Remediation Complications

Subsurface Cracking at Hazardous Waste Sites

A modeling study conducted at the Air Force Institute of Technology shows that subsurface cracks, either natural or due to the presence of chlorinated solvents such as trichloroethylene, may result in contamination of groundwater persisting for decades, even after most of the chemicals had been removed.

 

By Capt. James M. Bell, EIT, M.SAME, USAF, Lt. Col. John A. Christ, Ph.D., P.E., M.SAME, USAF, and Junqi Huang, Ph.D.

 


 

Dense Non-Aqueous Phase Liquids (DNAPLs) are organic liquids that are denser than water.

Many common DNAPLs—chlorinated solvents such as tetrachlorothylene, carbon tetrachloride and tricholoroethylene (TCE)—are used in a variety of industrial operations and their past use has resulted in groundwater contamination at a number of U.S. military installations. TCE contamination of drinking water at Camp Lejeune, N.C., for example, has been well documented and is suspected of being the cause of severe health effects to many that resided at the base over a 30-year period.

 


PERSISTENT PROBLEM

Figure 1A typical DNAPL-contaminated aquifer is shown in Figure 1. The contaminants (visible in red) migrate from the surface where they were intentionally or accidentally deposited down through the soil as a DNAPL. When the liquid reaches the water table it will continue sinking into the groundwater due to its density. As the DNAPL sinks, it leaves behind disconnected blobs and small amounts of residual DNAPL, called ganglia. The ganglia can remain in the pore spaces between the aquifer solids. As groundwater moves past, the DNAPL dissolves—resulting in a down-gradient groundwater plume that can be both spatially and temporally extensive.

DNAPL also will settle as pools on top of low permeability layers (clay layers) where it can dissolve and move by diffusion into the layer over time. This results in a relatively slow but persistent contaminant source. These low permeability layers may contain cracks as well. These cracks may be natural. Or, as has been shown through recent research at the University of Michigan, funded by the Department of Defense (DOD) Strategic Environmental Research and Development Program, the cracks may result from the presence of the DNAPL pool sitting atop the clay layer. In either case, DNAPL can be transported into the cracks and then diffuse into the low permeability matrix.

Contaminant in these low permeability layers may function as long lasting sources—even after the bulk of contamination in the aquifer has been removed. DNAPL in these low permeability layers will dissolve and diffuse into nearby flowing groundwater, continuing to contaminate down-gradient drinking water sources decades after the original source is removed.

 

MODELING STUDY

In a recent modeling study conducted at the Air Force Institute of Technology (AFIT), the subsurface storage and transport of a DNAPL, TCE, was investigated to see how its presence might impact the persistence of the down-gradient TCE plume, a factor that directly impacts risk to down-gradient human and environmental receptors.

A number of studies have found that the rate of dissolution from the DNAPL phase to the dissolved phase decreases with time. This trend is generally attributed to the change in surface area as the DNAPL ganglia dissolve. In the AFIT study, the impact of this decreasing rate of dissolution is accounted for when modeling the evolution and persistence of the resulting dissolved contaminant plume. The conceptual model used in the research included a monitoring well located 50-m down-gradient of a DNAPL source zone with a contaminant “pool” emplaced atop a cracked low permeability layer. The monitoring well tracks the simulated dissolved contaminant concentrations over time at that location. Parameters used in the model were based on an actual DNAPL-contaminated site at Oscoda, Mich.

The Department of Defense’s Groundwater Modeling System was used to implement the conceptual model. RT3D (a component model within the system) with a user-defined reaction model simulated DNAPL dissolution into the flowing groundwater and the resultant down-gradient dissolved contaminant transport.

The model assumed the DNAPL pool sat above the cracked low permeability layer for 10 years before it was removed. Cracks in the low permeability layer were assumed to contain DNAPL. Following the removal of the DNAPL pool, the DNAPL in the cracks continued to dissolve.

Figure 2Figure 2 depicts concentrations at the monitoring well 50-m down-gradient. This simulation was run assuming that the dissolution rate is constant; and that the dissolution rate changes with DNAPL saturation, a more realistic assumption. If the dissolution rate changes with DNAPL saturation, contaminant concentrations at the well persist much longer. This suggests that risks may persist long after the source has been removed. In fact, assuming that the regulatory maximum contaminant level indicates the water is safe to consume, the more realistic assumption of a dissolution rate that changes with DNAPL saturation results in an estimate that it will take at least twice as long until the water at the well is safe, compared to when assuming a constant dissolution rate. The reason is clear when comparing the source “half-life” for the two assumptions: constant versus decreasing dissolution rate. When a constant dissolution rate is assumed, it takes about two years for the mass of DNAPL in the cracks to fall below 50 percent of the initial mass. When the dissolution rate changes with DNAPL saturation, the model suggests it will take over 50 years for the mass to be reduced to 50 percent of its initial value.

It should be understood these results are based on a number of model assumptions. In particular, it assumes that the subsurface is relatively homogeneous—that the characteristics of the high and low permeability layers are assumed to be the same throughout space. Of course, this is a major simplification. In reality, the subsurface is heterogeneous, with large spatial variations in properties. Nevertheless, the simulations still provide valuable insights.

 


 A number of studies have found that the rate of dissolution from the DNAPL phase to the dissolved phase decreases with time. This trend is generally attributed to the change in surface area as the DNAPL ganglia dissolve.


  

BENEFICIAL RESEARCH

In this work, a time-variable dissolution rate in low permeability media was examined, using a simplified computer model of an aquifer to determine its influence on contaminant concentrations in groundwater down-gradient from a DNAPL-contaminated site. The model was specifically designed to consider the insights from the research project at the University of Michigan, which demonstrated that pooled DNAPL could crack low permeability material, such as clay. The effect of these simulated processes on the evolution and persistence of a dissolved contaminant plume demonstrates that they could result in higher down-gradient concentrations that last longer than expected. At sites where DNAPL is present, remediation managers should consider these implications when making site management decisions.

The AFIT study also demonstrates the value of modeling to provide insight into how subsurface transport processes might influence important factors such as down-gradient concentration, as well as to identify site parameters that may be considered for further characterization and research.

 

*This article was co-authored by Prof. Mark Goltz, Ph.D., P.E., F.SAME, Air Force Institute of Technology, and Prof. Avery Demond, Ph.D., P.E., University of Michigan. Financial support was provided by SERDP Project ER-1737. Any opinions expressed in this article are those of the authors and do not necessarily reflect the official positions and policies of the EPA, USAF, DOD, or the U.S. government.

 


 

Capt. James M. Bell, EIT, M.SAME, USAF, was a graduate student at the Air Force Institute of Technology and is now Flight Commander, Civil Engineering, 607th Support Squadron, Osan AB, Korea; 719-289-0991, or This email address is being protected from spambots. You need JavaScript enabled to view it..

Lt. Col. John A. Christ, Ph.D., P.E., M.SAME, USAF, is Associate Professor, Civil & Environmental Engineering, and Director, Commander’s Action Group, U.S. Air Force Academy, Colo.; 719-333-3739, or This email address is being protected from spambots. You need JavaScript enabled to view it..

Junqi Huang, Ph.D., is Hydrologist, National Risk Management Research Laboratory, U.S. Environmental Protection Agency; 580-436-8915, or This email address is being protected from spambots. You need JavaScript enabled to view it..