PollutionEngineering June 2012 : Page 22
Matrix Diffusion Challenges & Potential Solutions Contamination that seeps into low permeability soils provides tough treatment conditions. There is now a treatment alternative. By CHARLES NEWELL, ROBERT BORDEN and ED ALPERIN T he history h h i is t to ry of o f ch chlorinated h l lo ri i na t te d dra-solvent cleanup has dra matically changed over the years. Prior to the 1970s, the problems caused by chlorinated solvent releases were basi-cally unknown  . Then in the 1970s and 1980s, the presence of dissolved solvents in mobile groundwater was recognized. This was followed by the “DNAPL Paradigm” in the 1990s, where the chlorinated solvent concep-tual model was expanded to include the presence of Dense Non-aqueous Phase Liquids (DNAPL) in source zones. Despite expensive efforts by site owners, remediation specialists and researchers, complete cleanup of chlorinated solvent sites to drinking water standards, such as the Maximum Concentration Level (MCL), remains the exception rather than the rule [2, 3] . The difficultly in remediating chlo-rinated sites led researchers and reme-diation specialists to reexamine the basic processes controlling site cleanup. In the 22 Pollution Engineering JUNE 2012 Figure 1. Depiction of matrix diffusion process. During the loading period, a solvent plume moves through the transmissive sand, and then diffuses into the non-flowing low-permeability silts immediately above and below the plume. When the contaminants are removed from the transmissive sand (either naturally or by active remediation), the back diffusion period begins and contaminants move from the low permeability silts and re-contaminate the transmissive sand.
Matrix Diffusion Challenges And Potential Solutions
Contamination that seeps into low permeability soils provides tough treatment conditions. There is now a treatment alternative.
The history of chlorinated solvent cleanup has dramatically changed over the years. Prior to the 1970s, the problems caused by chlorinated solvent releases were basically unknown. Then in the 1970s and 1980s, the presence of dissolved solvents in mobile groundwater was recognized. This was followed by the “DNAPL Paradigm” in the 1990s, where the chlorinated solvent conceptual model was expanded to include the presence of Dense Non-aqueous Phase Liquids (DNAPL) in source zones.Despite expensive efforts by site owners, remediation specialists and researchers, complete cleanup of chlorinated solvent sites to drinking water standards, such as the Maximum Concentration Level(MCL) , remains the exception rather than the rule[2, 3].
The difficultly in remediating chlorinated sites led researchers and remediation specialists to reexamine the basic processes controlling site cleanup. In the 1970s, researchers recognized that dissolved contaminants could diffuse into lower permeability zones over time, gradually diffusing back out through a process called “matrix diffusion.” Unfortunately, the critical importance of this process in controlling site cleanup has been under appreciated.
Matrix diffusion is the process where contaminants present in higher permeability zones (such as sands or gravels) will diffuse into adjacent low permeability zones (most commonly silts or clays). Note that diffusion is a different process than dispersion; dispersion is the spreading of dissolved contaminants based on the movement of groundwater while diffusion is the movement of dissolved contaminants from high concentration areas to low concentration areas.Figure 1 shows how matrix diffusion works in heterogeneous aquifers.
Because the loading period (middle panel in Figure 1) can extend for decades, matrix diffusion can result in contaminants penetrating meters into the low permeability zones. The back diffusion period can extend much longer, as shown the following example.
Researchers at a site in Connecticut performed a detailed evaluation of a chlorinated solvent release. The site contained DNAPL in the source zone, so the site Owners and their consultants implemented a source isolation project using sealed sheet piling in 1995 (Figure 2). The down gradient Groundwater was then expected to achieve groundwater cleanup standards within a few years. While groundwater quality did improve , solvent concentrat ions appeared to level off and form a long tail. Detailed investigations of the aquifer down gradient by the research team Showed that the contaminant (TCE) had diffused almost 3 meters into the fine grained aquitard underlying the aquifer and was sustaining concentrations in the aquifer downgradient of the now-isolated source zone. They calculated that almost 3,000 kilograms of TCE was present in the aquitard and showed via numerical modeling that “the aquifer TCE will remain above the MCL for centuries.” Figure 2 shows concentration vs. time data from the area downgradient of the isolated source zone and the expected vs. observed patterns in concentration.
