a more dilute “plume” emanating beneath the window (Fig. 2C). However, As
concentrations remain locally high beneath the simulated wetland, both on the
leading and trailing edge of the high As plume, where flow is relatively diminished
but does not cease. On the trailing edge (closer to the Mekong River), As follows
flowlines above the dilute plume, and is thereby elevated in concentration just
below the clay layer. However, upstream of the dilute plume, the hydraulic
gradient between the surface and the underlying aquifer is lower than the base
case, resulting in lower porewater velocities and more accumulation of As in
solution along the vertical flowpath, thus resulting in higher As concentrations
within the underlying aquifer.
The results of these simulations are clear; only small differences in subsurface
heterogeneity, including natural textural variations or anthropogenic perturbations
(e.g., excavation) are required to completely alter the subsurface flow regime
and, in parallel, the subsurface spatial distribution of As. Such alterations in flow
should be considered prior to major alterations in the landscape or groundwater
extraction. Further, the simulations stress the importance of the relationship
between biogeochemical processes and hydrology. Future land use alterations in
the Mekong Delta will disrupt groundwater flow and hence the sources of As will
become even more obscured, shifting the spatial patterns of As concentration in
groundwaters. Thus, wells containing low As (