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H. Rubin et al.
6 Summary and Conclusions
The dissolution of entrapped NAPL and solute transport in a fractured permeable
formation was formulated and simulated based on a simplified conceptual model
of the fractured formation. An immobile residual NAPL saturation is assumed to
be uniformly distributed within the permeable blocks. The initial NAPL
saturation is small such that changing NAPL saturation can be assumed to have
negligible influence on the permeability of the blocks. A steady uniform
hydraulic gradient is applied across the domain producing flow through both the
fracture network and the permeable blocks, as well as mixing between the two
types of flow. Flow through the permeable blocks gradually dissolves the NAPL.
The variable NAPL saturation does influence the rate of interphase mass transfer,
due to changes in the interfacial contact area. The interphase mass transfer
coefficient is assumed to be proportional to 8;13. However, without losing the
general features of the calculation and analysis, some other possible relationships
can be used.
Mass balance expressions relating the dissolution of NAPL in the permeable
blocks, transport of solutes in the permeable block and fracture flows, as well as
mixing between these two types of flow are formulated within a dimensionless
framework. The mass balance equations are solved with explicit finite difference
schemes based on a numerical grid incorporating the permeable blocks and the
fracture network.
Numerical simulations were performed for a domain
incorporating 12 contaminated matrix sections and an initial saturation of 0.1.
Basic calculations refer to solute whose volumetric concentration of equilibrium is
env = 1.1xlO· 3 • Results are presented for a range of mobility numbers NM = 0.55 and dimensionless initial values of interphase mass transfer coefficients KfJ =
0.1 - 10. The interphase mass transfer coefficient is a major parameter controlling
the rate ofNAPL removal in an idealized fractured permeable formation.
The presence of fractures in a permeable medium can significantly change the
characteristics of the aquifer reclamation process. Flow through the fracture
network and mixing with the permeable block flow can enhance the rate ofNAPL
removal by providing pathways for the influx of uncontaminated water to drive
NAPL dissolution, as well as pathways for the removal ofNAPL solute.
There is a tradeoff between the treatment volume of aqueous phase and the time
period for complete removal of the NAPL. The treatment volume increases with
decreasing value of the mass transfer coefficient, decreasing mobility number, and
increasing the aquifer flow rate.
Simulations performed in this study indicate that addition of surfactants can
substantially reduce the time period of complete NAPL removal and the volume of
treated water. The proportional effect of the surfactant is almost identical for high
and low values of the interphase mass transfer coefficient. However the absolute
values of changes of reclamation time and volume of treated water are more
significant in cases oflow values of the interphase mass transfer coefficient.
H. Rubin et al.
6 Summary and Conclusions
The dissolution of entrapped NAPL and solute transport in a fractured permeable
formation was formulated and simulated based on a simplified conceptual model
of the fractured formation. An immobile residual NAPL saturation is assumed to
be uniformly distributed within the permeable blocks. The initial NAPL
saturation is small such that changing NAPL saturation can be assumed to have
negligible influence on the permeability of the blocks. A steady uniform
hydraulic gradient is applied across the domain producing flow through both the
fracture network and the permeable blocks, as well as mixing between the two
types of flow. Flow through the permeable blocks gradually dissolves the NAPL.
The variable NAPL saturation does influence the rate of interphase mass transfer,
due to changes in the interfacial contact area. The interphase mass transfer
coefficient is assumed to be proportional to 8;13. However, without losing the
general features of the calculation and analysis, some other possible relationships
can be used.
Mass balance expressions relating the dissolution of NAPL in the permeable
blocks, transport of solutes in the permeable block and fracture flows, as well as
mixing between these two types of flow are formulated within a dimensionless
framework. The mass balance equations are solved with explicit finite difference
schemes based on a numerical grid incorporating the permeable blocks and the
fracture network.
Numerical simulations were performed for a domain
incorporating 12 contaminated matrix sections and an initial saturation of 0.1.
Basic calculations refer to solute whose volumetric concentration of equilibrium is
env = 1.1xlO· 3 • Results are presented for a range of mobility numbers NM = 0.55 and dimensionless initial values of interphase mass transfer coefficients KfJ =
0.1 - 10. The interphase mass transfer coefficient is a major parameter controlling
the rate ofNAPL removal in an idealized fractured permeable formation.
The presence of fractures in a permeable medium can significantly change the
characteristics of the aquifer reclamation process. Flow through the fracture
network and mixing with the permeable block flow can enhance the rate ofNAPL
removal by providing pathways for the influx of uncontaminated water to drive
NAPL dissolution, as well as pathways for the removal ofNAPL solute.
There is a tradeoff between the treatment volume of aqueous phase and the time
period for complete removal of the NAPL. The treatment volume increases with
decreasing value of the mass transfer coefficient, decreasing mobility number, and
increasing the aquifer flow rate.
Simulations performed in this study indicate that addition of surfactants can
substantially reduce the time period of complete NAPL removal and the volume of
treated water. The proportional effect of the surfactant is almost identical for high
and low values of the interphase mass transfer coefficient. However the absolute
values of changes of reclamation time and volume of treated water are more
significant in cases oflow values of the interphase mass transfer coefficient.