Matrix diffusion can have profound effects on remediation of chlorinated solvent sites. Matrix diffusion emerges as a key problem as sites get older or remediated:
Clearly, chlorinated solvent releases evolve with time. In the initial state, the primary issue is presence of DNAPL in source zones. With time, DNAPL is depleted through dissolution and/or volatilization.However, plumes form and contaminants may be slowly driven into lower permeability zones via diffusion and slow advection, a process that “increases the entropy” (the disorder) of the site and makes it more difficult to clean up. …A common feature at late stage sites is a large dilute groundwater plume with chlorinated solvents concentrations in the range of 10s to 100s of ìg/L. Furthermore, at late stage sites little remains to differentiate source zones and plumes; rather what is left is a zone that has elements of a continuing source and elements of a plume.
Several research initiatives are underway to find better ways to characterize, understand and model matrix diffusion processes. A research consortium of Colorado State University (CSU), the University of Guelph, the University of Kansas and GSI Environmental are performing basic research comprised of lab work, field testing and modeling. GSI Environmental and CSU are developing the Matrix Diffusion Toolkit, a spreadsheet based modeling tool that will be available in late 2012 to help users determine if matrix diffusion is important at Their site, and what the potential impacts may be. Several groups are focusing on how to remediate the low-permeability zones once they are charged up with solvents. This includes the application of shear-thinning fluids that help force injected amendments into lower-permeability zones. Others are focusing on how to remediate the low-permeability zones once they are charged up with solvents.
Current thinking by the research community and by remediation experts is that some technologies may be better suited at removing the contaminants from the low permeability zones than others. For example, thermal technologies are not as affected by aquifer heterogeneity as injection-based technologies.Additionally, technologies such as excavation or deep soil mixing (also called ZVI Clay) can be used to disrupt or render harmless the low-permeability zones loaded with contaminants from matrix diffusion.
An emerging concept for managing low-permeability zones is Sustained Treatment. Sustained treatment recognizes that the on-going treatment effect of some technologies extends for a much longer period than other technologies (Figure 3). While thermal technologies or in-situ chemical oxidation may only be actively treating during and shortly after the actual treatment period, enhanced bioremediation may last several years because it can 1) leave behind biomass that will decay and serve as a type of electron donor after active injection is over (this is called endogenous decay); and 2) the addition of bioremediation chemicals can charge up naturally occurring minerals in the subsurface that will abiotically treat chlorinated solvents long after the biodegradation project is over.
The importance of sustained treatment is that while it is difficult to remove contaminants that have diffused into low permeability silts and clays, the contaminants can be treated while they diffuse out. In other words, the goal is long-term interception of contaminants using technologies that only have to be infrequently applied.
Improving treatment today
Enhanced anaerobic bioremediation is one of the best methods of treating Chlorinated solvents. However, injection- based technologies can only work if the treatment reagent is effectively delivered and contacts the contaminant. This is particularly challenging in heterogeneous aquifers where contaminants have diffused into lower permeability Zones that are difficult to treat. However, distribution and resulting treatment can be improved through better injection system design. A 2009 paper showed that 3D flow and transport models could be used to optimize injection system designs and distribute the electron donor throughout more of the aquifer.
Figure 4a shows the distribution of a slow release electron donor from an emulsified oil substrate (EOS) in a heterogeneous aquifer under typical injection techniques (poor treatment).The blue areas are treated zones where the EOS is in direct contact with the contaminant, providing optimum treatment.
However the white areas do not have EOS, so treatment will be limited by slow mass transfer between the treated and untreated zones. Figure 4b shows the effect of using 3D flow and transport models to optimize injection system designs to effectively distribute the EOS throughout the aquifer resulting in greater contact between the electron donor and the contaminated matrix.
While it is not possible to treat every single spot, improving treatment can accelerate cleanup and reduce the time required for site closure. Figure 5 shows the effect of increasing the fraction of aquifer in direct contact with electron donor (volume contact efficiency) on the estimated time to reduce contaminant mass by 95 percent for a moderated sorbing contaminant such as TCE (retardation factor R = 3). Increasing the contact efficiency from 19 percent to 55 percent should reduce the cleanup time from 65 years to 12 years. For an electron donor such as EOS that can last more than five years, only two or three injections may be required to achieve closure.
