United States Environmental Protection Agency Robert S. Kerr Environmental Research Laboratory Ada OK 74820 Research and Development EPA/600/SR-93/022 April 1993 Project Summary DNAPL Site Evaluation Robert M. Cohen and James W. Mercer Dense nonaqueous-phase liquids (DNAPLs), especially chlorinated sol- vents, are among the most prevalent subsurface contaminants identified in ground-water supplies and at waste dis- posal sites. There are several site-char- acterization issues specific to DNAPL sites including (a) the risk of inducing DNAPL migration by drilling, pumping or other field activities; (b) the use of special sampling and measurement methods to assess DNAPL presence and migration potential; and (c) devel- opment of a cost-effective character- ization strategy that accounts for DNAPL chemical transport processes, the risk of inducing DNAPL movement during field work, and the data required to select and implement a realistic rem- edy. This manual provides information to address these issues and describes and evaluates activities that can be used to determine the presence, fate, and transport of subsurface DNAPL contamination. The manual discusses the scope of the DNAPL problem, the properties of DNAPLs and subsurface media affecting DNAPL transport and fate, objectives and strategies for DNAPL site characterization, invasive and non-invasive methods of site char- acterization, and laboratory methods for characterizing fluid and media proper- ties. The manual concludes with sev- eral case histories illustrating problems specific to DNAPL sites and priority research needs for improving DNAPL site characterization. This Project Summary was developed by EPA's Robert S. Kerr Environmental Research Laboratory, Ada, OK, to an- nounce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at the back). Introduction Dense nonaqueous phase liquids (DNAPLs), such as some chlorinated sol- vents, creosote-based wood-treating oils, coal tar wastes, and pesticides, are im- miscible fluids with a density greater than water. As a result of widespread produc- tion, transportation, utilization, and disposal of hazardous DNAPLs, particularly since 1940, there are numerous DNAPL con- tamination sites in North America and Eu- rope. The potential for serious long-term contamination of groundwater by some DNAPL chemicals at many sites is high due to their toxicity, limited solubility (but much higher than drinking water limits), and significant migration potential in soil gas, groundwater, and/or as a separate phase (Figure 1). DNAPL chemicals, es- pecially chlorinated solvents, are among the most prevalent ground-water contami- nants identified in ground-water supplies and at waste disposal sites. The subsurface movement of DNAPL is controlled substantially by the nature of the release, the DNAPL density, interfa- cial tension, and viscosity, porous media capillary properties, and, usually to a lesser extent, hydraulic forces. Below the water table, non-wetting DNAPL migrates pref- erentially through permeable pathways such as soil and rock fractures, root holes, and sand layers that provide relatively little capillary resistance to flow. Visual detec- tion of DNAPL in soil and ground-water samples may be difficult where the DNAPL is transparent, present in low saturation, or distributed heterogeneously. These fac- tors confound characterization of the move- ment and distribution of DNAPL even at sites with relatively homogenous soil and a known, uniform DNAPL source. The dif- ficulty of site characterization is further compounded by fractured bedrock, het- erogeneous strata, multiple DNAPL mix- tures and releases, etc. Obtaining a detailed delineation of sub- surface DNAPL, therefore, can be very costly and may be impractical using con- ventional site investigation techniques. Furthermore, the risk of causing DNAPL migration by drilling or other actions may be substantial and should be considered prior to commencing field work. Although DNAPL can greatly complicate site char- Printed on Recycled Paper ------- Dissolved Contaminant Plumes Figure 1. DNA PL chemicals are distributed in several phases: dissolved in groundwater, adsorbed to soils, volatilized in soil gas, and as residual and mobile immiscible fluids (modified from Huling and Weaver, 1991;WCGR, 1991). acterization, failure to adequately define its presence, fate, and transport can re- sult in misguided investigation and re- medial efforts. Large savings and environmental benefits can be realized by conducting studies and implementing remedies in a cost-effective manner. Cost-effective DNAPL site management requires an understanding of DNAPL prop- erties and migration processes, and of the methods available to investigate and in- terpret the transport and fate of DNAPL in the subsurface. Lighter-than-water NAPLs (LNAPLs) which do not sink through the saturated zone, such as petroleum products, are also present and cause ground-water con- tamination at numerous sites. Although many of the same principles and con- cerns apply at both LNAPL and DNAPL sites, LNAPL site characterization is not specifically addressed in this document. Objectives This manual is designed to guide inves- tigators involved in the planning and imple- mentation of characterization studies at sites suspected of having subsurface con- tamination by DNAPLs. Specifically, the document is intended to Summarize the current state of knowledge for characterizing DNAPL-contaminated sites; Develop a framework for planning and implementing DNAPL site char- acterization activities; Provide a detailed discussion of the types of data, tools, and methods that can be used to identify, char- acterize, and monitor DNAPL sites, and an analysis of their utility, limi- tations, risks, availability, and cost; Identify and illustrate methods, in- cluding the development of concep- tual models, to interpret contaminant