Effects of Dispersion Parameters and Low Permeability Porous Media on
the Contaminant Transport Behavior in the Aquifer System: A Numerical
Study
Abstract
In this study, contaminant transport behaviour in the aquifer system
(140 m × 180 m × 5 m) was analyzed using a 3-D groundwater flow and
contaminant transport model viz. MODFLOW2005 and MT3DMS. The impacts of
hydrodynamic dispersion parameters on the conservative contaminant plume
dynamics were analyzed for homogeneous and heterogeneous aquifer systems
with low permeability porous media (LPPM). The spatio-temporal
distribution of contaminant concentration and breakthrough curves (BTCs)
at 12 observation wells were used to analyze the transport dynamics due
to conservative contaminant released from a single point source over a
hypothetical study area for a period of 1 year (365 days). Results from
MODPATH show a significant variation in the pathway of groundwater for
homogeneous and heterogeneous aquifer systems. During the source loading
period, a very low value of concentration of order 10-9 mg/m3 was
observed in the LPPM region. The spatial distribution of contaminant
plume for aquifer system with LPPM varied largely as compared to
homogeneous aquifer system. The maximum value of concentration in the
aquifer with LPPM was found to be ~40% higher than the
homogeneous system after source removal. After the source removal, the
maximum value of 1.98 mg/m3 was observed for the homogeneous system at a
location away from pumping and extraction well after 730 days; however,
for a heterogeneous system with LPPM, the maximum value of 2.57 mg/m3
was observed. An early breakthrough was observed for αL= 54 m as
compared to αL= 9 m for homogeneous aquifer system, clearly depicting
the effect of longitudinal dispersivity on BTC. However, effects of
dispersivity on the rising and falling limbs of the BTC were negligible
for heterogeneous aquifer system with LPPM. Further, an impact of LPPM
and longitudinal dispersivity on the peak concentration value at
observation well (OBS-7) was undistinguishable. The numerical
simulations carried out in this study mimic the realistic heterogeneous
aquifer conditions and highlighted the relevance of LPPM and associated
transport processes on contaminant transport dynamics at field-scale,
which was usually overlooked.