Mathew et al. (2025) Delineation of seawater intrusion zone using geophysical and geochemical approach – a case study of North Chennai coastal aquifer, India

Identification

• Journal: Water Practice & Technology
• Year: 2025
• Date: 2025-10-18
• Authors: Riya Ann Mathew, Marykutty Abraham
• DOI: 10.2166/wpt.2025.140

Research Groups

• Arcadis U.S. Inc., Austin, TX, USA
• Evana Environmental Solutions, Chennai, India

Short Summary

This study delineates seawater intrusion (SWI) zones in the North Chennai coastal aquifer, India, using an integrated geophysical (resistivity survey) and geochemical approach, supported by GIS mapping. The findings confirm high and moderate SWI extending up to 13.7 km and 16.5 km inland from the coast, respectively, with groundwater quality largely unsuitable for drinking and irrigation.

Objective

• To demarcate seawater intrusion (SWI) zones in the coastal regions based on resistivity surveys, geochemical studies, and GIS mapping.
• To propose remedial measures for the sustainable management of coastal aquifers.

Study Configuration

• Spatial Scale: North Chennai coastal aquifer, India, located between the Araniyar and Kosasthalaiyar rivers. The study area spans 295 km², geographically situated between 80°08′44.85″ and 80°20′13.86″ east longitudes and 13°12′25.33″ and 13°22′39.37″ north latitudes, with its western boundary approximately 22 km from the east coast.
• Temporal Scale: Geochemical analysis of groundwater quality parameters was conducted over ten years (2005–2015) for pre- and post-monsoon periods. Geophysical field surveys were carried out during the dry season.

Methodology and Data

• Models used:
  • RES2DINV software (for interpretation of apparent resistivity values and layer thickness).
  • Zondres2D software (for interpretation of apparent resistivity values).
  • GIS software (for spatial distribution mapping and interpolation of water quality parameters).
• Data sources:
  • Geophysical data: Vertical Electrical Soundings (VES) at 33 locations using Schlumberger configuration with a DDR3 resistivity meter (IGIS, Hyderabad, India), employing a current electrode half-spacing (AB/2) of 75 meters. Pumping tests were conducted at two representative locations (Minjur and Ponneri) to verify aquifer characteristics.
  • Geochemical data: Groundwater samples collected from corresponding VES locations were analyzed for pH, total dissolved solids (TDS), electrical conductivity (EC), bicarbonate, calcium, magnesium, sodium, chloride, sulphate, and nitrate, following APHA methods (2005).
  • Remote sensing data: IRS-P6, LISS-4, and Pan merged data were utilized to prepare geological, geomorphological, and land use maps.
  • Ancillary data: Hydrological, hydrogeological, and hydrometeorological data were collected from government departments and field observations.

Main Results

• Groundwater quality was found to be unsuitable for drinking purposes in 64% of the study area and for irrigation purposes in 85% of the area, primarily due to high sodium concentrations from salinity intrusion.
• The geophysical resistivity survey, corroborated by geochemical studies, indicated high saline water intrusion (resistivity < 15 Ωm, EC > 2,000 µS/cm, Cl > 350 mg/L, Na/Cl < 0.6) up to 13.7 km from the coast.
• Moderate saline water intrusion (resistivity 16–35 Ωm, EC 600–2,000 µS/cm, Cl 200–350 mg/L, Na/Cl 0.6–0.86) was observed up to 16.5 km from the coast.
• Freshwater conditions (resistivity > 36 Ωm, EC < 600 µS/cm, Cl < 200 mg/L, Na/Cl > 0.86) were identified beyond 16.5 km from the coast.
• The depth of seawater intrusion varied between 2.4 m and 64.78 m below ground level, with the second (saturated) and third aquifer layers showing significant intrusion, particularly in the eastern part.
• The eastern and middle portions of the study area are predominantly contaminated by saline water intrusion, influenced by factors such as the Bay of Bengal, Pulicat Salt Lake, industrial effluents, and subsurface formations.

Contributions

• Provided a comprehensive delineation of seawater intrusion zones in the North Chennai coastal aquifer by integrating geophysical resistivity surveys, geochemical analysis, and GIS techniques.
• Demonstrated the efficacy of geophysical methods as a valuable and less laborious alternative to traditional geochemical approaches for estimating aquifer parameters and detecting saline water intrusion.
• Quantified the spatial (up to 16.5 km inland) and depth-wise (2.4 m to 64.78 m below ground level) extent of seawater intrusion in the study area.
• Highlighted the critical need for proper management strategies, including controlled groundwater withdrawals, artificial recharge, and structural arrangements (e.g., injection barriers, check dams), to mitigate SWI and safeguard coastal aquifers.

Funding

Not explicitly mentioned in the provided text.

Citation

@article{Mathew2025Delineation,
  author = {Mathew, Riya Ann and Abraham, Marykutty},
  title = {Delineation of seawater intrusion zone using geophysical and geochemical approach – a case study of North Chennai coastal aquifer, India},
  journal = {Water Practice & Technology},
  year = {2025},
  doi = {10.2166/wpt.2025.140},
  url = {https://doi.org/10.2166/wpt.2025.140}
}