fate and transport at DNAPL sites based on the data collected; Assess new and developing site characterization methodologies that may be valuable and identify addi- tional research needs; and, Review the scope of the DNAPL contamination problem, the proper- ties of DNAPLs and media, and DNAPL transport processes to pro- vide context for understanding DNAPL site characterization. The primary goal of this manual is to help site managers minimize the risks and maximize the cost-effectiveness of site in- vestigation/remediation by providing the best information available to describe and evaluate activities that can be used to determine the presence, fate, and trans- port of subsurface DNAPL contamination. Outlook Remedial activities at a contaminated site need to account for the possible pres- ence of DNAPL. If remediation is imple- mented at a DNAPL site, yet does not consider the DNAPL, the remedy will un- derestimate the time and effort required to achieve remediation goals. Thus, adequate site characterization is required to under- stand contaminant behavior and to make remedial decisions. There is no practical cookbook approach to DNAPL site investigation or data analy- sis. Each site presents variations of con- taminant transport conditions and issues. Although there are no certain answers to many of the DNAPL site evaluation is- sues, this manual provides a framework for their evaluation. Conclusions and Recommendations As awareness of DNAPL contamination increased in the 1980s, research was con- ducted to better understand the behavior of DNAPL in the subsurface. Much of this research was an expansion of the investi- gations performed by Schwille (1988). DNAPL research is currently focusing on remediation (National Center for Ground Water Research, 1992). Throughout this progression of DNAPL research, relatively little effort has been expended on devel- oping new site characterization tools or methods for DNAPL sites. What has generally occurred at DNAPL sites is that tools and techniques utilized at contamination sites in general have been applied with varying degrees of suc- cess. Additionally, some new tools and methods have been developed and oth- ers have been adapted to better satisfy the requirements of a DNAPL site investi- gation. Site characterization strategies have also evolved to more closely match the special concerns and risks posed by DNAPL presence. Despite substantial progress, additional research on DNAPL site characterization tools and methods is warranted utilizing a variety of venues: laboratories, controlled field sites with emplaced DNAPL, and un- controlled contamination sites. Additional research and technology transfer efforts should focus on 1. Well drilling techniques to demon- strate the isolation of DNAPL zones through the use of double-cased wells or other techniques; 2. Well and boring abandonment tech- niques to demonstrate the efficacy of different grouting mixtures and ------- methods to prevent preferential ver- tical fluid migration; 3. The utility of surface and borehole geophysical methods to better char- acterize DNAPL presence and dis- tribution, and stratigraphic controls on DNAPL movement; 4. The utility of soil gas surveying to better characterize NAPL presence and related chemical migration; 5. Methods to determine in-situ NAPL saturation (e.g., borehole geophys- ics, simple quantitative sample analysis); 6. Techniques to determine field-scale constitutive relationships between saturation, capillary pressure, and relative permeability; 7. Practical field or laboratory tech- niques to delineate mobile DNAPL from DNAPL in stratigraphic traps from DNAPL at residual saturation; 8. Additional cost-effective methods to determine NAPL presence, compo- sition, and properties; 9. Techniques to better define site stratigraphy, heterogeneity, and fracture distributions; 10. The long-term capacity of capillary barriers (e.g., clayey soil layers) to prevent DNAPL movement, includ- ing methods for determining barrier continuity and time-dependent as- pects of DNAPL-mineral structure and wettability interactions; 11. Identifying the limited characteriza- tion efforts required to determine and implement appropriate reme- dial measures at DNAPL contami- nation sites; 12. Further optimization of character- ization strategies given different source, hydrogeologic, risk and rem- edy considerations; and, 13. Refinement of pilot test designs, pro- tocols, and monitoring requirements to determine the feasibility and/or technical impracticali'., f alterna- tive remedial measure.-, References Huling, S.G. and J.W. Weaver, 1991. Dense nonaqueous phase liquids, USEPA Ground Water Issue Pa- per, EPA/540/4-91/002, 21 pp. National Center for Ground Water Re- search, 1992. Extended abstracts, Proceedings of the Subsurface Res- toration Third Internationa! Confer- ence on Ground Water Quality Research, Rice University, Hous- ton, Texas, 343 pp. Schwille, F., 1988. Dense Chlorinated Solvents in Porous and Fractured Media, Lewis Publishers, Chelsea, Michigan, 146 pp. WCGR, 1991. Dense, Immiscible Phase Liquid Contaminants (DNAPLs) in Porous and Fractured Media, A Short Course, DNAPL Short Course notes, October 7-10, Kitchner Ontario, Canada, Waterloo Center for Groundwater Research, Univer- sity of Waterloo. ------- Robert M. Cohen and James W. Mercer are with GeoTrans, Inc., Sterling, VA20166. John E. Matthews is the EPA Project Officer (see below). The complete report, entitled "DNAPL Site Evaluation," (Order No. PB93-150217; Cost: $44.50, subject to change), will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Robert S. Kerr Environmental Research Laboratory U.S. Environmental Protection Agency Ada, OK 74820 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 Official Business Penalty for Private Use $300 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 EPA/600/SR-93/022 ------- |