Geophysical and Seawater intrusion models to distinguish Modern and Palaeo salinity in the Central Godvari Delta, Andhra Pradesh, India

Abstract:

Central Godavari Delta is located in the East coast of Andhra Pradesh along Bay of Bengal. Ample surface water is made available for irrigation and aqua culture through well distributed canals drawn from Godavari River since last 150 years. Groundwater in the area is highly saline though the groundwater levels are very shallow ranging from 1 to 3 m below ground level.

Integrated Electrical Resistivity Tomograms (ERT), hydrochemical (pH, TDS, Ca²⁺, Mg²⁺, K⁺, F^-, Cl^-, SO₄^2-, NO₃^-, HCO₃^- and CO₃^-), isotopic (Br^- and δ¹⁸O ) and density dependant solute tranport (SEAWAT) modelling studies have been carried out for four years (2006, 2007, 2014 and 2015) to identify the salinity sources and to understand the possible extent of seawater intrusion. The integration of all these data sets revealed that coarse grained sands exhibits resistivity of 4-20 Ωm forming the surface layer, clay layer exhibits < 1 Ωm mostly present as the second layer at many locations. However, similar resistivity values are also associated with soils saturated with sea water. Clay with fine sand occurs as the third layer with a resistivity of 1- 4 Ωm.

The different mixing models ((TDS vs. (Na²⁺+ K⁺) and (Ca²⁺+Mg²⁺), (Na⁺-Cl^- ) vs. Ca²⁺+Mg²⁺-HCO₃^--SO₄^2-)) and ionic ratios ( Na²⁺/Cl^-, SO₄^2-/Cl^-, Mg²⁺/Ca²⁺_, Mg²⁺/Cl^- and Cl^-/Br) and δ¹⁸O does not reflect any modern seawater signatures. These models indicated that salinity in the shallow wells is due to dissolution of evaporitic minerals and ion exchange processes. In the pumping wells the salinity is due to upconing of entrapped sea water that belongs to Palaeo origin and wells located near the coast and mudflats is due to physical mixing of marine water. The estimated regional groundwater balance using SEAWAT model indicate significant amount of submarine groundwater discharge as outfall to the Bay of Bengal. Assuming observed hydrological conditions, no considerable advance in seawater intrusion would be expected into the delta region.