PNNL-13501 - Pacific Northwest National Laboratory

Summary and Conclusions
Over the past few years the Laboratory has made great
strides in advancing the use of In Situ Redox
Manipulation technology for the transformation/
degradation of redox-sensitive contaminants such as
chromate and trichloroethylene. The technology involves
the injection of a dithionite solution that transforms
(structural) Fe(III) to Fe(II), thus establishing a reductive
Fe(II) barrier in the flow path of the plume. Large-scale
field implementation of the In Situ Redox Manipulation
technology is now under way at Hanford for chromate
remediation. Given the large magnitude of the operation,
it is critical to evaluate the performance and costeffectiveness
of any proposed design for the barrier. The
main components of the cost model are the drilling costs,
the cost of reagents, the cost of waste disposal, and the
labor costs. The key design variables are the number of
wells, their flow rate, and the number of regenerations of
the barrier. The design variables affect the total cost in
different ways. Increasing the number of wells will
decrease the total amount of dithionite required and will
generate less waste. A smaller barrier width will decrease
the loss and wastage of reductive capacity, but will
increase the costs related to more frequent regeneration.
We developed a code for optimal selection of key design
parameters that would yield a robust design and also
minimize the total cost of the remediation effort. The cost
model is driven by the process model consisting of two
parts: 1) the creation of the barrier by injecting the
reducing agent, and 2) the reoxidation of the barrier by
the contaminant (and also oxygen if aerobic) in the
invading groundwater. We also considered the loss of
reducing capacity that can occur due to rainfall events and
due to the diffusion of oxygen from the atmosphere. The
solution to the reoxidation submodel was combined with
the known solution to the reduction submodel to develop
an analytic objective function (cost of the operation) in
terms of the key decision variables (assuming a
homogeneous system). We developed an engineering
design tool to obtain optimal design parameters and also
their sensitivity to uncertainties in the physiochemical
description of the problem.
Publication
Chilakapati A, MD Williams, SB Yabusaki, CR Cole, and
JE Szecsody. 2000. “Optimal design of an in situ Fe(II)
barrier: Transport limited reoxidation.” Environmental
Science and Technology (submitted).
Earth System Science 199