ORIGIN OF ARSENIC RICH, YOUNG GROUNDWATER IN DEEP TUBEWELLS IN THE CENTRAL SOUTHWESTERN BENGAL BASIN

Groundwater extracted from deeper than 150 m often provides a safe alternative to shallow groundwater that is highly contaminated by arsenic in the Bengal Basin. Generally, deep groundwater is tens of thousands of years old and As-safe. However, based on field kit testing we found high As (>100 ppb) in 14 deep (150 - 240 m) groundwater samples collected along a 100-km long, transboundary, latitudinal transect in the central-SW Bengal Basin. Dissolved inorganic carbon in this high-As water has radiocarbon ages that are younger (1.7±1 ka) than the low-As deep wells (20±10 ka) found in areas 50 km south of this transect and elsewhere in Bangladesh. Based on available sediment age data and stratigraphic markers, we show that the aquifer sediments deeper than 150 m are older than Holocene suggesting downward migration of the shallow groundwater is the likely cause of the high As. Pumping-induced vertical migration is possible and could be important locally, but is unlikely to explain the observed regional pattern of groundwater age because deep pumping in this region is not widespread and occurs shallower than most of the high-As samples. The mega-city of Kolkata is located 100 km to the southwest and pumps groundwater mostly from depths shallower than 120 m. Irrigation pumping is also shallower than 100 m. Our analysis of an extensive set of lithologs shows that the aquifer system in this region is composed of sands, gravelly sands, and gravels with little or no clay down to a maximum depth of >300 m underlying a 0-20 m thick surficial silt/clay layer. This results in an overall higher permeability and lower vertical anisotropy in this area than elsewhere in the Basin. Numerical modeling indicates that this stratigraphic feature together with local topography creates a natural deep flow system in this region. Groundwater recharges in local topographic highs, is drawn deep through this high-permeability layer, and then flows upward and discharges into local topographic lows, usually in streams. Backward particle tracking suggests that the advective groundwater travel time from recharge locations to the 150-240 m depth interval is similar to that of the C-14 age.