EXPLORATION OF ARSENIC AND MANGANESE SAFE SHALLOW AQUIFERS IN THE WESTERN BENGAL BASIN

Identification of arsenic (As) safe aquifers based on the color of the sediments have been explored in some of the studies in Bengal basin. However, there is limited knowledge regarding distribution of the As safe aquifers and dubiousness regarding their usage considering the high levels of manganese (Mn) in groundwater reported in earlier studies. In this study a total number of 50 boreholes were drilled at two study sites (~25 km² each) in Western Bengal Basin (West Bengal state, India) to investigate the shallow aquifer sediments and groundwater. In the first site, North 24 Parganas district (NP), the lithologic investigation reveals that the continuous brown-colored sand (BS) was found at a depth between 40 – 60 m, with an average thickness of ~15m. They are overlain by grey-colored clay that are overlain by grey-colored sand (GS). On the contrary, the lithology of the shallow aquifer sediments in Nadia district (ND) is predominated by GS, with only some discontinuous lenses of BS in some of the boreholes. Hydrogeochemical examinations were conducted of the groundwater samples collected from the existing tube wells near the drilling sites. In both NP and ND, 91% and 78% of the samples collected from GS have elevated levels of groundwater As (≥ 10 ppb), whereas only 15% and 18% of the tube wells installed in BS of NP and ND, respectively, have elevated As. Also, 85% and 82% of the wells at BS are Mn safe (< 400 ppb) in NP and ND, respectively. This preliminary study suggests that the redox condition in most of the groundwater hosted by BS are not Mn-reductive, whereas high concentration of iron (Fe) (>3mg/L) in the groundwater yielded by GS suggest they are at Fe reducing stage. This study demonstrates that BS aquifers with low As and Mn concentration also exists and can be exploited for rural drinking water supply. However, in many areas, they may be present in small lenses and may not serve as continuous aquifers that can be targeted as sustainable drinking water source.