Authors:
Ph.D, P.E. Dave T. Adamson | GSI Environmental Inc. | United States
Henrik Engdal Steffensen | NIRAS A/S | Denmark
Ph.D, P.E. Charles J. Newell | GSI Environmental Inc. | United States
Niels D. Overheu | Capital Region | Denmark
Mads Terkelsen | Capital Region of Denmark, Center for Regional Development | Denmark
Peder Johansen | Capital Region | Denmark
Line Moerkebjerg Fischer | Capital Region | Denmark
Charlotte Riis | NIRAS A/S | Denmark
Anders G. Christensen | NIRAS | Denmark
Background
Matrix diffusion is a general term that is used to describe the transport of contaminants in heterogeneous media where diffusion processes play a major role in the storage and release of contaminants from low permeable layers. Recently, there has been an increased recognition that matrix diffusion processes are a significant factor controlling the success of groundwater remediation. New field techniques and site characterization approaches have, consequently, been developed to measure contaminants that have diffused into low permeability (“low-k”) zones and assess their impact on groundwater quality.
With the knowledge that significant accumulation of contaminants can occur in low-k zones at sites, there has been the concurrent desire to apply groundwater models to estimate future impacts to groundwater affected by matrix diffusion processes. However, most conventional groundwater transport models either do not have the capability to model matrix diffusion, or if they do, cannot simulate this process accurately.
This leads to a risk of overlooking an essential driver for contaminant migration and underestimating source longevity. Including matrix diffusion processes in conceptual site models (CSMs) thus have a direct impact on risk assessment as well as on remedial strategy and design.
Objective
The objective of this project is to investigate the extent of matrix diffusion at three contaminated sites in Denmark and the implications on risk assessment and remedial strategy at these sites. The project is conducted in three phases, including focused site investigations, modelling and finally deduction of general recommendations for means to achieve an understanding of matrix diffusion in site investigations and risk assessments.
Conclusion
Detailed matrix diffusion characterization has been carried out at three sites and has included MIHPT logs, depth specific water sampling and soil coring with detailed subsampling. Investigations has been targeted at studying the low permeable layers and their interfaces to transmissive zones. Based on the findings, groundwater transport modeling will be performed for two of the sites to evaluate the potential future impact of matrix diffusion on groundwater quality.
The modeling approach will be using a conventional numerical model, MODFLOW-MT3D, but one that is configured to model a 2-dimensional slice of the subsurface in the X (in the direction of groundwater flow) and Z (vertically downward) directions, with extremely high grid resolution (centimeters).
There is, however, some uncertainty on whether the numerical modeling will be impractical as computer run times may be too long. In that case, we will rely on a new analytical matrix diffusion modeling tool developed by GSI for the U.S. Department of Defense, the Matrix Diffusion Toolkit (MDT) and adapt the models in the matrix diffusion toolkit to model the two sites. This toolkit has two separate analytical models that have the advantage of very fast run-times on personal computers.
Data from the MDT simulations (eg. Flux and time) can later be used as input data in conventional Numerical groundwater models (eg. MODFLOW) for evaluation of risk assessments, remediation criteria, remediation time frames etc.
Finally, based on the findings for all three sites, general recommendations will be deducted on how to approach remedial investigations and risk assessment at other contaminated sites to include matrix diffusion processes in the CSM.
The modeling will be carried out in December 2014 –March 2015 followed by the deduction of general recommendations. The results will be available in due time to be presented at the conference in June 2015.