Vulnerability of Deep Groundwater in the Bengal Basin to Contamination: the Role of Physical and Chemical Aquifer Heterogeneity and Pumping

Abstract:

Deep (≥150 m) groundwater is an indispensable resource in the Bengal Basin providing arsenic (As)-safe drinking water to millions of people. Simplifying the highly heterogeneous fluvio-deltaic Bengal Basin aquifer system into a homogeneous-anisotropic system, basin-scale modeling has shown that the deep groundwater can be utilized safely for millennia over much of the arsenic-affected area if irrigation remains shallow. However, the extent to which physical (variations in hydraulic conductivity, K) and geochemical (variations in sorption, Kd) heterogeneity as well as pumping affect the timescale of downward migration of contaminated shallow groundwater is unknown. Here we imbed detailed heterogeneous representations of physical and chemical aquifer properties and pumping rates within a basin-scale numerical groundwater flow model. Advective particle tracking shows that physical heterogeneity does not significantly affect the mean and median travel time of As transport to 150 m depth. However, it reduces the travel time for the fastest 10% particles by a factor of two, on average. In some cases, heterogeneity causes a shift in recharge locations from As-safe to As-unsafe areas. The distribution of deep pumping does not affect vulnerability as along as the irrigation pumping remains shallow, supporting conclusions from previous studies. Without sorption, >80% of the aquifer area at 150 m depth becomes contaminated after 1000 years. This reduces to <1% when either uniform sorption or sand-only sorption are considered. However, should As sorb only onto the less permeable aquifer materials (i.e. silts and clays), 30% of the area becomes contaminated on average. This suggests that both physical and chemical heterogeneity, as well as their correlation, are critical for managing deep groundwater in the Bengal Basin.