimate Si fluxes through saline SGD as:

$$sSGD_{Si} = ([DSi]_{sgw} - [DSi]_{seawater}) * Y_{ssgd} \quad (7)$$

sSGD<sub>Si</sub> is the DSi flux through saline SGD, [DSi]<sub>sgw</sub> and [DSi]<sub>seawater</sub> is the dissolved silicon concentration in saline groundwater (based on high conductivity and Cl) and seawater (Singh et al., 2015), and Y<sub>ssgd</sub> is the saline SGD water flux, assumed to be close to reported total SGD flux along the Indian coast. The estimated sSGD<sub>Si</sub> ranges between 5.1 and 48.6 GmolSi yr<sup>-1</sup>, 5%–56% of the Si flux exported by Indian rivers (Mangalaa et al., 2017). This is likely an overestimate, given the saline groundwater end member from the present work comprises a mix of saline SGD and fresh SGD. In addition, DSi of saline SGD can be sourced from the dissolution of lithogenic phases and biogenic silica in the subsurface region, and only the lithogenic dissolution can be considered as a new Si source to the coastal ocean (Anschutz et al., 2009; Ehlert, Doering, et al., 2016; Rahman et al., 2019). However, distinguishing new versus recycled Si in saline SGD and understanding the isotopic impact in the coastal ocean is outside the scope of the present work. Overall, we observe a minor impact of fresh groundwater discharge on the silicon isotopic composition and DSi flux to the Indian coastal Ocean.

## 5. Conclusion

Coastal groundwater from the Indian subcontinent displays large variability in DSi (80–1,350 µM) and δ³⁰Si values (−1.1–4.5 %c). The average δ³⁰Si value of Indian coastal groundwater (0.8 ± 1.1%c, 1SD, n = 85) is comparable to previously published estimate of global groundwater (0.8 ± 0.8%c, n = 117), and is significantly lower than the average δ³⁰Si of Indian rivers (1.5 ± 0.6%c, ±1SD, n = 152). The observed variability in δ³⁰Si of the groundwater samples do not show any correlation with the aquifer lithology. The coastal alluvial aquifers encompass the largest variability observed in both DSi and δ³⁰Si values. The coastal groundwater from low rainfall regions (<1,200 mm/yr) exhibits on average 0.8%c heavier δ³⁰Si values compared to high rainfall regions (>2,000 mm/yr), with significantly enriched DSi. The DSi in groundwater is sourced from incongruent weathering of primary silicate minerals, with few alluvial and lateritic aquifers exhibiting signatures of dissolution from secondary phases, for example, clays and amorphous aluminosilicates. Only a few coastal groundwater exhibit
signatures of seawater intrusion, with a fraction of seawater ranging from 4% to 14%, with wide range of DSi (150–680 µM) and δ³⁰Si (0.4–4.5‰) values. The δ³⁰Si composition of most of the coastal groundwater can be explained by a steady-state model, with a dynamic equilibrium between the Si supply and incorporation into secondary Si-bearing phases and a fractionation factor consistent with the formation of smectite and kaolinite type clays. The heaviest δ³⁰Si values (>2‰) followed a Rayleigh model with a low Ge/Si ratio, which can be explained by fractionation during plant uptake in irrigated regions. Finally, we estimated the total Si flux through fresh SGD to be 2.1 Gmol yr⁻¹, which would correspond to <1% of riverine input to the North Indian Ocean and imparts no significant isotopic impact despite having a higher DSi and lower δ³⁰Si values. Overall, groundwater systems form a highly heterogeneous reservoir of Si, exhibiting a 5.5‰ variability in δ³⁰Si composition, incorporating signatures from various abiotic and biotic processes.

## Data Availability Statement

The data set for this article can be found online at https://doi.org/10.5281/zenodo.14730770 (Kavil et al., 2025).

## Acknowledgments

The current work is funded by CEFIPRA-IFCPAR support (NUnderGruND, project #5907). We acknowledge the NIO India team for providing support in the field campaign and groundwater sampling. The authors thank Moustafa Belhadj Senini and Frédéric Candaudap for their support to HR-ICPMS analyses, Hemanth Moger from IFCWS for IC analysis, Pierre Burckel (IPGP) and Irina Djouraev (ALYSES facility IRD-SU) for the help with ICPMS to check for Si chemistry and Mélanie Longchamp (GESE platform, SU) for cation analyses. The authors thank Bobbili Sridevi for their help with the figures. SPK thanked IRD-Campus France for providing a Ph.D. scholarship and constant support to carry out the work. The authors would like to thank M. Benrahmoune (LOCEAN-IPSL) for their help in sample processing and clean lab management at LOCEAN, Sorbonne University.

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Erratum

The originally published version of this article omitted the contributions for coauthors Damien Cardinal, Jean Riotte, and Arnaud Dapoigny. Their contributions are as follows: Damien Cardinal contributed to Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing – review & editing, and Project administration; Jean Riotte contributed to Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing – review & editing, and Project administration; and Arnaud Dapoigny contributed to Methodology, Validation, and Formal analysis. This may be considered the authoritative version of record.