Our idealized sediment profile, consisting of 12 m of clay with 48 m of
underlying sandy aquifer (Fig. 1), was used to construct a coupled hydrologicalbiogeochemical
2-D simulation of our field area, using calibrated biogeochemical
reactions (Kocar, 2008) and known hydrologic parameters (Kocar et al., 2008).
A grid of 35 horizontal and 36 vertical cells were used, with the upper 12 m in the
vertical direction discretized in 1 m intervals, and the lower (aquifer) portion of
the profile discretized in 4 m intervals. Two constant head boundaries were
assigned to simulate previously determined hydrologic conditions (Fig. 1). The
zone of outflow (the Mekong River) was assigned a constant head of 5.0 mbg, and
the zone of inflow (the interior wetlands) was assigned a constant head of 3.0 mbg
to achieve groundwater residence times simulated by Benner et al. (2008). All
biogeochemical processes are driven by the oxidation of organic matter, and
occur within the first 12 m (overlying aquitard) of the profile, where active
release of As is known to occur. Thus, the primary variable controlling the extent
of As release is organic matter oxidation, which is calibrated to observed spatial
patterns of dissolved organic carbon (DOC). The simulation time was 4000 years,
enough time for the aquifer to be flushed several times. In another simulation, we
insert a window of high hydraulic conductivity into the overlying surficial clays.
This window is placed within the surficial zone of As release to demonstrate the
influence of subsurface heterogeneity on As release and transport. For more
simulation details, please see the previous report by Kocar (2008).