Contaminant Fate and Reactive Transport in Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 874

Fig. 19.12 Focusing of pathlines into high permeability zones included into the simulation
domain. Pathlines computed using MODPATH (Pollock 1994 )
distribution (Werth et al. 2006 ). When flow is focused in a high-permeability zone
the velocity increases and the distance required for a solute to cross a given num-
ber of flow lines decreases. This focusing results in an enhancement of transverse
mixing and, therefore, of reactions occurring at the plume fringe (Fig. 19.12 ). These
concepts were experimentally demonstrated at the laboratory scale, in tank systems
with both instantaneous abiotic (Rolle et al. 2008c , 2009 ) and microbially mediated
reactions (Bauer et al. 2009a ).
In the present scenario modeling a rectangular high-permeability lens, 175 m
long and 0.5 m thick, was included in the simulation domain. Different model runs
were performed using different hydraulic conductivity ratios between the high-
permeability lens (K L ) and the surrounding aquifer sediments (K). Also, at this
larger scale, the focusing of the reactive fringe caused a significant increase of
mixing and reaction of toluene and oxygen. For example, a ratio of hydraulic con-
ductivities (K L /K) of 4 was sufficient to result in a steady state plume approximately
100 m shorter with respect to the base case steady state plume length (Fig. 19.8a ).
This example conceptually illustrates the effect of mixing and reaction enhancement
due to flow focusing in a simplified aquifer geometry and hydraulic conductivity
field at the typical scale of groundwater contamination. Under natural conditions,
often characterized by a high degree of physical heterogeneity, a high variability
of the mixing and reaction enhancement is likely to be expected. As shown by
Wer th e t a l . ( 2006 ), this variability reflects the extent to which the reactive fringe
of the plume is focused into different high-permeability inclusions and entails a
considerable variability in the prediction of reactive transport (Cirpka et al. 2008 ).
Besides the flow focusing in high permeability formations embedded into a
less permeable porous medium (e.g. coarse sand and gravel lenses and layers in
a fine sand aquifer), other hydrogeological configurations and/or stresses such as
the presence of impervious bodies (e.g. constructions), the pumping from a partially
penetrating well, et cetera, can result in focusing of groundwater flow. In Fig. 19.13 ,