Hydrogeological processes controlling the release of arsenic in parts of 24 Parganas district, West Bengal

摘要

A study was conducted to understand the hydrogeological processes dominating in the North 24 Parganas and South 24 Parganas based on representative 39 groundwater samples collected from selected area. The abundance of major ions was in the order of Ca~(2+) ＞ Na~+ ＞ Mg~(2+) ＞ K~+ ＞ Fe~(2+) for cations and HCO_3~- ＞ PO_4~(3-) ＞ Cl~- ＞ SO_4~(2-) ＞ NO_3~- for anions. Piper trilinear diagram was plotted to understand the hyd-rochemical facies. Most of the samples are of Ca-HCO_3 type. Based on conventional graphical plots for (Ca + Mg) vs. (SO_4 + HCO_3) and (Na + K) vs. Cl, it is interpreted that silicate weathering and ion exchange are the dominant processes within the study area. Previous studies have reported quartz, feldspar, illite, and chlorite clay minerals as the major mineral components obtained by the XRD analysis of sediments. Mineralogical investigations by SEM and EDX of aquifer materials have shown the occurrence of arsenic as coating on mineral grains in the silty clay as well as in the sandy layers. Excessive withdrawal of groundwater for irrigation and drinking purposes is responsible for fluctuation of the water table in the West Bengal. Aeration beneath the ground surface caused by fluctuation of the water table may lead to the formation of carbonic acid. Carbonic acid is responsible for the weathering of silicate minerals, and due to the formation of clay as a product of weathering, ion exchange also dominates in the area. These hydrogeological processes may be responsible for the release of arsenic into the groundwater of the study area, which is a part of North 24 Parganas and South 24 Parganas.