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PI-Submitted Research Highlights for
Subsurface Biogeochemical Research Program

Predictive Numerical Modeling Provides Insights into Changes in Contaminant Mobility under Increased and Extreme Precipitation Scenarios

Haruko Wainwright

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July 8, 2019

Untangling the complex interactions between contaminant dilution and re-mobilization accompanying significant precipitation increases through numerical simulation.

The Science
Climate change—through precipitation regime shifts or extreme precipitation events—can have a significant impact on the mobility of residual contaminants at sites where remediation solutions and management are based on an expected range of site conditions. This study used numerical simulations to evaluate and quantify the impact of such shifts or events; in particular, the competing factors of dilution and re-mobilization. Results showed that contaminant concentrations immediately decreased following extreme precipitation events due to dilution, but subsequently increased several years later due to re-mobilization of contaminants from the source zone.

The Impact
The impact of changes in contaminant mobility and concentration due to extreme precipitation and shifts in the precipitation regime were found to last for several decades, depending on monitoring well locations, performance metrics and site conditions. The results of this study suggested critical considerations for the design of long-term engineered systems such as surface capping structures, and for not only monitoring their efficacy, but also for defining threshold levels of precipitation that could drastically alter the system behavior.

Summary
Through numerical modeling of un-saturated/saturated flow and transport, a team of scientists evaluated the effect of increasing and decreasing precipitation, as well as the impact of potential failure of surface barrier systems. The approach was demonstrated using a case study involving the simulation of the transport of non-reactive radioactive tritium at the U.S. Department of Energy's Savannah River Site F-Area. Results showed that such hydrological changes significantly impact groundwater concentrations.  After an initial dilution effect, the modeling results identified a significant concentration increase some years later as a consequence of contaminant mobilization. Threshold levels of precipitation were identified, above which the contaminant concentration/exports were affected. The results suggest the importance of source zone monitoring to detect re-mobilization and highlight surface barrier design requirements needed to reduce the impact of hydrological changes.

Contacts
BER Program Manager 
Paul Bayer
Department of Energy
Paul.Bayer@science.doe.gov

Principal Investigator
Haruko Wainwright
Lawrence Berkeley National Laboratory
HMWainwright@lbl.gov

Funding
This material is based upon work supported as part of the ASCEM project, which is funded by the U.S. Department of Energy (DOE) Office of Environmental Management, and as part of the Lawrence Berkeley National Laboratory (LBNL) Science Focus Area, which is funded by the Office of Biological and Environmental Research within the DOE Office of Science, both under Award Number DE-AC02-05CH11231 to LBNL This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science user facility supported by the DOE Office of Science under Contract No. DE-AC02-05CH11231. The first and second authors acknowledge the support by the National Science Foundation under Grant No. 1654009.

Publications
Libera, A., de Barros, F. P., Faybishenko, B., Eddy-Dilek, C., Denham, M., Lipnikov, K., Moulton, J. D., Maco, B. & Wainwright, H. (2019). Climate change impact on residual contaminants under sustainable remediation. Journal of Contaminant Hydrology 226, 103518 (2019). [DOI:10.1016/j.jconhyd.2019.103518].

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