GROUNDWATER CHEMISTRY AND ARSENIC MOBILIZATION IN SHALLOW AND DEEP AQUIFER – A FIELD SCALE STUDY IN WEST BENGAL, INDIA

Arsenic (As) contamination in groundwater is a crucial water quality problem in many parts of the world, notably in Southeast Asia (e.g. India, Bangladesh, China, Taiwan and Vietnam). A large section of the rural population is now suffering with arsenocosis, where the population heavily relies on groundwater for their domestic purpose. This study deals with groundwater quality and As mobilization behavior in the shallow (<50 m) and deeper (>100 m) aquifers in Nadia district. The groundwaters from shallow aquifers are enriched with redox sensitive species, such as HCO₃^-, DOC, PO₄^-3 and NH₄⁺ whereas the deeper aquifer contains relatively low As (in few wells >50 ppb), low to very low Fe, high HCO₃^-, low PO₄^-3, DOC and NH₄⁺. The dissolve Fe(II) is relatively high in both shallow and deep aquifer. The NO₃^-, SO₄^-2 and DO are relatively low in shallow aquifer whereas high in the deeper aquifer. The As speciation study reveals a varied combination of As (III)/As_T in groundwater and having little relationship with other measured water quality parameters in shallow aquifer whereas the relationship is more predominant in the deeper aquifer. The sediment As_T and Fe_T content is moderately uniform in both the aquifer whereas organic matter content largely varies. This study demonstrates that the As mobilization processes are different for shallow and deep aquifer in Nadia district. In the shallow aquifer As release to the groundwater is due to reduction of Fe oxides/hydroxides when anoxic condition develops in the aquifer. Local recharge, particularly rich in dissolve organic matter is an important issue for As mobilization in the shallow aquifer, where ponds and sanitation pattern plays a major role. However, in the deeper aquifer As mobilization is mostly governed by the carbonate dissolution reaction, which also includes the role of pCO₂ in mobilization process.