of the Problem
Successful in situ bioremediation requires adequate
mixing of groundwater within the treatment
Mixing delivers food and other metabolic requirements
to the active microorganisms and removes their waste
products. Mixing is especially critical when the
treatment plan calls for adding supplements or microorganisms
to the aquifer. Cleanup will not be effective unless
those additives are adequately dispersed.
Peter Kitanidis is evaluating technologies that promote
aquifer mixing. He and his
research team are developing tools that will help
practitioners design effective chemical delivery
and mixing systems.
Strategies for Cost-Effective Chemical Delivery and
Mixing for Bioremediation
In situ bioremediation plans often call for injection and extraction
wells or other technologies that will ensure delivery and removal
of the reactants and products of the cleanup reactions. However,
it can be difficult to anticipate their effectiveness in the
heterogeneous conditions of an aquifer. WRHSRC researcher Peter
Kitanidis and his research team are working on this problem.
They are are developing computer models that
help practitioners visualize aquifer conditions and
design treatment systems that enhance aquifer
on highlighted words and illustrations
or links will pop up.
Dr. Kitanidis, a professor of Environmental Engineering at Stanford
University, comments that mixing is an understudied part of bioremediation, “Laboratory
studies often focus at the bench scale and do not consider the
difficulties involved in scaling up.” Several factors make
mixing a difficult process to anticipate. First, aquifers are
heterogeneous. Variation in the geology of the aquifer will influence
characteristics such as dispersion and sorption and it can be
prohibitively expensive to determine geologic characteristics
in detail. Second, Kitanidis points out that there is a lack
of process understanding of mixing in aquifers. Most mixing studies
have focused on surface conditions where flow is turbulent. However,
in the porous media of a geologic formation viscosity dominates
and suppresses turbulence. The slow rate of mixing in non-turbulent
systems also means that it is likely to be the limiting factor
for chemical reactions. Third, mass transport in a formation
is influenced by external factors that may be hard to predict.
For example, the regional flow field, may respond to changes
in nearby river stage.
A Case Study at Oak Ridge National Laboratory
In recent years, Kitanidis and his team have informed their
mixing studies with work at a field site at Oak Ridge National
Laboratory. The site is a good place to study mixing because
the aquifer geology, chemistry, and flow field are extremely
complex. The goal at the site is for chemical reduction of Uranium
(VI) to less mobile Uranium (IV). The aquifer is highly weathered
saprolite and, in addition to the high uranium concentration,
the pH is exceptionally
low (~3.5) and nitrates are exceptionally high (~10 g/L). Nitrates
need to be removed and pH needs to be raised in a way that permits
biological activity but prevents metals from precipitating and
clogging the porous medium. In addition, the regional flow field
is influence by a nearby stream so that the flow rates and direction
change with rainfall events.
A team of researchers from Oak Ridge National Laboratory and
Stanford University (under the direction of Craig
Criddle and Philip
developed a cleanup process involving both above ground and
in situ treatment steps. Injection and
extraction wells are set up to create two recirculating loops
(Figure 1 and Figure 2). The
outer loop captures contaminated water and brings it to the surface
removal of metals and nitrogen. Once it
is reinjected, it creates a “protected” zone around
the inner loop where the aquifer chemistry is appropriate for
microbial uranium reduction.
Kitanidis and his team developed mathematical models of the
flow, transport and biogeochemistry of the system. The models
enable the team to visualize injection, extraction, and recirculation
patterns and compare predictions with the results of experiments
and field tests. They also enable determination of breakthrough
curves and residence times for the reactants and products involved
in the cleanup reactions.
Stanford University graduate students Mike
Fienen and Jian
Luo designed the models as functions that
run in the mathematical software MATLAB ®.
They describe the first module, called Complex Capture Zone
Routine or ComCZAR, in a forthcoming article in the Journal of
Hydrology. A public use version of the model is available online
( http://www.stanford.edu/~fienen/software/ ).
ComCZAR is a two-dimensional, analytical model that creates an
image of streamlines showing
capture zones for wells in a layered homogenous aquifer with
anisotropic transmissivity and a regional flow field.
Fienen, Luo, and Kitanidis developed a second module for delineation
of capture, release, and recirculation zones. This series of
MATLAB algorithms also enables the user to create isochrons (contours
of equal travel time) and breakthrough curves for extraction
wells. The authors are preparing a paper on this module and a
version is available online at (http://www.stanford.edu/~jianluo/software/ ).
and his team hope these models can be applied to other field
sites where injection and extraction wells will be implemented.
They believe that a better understanding of aquifer mixing a
suite of effective computer models can help practitioners develop
more cost-effective bioremediation systems. Over the next
year, they plan
to develop a manual on chemical delivery and mixing and the
For More Information
Peter Kitanidis, or refer to the following references:
Fienen, M. N., Luo, J. and Kitanidis, P. K., Semi-analytical,
homogeneous, anisotropic capture zone delineation. Journal
of Hydrology (in review), 2004. software
Luo, J., Fienen,
M. N. and Kitanidis, P. K., A MATLAB Implementation for
Two-Dimensional Steady-State Groundwater Flow Field
Created by Multiple Extraction/Injection Well Pairs.
(To be submitted). software
Luo, J., Cirpka, O.A., Wu, W-M.,
Fienen, M.N., Jardine, P.M., Mehlhorn, T.L., Watson,
D.B., Criddle, C.S., and
Kitanidis, P.K. Mass-Transfer Limitation for Nitrate
Removal in a Uranium-Contaminated Aquifer at Oak Ridge,
Water (submitted), 2004.
Luo, J. and Kitanidis, P. K.,
Fluid residence times within a recirculation zone created
by an extraction-injection
well pair. Journal of Hydrology, 295(1-4): 149-162,
Luo, J., Fienen, M. N. and Kitanidis, P. K.
3-D Groundwater Flow Modeling For the Oak Ridge Reservation
Finite-Volume Method on An Unstructured Grid System.
the International Groundwater Symposium, Berkeley,
California, March 25-28, 2002.