The good news is that cleanup times can be reduced by designing and installing better injection systems. The bad News is that better injection systems may increase up-front costs. Fortunately, the Department of Defense Environmental Science and Technology Certification Program (ESTCP) has supported the development of simple-to-use spreadsheet tools to help develop an optimum design for a site (search for “emulsion design tool” at www.serdp.org to download a copy). Figure 6 shows the effect of improving contact efficiency on cumulative net present value (NPV) based on 5 percent rate of return. Lower design contact efficiency (20 percent) results in lower initial costs, but extends the operating period to 60 years. Higher design contact efficiency (55 percent) results in higher initial costs, but lower life-cycle costs due to the more rapid site closure.
Reducing future costs
A significant part of managing heterogeneous sites is maintaining an anaerobic bioremediation system for years or even decades. The cost analysis presented above assumes that a substrate prepared with traditional soybean oil would have to be re-injected once every five years. If the frequency could be reduced to once Every 10 or 20 years, this could significantly reduce long-term management costs by eliminating additional mobilizations and material costs.
Different substrates were studied to identify electron donors that would gradually ferment to hydrogen and acetate, providing a slow, nearly constant supply of electron donor to maintain reducing conditions at sites where matrix diffusion was a problem. The different substrates were incubated with a rich inorganic nutrient broth at 37ºC instead of 15ºC to accelerate fermentation and generate results within a few years. Figure 7 shows that nearly all traditional substrates (chitin, lactate and vegetable oils) were depleted in one year or less. However, several extended release substrates (ER4, ER5 and ER6) performed very well, releasing electron equivalents more than three times as long as soybean oil. Emulsified soybean oil has been shown to last 3 to 5 years Or more at many sites. Based on the elevated temperature results, ER4, ER5 and ER6 could potentially last 10 to 20 years, depending on site conditions.
Robert (Bob) Borden, Ph.D., P.E. is principal environmental engineer at Solutions-IES Inc., headquartered near the Research Triangle Park. Bob is available at rcborden@solutionsies.Com.
1. Sale, T., C. Newell, H. Stroo, R. Hinchee, and P. Johnson, (2008), Frequently Asked Questions Regarding Management of Chlorinated Solvent in Soils and Groundwater, Developed for the Environmental Security Testing and Certification Program (ER-0530). <www.gsinet.com/Publications/papers2.asp>
2. McGuire, T.M., J.M. McDade, and C.J. Newell, 2006. Performance of DNAPL Source Depletion Technologies at 59 Chlorinated Solvent-Impact Sites, Ground Water Monitoring and Remediation, Vol 26, No. 1, pg 73-84.
3. Interstate Technology and Regulatory Council, 2012. Integrated DNAPL Site Strategy Technology/Regulatory Guidance ITRC Integrated DNAPL Site Strategy Team.<www.itrcweb.org/guidancedocument. asp?TID=82>
4. Chapman, S.W. and B.L. Parker. 2005. Plume Persistence Due to Aquitard Back Diffusion Following Dense Nonaqueous Phase Liquid Removal or Isolation, Water Resource Research, Vol. 41, No. 12, W12411.
5. Sale, T. and C. J. Newell, 2011. A Guide for Selecting Remedies for Subsurface Releases of Chlorinated Solvent Sites.ESTCP Project ER-05 30. Environmental Security Technology Certification Program, Washington DC. <http://serdp-estcp.org/ Program-Areas/Environmental-Restoration/ Contaminated-Groundwater/Persistent- Contamination/ER-200530>
6. Zhong L, Szecsody J, Oostrom M, Truex M, Shen X, Li X, 2011. Enhanced remedial amendment delivery to subsurface using shear thinning fluid and aqueous foam. J Hazard Mater. 2011 Jul 15;191(1-3):249-57.Epub 2011 Apr 23.
7. Newell, C.J., 2011. Three Active Remediation Alternatives for Low Permeability Zones: Heat, Augers, and Sustained Treatment.Presentation given at SERDP/ESTCP Partners in Environmental Technology Technical Symposium & Workshop, Washington D. C. Nov. 29- Dec. 1, 2011, <http://symposium2011.Serdp-estcp.org/Technical- Sessions>
8. Adamson, David T., McGuire, Travis M., Newell, Charles J., and Stroo, H. ., 2011.Sustained Treatment: Implications for Treatment Timescales Associated with Source Depletion Technologies, Volume 21, Issue 2, Spring 2011, Pages: 27–50
9. Clayton, M. H., and R. C. Borden, Numerical Modeling of Emulsified Oil Distribution in Heterogeneous Aquifers, Ground Water, 47(2): 246–258, 2009.
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