Evaluation of water quality, Hydro-geochemistry of Confined and Unconfined aquifers and irrigation water quality in Digha Coast of West Bengal, India (A case study)

Evaluation of ground water quality and hydro-geochemistry of confmed and unconfmed aquifer (with respect to provisional guideline values of WHO 2004, geochemical classifications, ion exchange processes and mechanisms controlling ground water chemistry) and evaluation of surface water quality (with respect to different sodium hazards, Ca²⁺/Mg²⁺ ratio and common phytotoxic anions) for irrigation purpose in a coastal area were the aims of the present study. Accordingly ground and surface water samples were collected from confmed, unconfmed aquifers and irrigation water tanks of Digha coast and its adjoining area of West Bengal, India in post-monsoon and pre-monsoon seasons. Quality of the water samples collected from the confmed aquifer found satisfactory in comparison with water samples collected from the unconfmed aquifer. Geochemical nature of confmed aquifer was earth alkaline with increased portion of alkalis with prevailing bicarbonate, followed by chloro-alkaline disequilibrium type of ion-exchange process. Chemical weathering of rock forming minerals was the major driving force controlling confmed aquifer water chemistry. Hydro-geochemistry of unconfmed aquifer in post-monsoon season was alkaline with bicarbonate and sulfate-chloride and earth alkaline with increased portion of alkalis with prevailing bicarbonate where as in pre-monsoon season hydro-geochemistry of the unconfmed aquifer shifted towards alkaline with sulfate and bicarbonate. Ion exchange process in this aquifer showed complete dominance of base exchange reaction in both the seasons. Chemical weathering and chemical weathering along with evaporation were the two major driving forces controlling the water chemistry of the unconfmed aquifer in postmonsoon and pre-monsoon seasons respectively. RSC (residual sodium carbonate), %Na⁺, SAR (sodium adsorption ratio), Ca²⁺/Mg²⁺ ratio and concentration of cr, B⁻, P⁻ in irrigation water revealed that the majority of surface water samples were not good enough for irrigation in pre-monsoon season in comparison to that of post-monsoon season.