t Groundwater for domestic and irrigation purposes
is produced primarily from shallow parts of the
Bengal Basin aquifer system (India and Bangladesh),
which contains high concentrations of dissolved arsenic
(exceeding worldwide drinking water standards), though
deeper groundwater is generally low in arsenic. An
essential first step for determining sustainable management
of the deep groundwater resource is identification of
hydrogeologic controls on flow and quantification of
basin-scale groundwater flow patterns. Results from
groundwater modeling, in which the Bengal Basin aquifer
system is represented as a single aquifer with higher
horizontal than vertical hydraulic conductivity, indicate
that this anisotropy is the primary hydrogeologic control
on the natural flowpath lengths. Despite extremely low
hydraulic gradients due to minimal topographic relief,
anisotropy implies large-scale (tens to hundreds of kilometers)
flow at depth. Other hydrogeologic factors,
including lateral and vertical changes in hydraulic conductivity,
have minor effects on overall flow patterns.
However, because natural hydraulic gradients are low, the
impact of pumping on groundwater flow is overwhelming;
modeling indicates that pumping has substantially
changed the shallow groundwater budget and flowpaths
from predevelopment conditions.