DNAPL remediation of fractured rock evaluated via numerical simulation
Abstract
Fractured rock formations represent a valuable source of groundwater and can be highly
susceptible to contamination by dense, non-aqueous phase liquids (DNAPLs). The goal
of this research is to evaluate the effectiveness of three accepted remediation technologies
for addressing DNAPL contamination in fractured rock environments.
The technologies under investigation in this study are chemical oxidation, bioremediation,
and surfactant flushing. Numerical simulations were employed to examine the
performance of each of these technologies at the field scale. The numerical model
DNAPL3D-RX, a finite difference multiphase flow-dissolution-aqueous transport code
that incorporates RT3D for multiple species reactions, was modified to simulate fractured
rock environments. A gridding routine was developed to allow the model to accurately
capture DNAPL migration in fractures and aqueous phase diffusion gradients in the
matrix while retaining overall model efficiency. Reaction kinetics code subroutines
were developed for each technology so as to ensure the key processes were accounted for
in the simulations. The three remedial approaches were systematically evaluated via
simulations in two-dimensional domains characterized by heterogeneous orthogonal
fracture networks parameterized to be representative of sandstone, granite, and shale.
Each simulation included a DNAPL release at the water table, redistribution to pools and
residual, followed by 20 years of ‘ageing’ under ambient gradient conditions. Suites of
simulations for each technology examined a variety of operational issues including the
influence of DNAPL type and remedial fluid injection protocol. Performance metrics included changes in mass flux exiting, mass destruction in the matrix versus the fractures,
and percentage of injected remedial fluid interacting with the target contaminant.
The effectiveness of the three remediation technologies covered a wide range; the mass of
contaminants destroyed were found to range from 15% to 99.5% of the initial mass
present. Effectiveness of each technology was found to depend on a variety of critical
factors particular to each approach. For example, in-situ chemical oxidation was found
to be limited by the organic material present in the matrix of the rocks, while the
efficiency of enhanced bioremediation was found to be related to factors such as the
location of indigenous bacteria present in the domain and rate of bioremediation.
In the chemical oxidation study, the efficiency of oxidant consumption was observed to
be poor across the suite of scenarios, with greater than 90% of the injected permanganate
consumed by natural oxidant demand. This study further revealed that the same factors
that contributed to forward diffusion of contaminants prior to treatment are critical to this
remediation method as they can determine the extent of contaminant destruction during
the injection period.
Bioremediation in fractured rock was demonstrated to produce relatively good results
under robust first-order decay rates and active microorganisms throughout the fractures
and matrix. It was demonstrated that under ideal conditions, of the total initial mass
present, up to 3/4 could be reduced to ethene, indicating bioremediation may be a
promising treatment approach due to the effective penetration of electron donor into the matrix during the treatment period and the ongoing treatment that occurs after injection
ceases. However, when indigenous bacteria was assumed to exist only within the
fractured walls of sandstone, it was found that under the same conditions, the rate of
dechlorination was 200 times less than the Base Case. Since the majority of the mass
resided in the matrix, lack of bioremediation in the matrix significantly reduced the
effectiveness of treatment.
Surfactant treatment with Tween-80 was proven to be a relatively effective technique in
enhanced solubilisation of DNAPL from the fractures within the domain. However, by
comparing the aqueous and sorbed mass at the start and end of the Treatment stage, it is
revealed that surfactant treatment is not efficient in removing these masses that reside
within the matrix. Furthermore, DNAPLs identified in dead end vertical fractures were
found to remain in the domain by the end of the simulations across all scenarios studied;
indicating that the injected surfactant experiences difficulty in accessing DNAPLs
entrapped in dead end fractures.
Altogether, the results underscore the challenge of restoring fractured rock aquifers due
to the field scale limitations on sufficient contact between remedial fluids and in situ
contaminants in all but the most ideal circumstances.