How Do We Know Remediation Is Working?
Sources: Susan Hubbard & Dan Hawkes
ESD Environmental Geophysics Team Advances the Ability to Remotely Quantify Contaminant Remediation Processes, Using Laboratory, Field, and Numerical Biogeophysical Studies
The production and testing of nuclear weapons in the U.S. has created a vast volume of subsurface metal and radionuclide contamination, much of which can be found at so-called superfund sites. That 73 million U.S. citizens live within 4 miles of a superfund site only adds to the urgency and need for remediation. The U.S. Department of Energy has responded by taking on the responsibility to locate, monitor, and ultimately clean up these sites, but the challenge is formidable. In situ remediation strategies have already started to be applied at these locations, yet it is often difficult to assess the response of the subsurface to these treatments (or their ultimate remediation efficacy) using conventional wellbore-based methods, because of the range in scales and the complexity of the governing hydrological and biogeochemical processes. This difficulty is exacerbated by feedbacks that occur between remediation-induced biogeochemical transformations and flow characteristics.
The ESD Environmental Geophysical group is exploring how to quantify spatiotemporal variations in remediation-induced biogeochemical transformations using spatially extensive (but indirect) geophysical data combined with direct (but sparse) wellbore data. Figures modified from Williams et al., 2009.
These challenges are a central concern to ESD Deputy Director (and Environmental Remediation and Water Resources Program Director) Susan Hubbard and her colleagues, as they conduct research within DOE-LBNL’s Sustainable Systems Scientific Focus Area (Sustainable System SFA). Hubbard and the ESD Environmental Geophysics Group have been using biogeophysical methods, involving time-lapse geophysical techniques, to remotely monitor biogeochemical transformations associated with remediation treatments, such as the generation of gases, precipitates, or biofilms, with a recent focus on the use of the spectral induced polarization (SIP) method. This method measures the electrical resistivity and phase shift between an induced electrical current and recorded voltage. Because the frequency-dependent SIP response is sensitive to grain/pore fluid interface properties that are often altered during remediation, ESD scientists have been exploring the potential of using SIP data to quantify remediation-induced biogeochemical transformations. As noted below, over the past few years the team has conducted laboratory investigations, field studies, and numerical studies using the time-lapse SIP method to remotely quantify complex subsurface processes associated with remediation treatments.
- Laboratory experimental studies have been performed to quantify the geophysical response to the evolution of precipitates in porous media under controlled conditions.
- Recent analysis and interpretation of field-scale SIP data are beginning to reveal the complexity of biogeochemical transformations that occur during remediation in heterogeneous subsurface environments.
- To use the correlations between geophysical signatures and subsurface biogeochemical transformations for quantitatively estimating biogeochemical states and transformations, other ESD research has focused on developing estimation frameworks that can integrate the spatially extensive (but indirect) geophysical data with the direct (but sparse) wellbore measurements, both collected over time during a remediation treatment.
Together, these studies provide the foundation and methodology needed for noninvasive quantitative field-scale estimation of biogeochemical parameters over space and time. Although the discussions provided here focus on the use of these methods for improving understanding and assessment of coupled biogeochemical-hydrological processes associated with environmental remediation, the LBNL team has also been exploring their potential for advancing the understanding of a variety of other induced and coupled subsurface processes, such as those associated with carbon sequestration and microbially enhanced hydrocarbon recovery.