Seawater intrusion into groundwater and its impact on irrigation and agriculture: Evidence from the coastal region of West Bengal, India

Abstract The present study aims to trace out the signatures of seawater intrusion into the groundwater and the irrigation hazards in the coastal region of West Bengal, India based on 56 groundwater samples. The result reveals that 48.21 % of the groundwater samples have exhibited the values of seawater mixing index greater than 1 indicating the effect of seawater mixing into the fresh groundwater. The electrical conductivity (EC) in the studied samples ranges from 338 to 4330 μ S/cm and as per the United States Salinity Laboratory (USSL) classification of EC for irrigation water quality, about 31% of samples come under medium (251-750 μ S/cm), 53% under high (751-2250 μ S/cm), and 16 % under very high (2251-5000 μ S/cm) salinity hazard classes. Besides, the potential salinity (PS) reveals that about 52 % of samples have high salinity problems while 16 % are moderately suitable (water may be used for medium to coarse-textured soils) and 32 % are suitable which can safely be used for fine and medium, and coarse-textured soils. The problem of salinity intrusion acts as a catalyst to impair agricultural practice. For example, the average yield of rice production is low (1.5 - 2.0 t ha−1) for South 24 Parganas compared to the neighbouring districts. The study finds that ∼ 49 % of samples having Na + /Cl − ratio greater than 1 indicates that silicate weathering is the dominant process controlling groundwater hydro-chemistry and sodium concentration. Moreover, a higher sea-level rise (3.90 ± 0.13 mm a−1) , frequent storm surge (142 events of moderate to severe cyclone during 1582 to 1991) and overexploitation of groundwater and fall of piezometric level (e.g. 10.96 m in 1956–2005 at Gobra, Kolkata) are major driving forces of seawater intrusion.