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An Introduction to the Effect of Heterogeneities on the Characterization and Remediation of Alluvial Geosystems R. E. JACKSON INTERA Inc., 9111A Research Blvd., Austin, TX 78758

Key Terms: Geosystem, Non-Aqueous Phase Liquid (NAPL), Alluvium, Heterogeneity INTRODUCTION Heterogeneous granular sediments—sands and gravels —form the most important water-supply aquifers in many areas of North America. In the past 60 years many such aquifers have become contaminated with nonaqueous phase liquids (NAPLs) and the dissolved phases associated with them. These include glacial outwash deposits in New England, Ohio, and Ontario, the floodplain alluvium along the Columbia and Fraser rivers, the buried channel aquifers beneath the U.S. Midwest and the Canadian prairies, and the huge coalescing alluvial-fan aquifers of the U.S. Southwest basin-andrange province. These heterogeneous granular sediments are referred to in this article as ‘alluvial geosystems’ when contaminated by NAPLs. The term geosystem comprises that system composed of alluvial aquifer materials, contaminated groundwater, NAPLs, and any capillary barrier that traps or redirects migrating NAPLs. This term might be compared with the oilfield term reservoir but reflects the smaller scale, absence of lithification, and shallower depth of these aquifers, the irregular distribution of the NAPLs, as well as its much lower average NAPL saturations than are displayed by crude oil in petroleum reservoirs. Furthermore, the immiscible fluids in these pages of this journal refer to not just lighter-than-water, refined fuel hydrocarbons (LNAPLs) but also to denser-than-water, chlorinated solvents, coal tar, and creosote (DNAPLs). Hydrogeologists working before World War II, such as Meinzer (1923) and Tolman (1937), sought to quantify the relationship between textural heterogeneities and groundwater flow mainly by reference to the experimental studies of Allen Hazen in the 1890s. Hazen had shown the importance of the ‘effective diameter’ of grain size on hydraulic conductivity, although it fell short when applied to nonuniform mixtures of sands (Tolman, 1937, p. 202). Krumbein and Monk (1943) and Masch and Denny (1966) later addressed this relationship between nonuniformity in grain size (i.e., the variation in particle diameter) and permeability and hydraulic

conductivity. Once hydraulic testing of aquifers had become commonplace, Tolman (1937, p. 214) advised hydrogeologists that ‘‘the heterogeneity of alluvial deposits may render this method of permeability determination [the aquifer pumping test] more reliable than the laboratory method [the permeameter test] which is limited to one type of material in each test and results are then applied to assumed field conditions.’’ However, it was the use of hydrogeological tracers— in particular, radionuclides and dyes—that showed hydrogeologists how important heterogeneous sedimentary units would be at the field scale in the transport of contaminants through groundwater flow systems. It was the U.S. Geological Survey (USGS) that laid the groundwork for our present understanding of the heterogeneity of alluvial systems by undertaking research on behalf of the U.S. Atomic Energy Commission, now the U.S. Department of Energy. Looking back at research published in the 1950s on bomb-tritium and radioactivewaste migration, the USGS’ Theis (1963) pointed out the very large difference in the measured values of tracer dispersion in the field when compared with the much smaller values measured in laboratory soil columns. He concluded, ‘‘the reason for the increased longitudinal dispersion must lie in the wide distribution of permeabilities in any suite of sedimentary beds.’’ Theis’ interest in this matter appears to have led to the series of fundamental studies of the dispersion process by the USGS (e.g., Ogata and Banks, 1961; Simpson, 1962; Skibitzke and Robinson, 1963; and Ogata, 1970). These studies in turn led to further studies that sought to establish a clear link between aquifer heterogeneity and field-scale dispersion as measured by the length-scale parameter, the dispersivity (see Pickens and Grisak, 1981; Gelhar et al., 1985; and Neuman, 1990). However, interconnecting high-permeability units that preferentially channel contamination make ‘‘it impossible to use the standard advection-dispersion equation with effective dispersivity values’’ (Anderson, 1990). More recent attempts to address aquifer heterogeneity using stochastic processes cannot be said to have had a profound effect on the present practice of contaminant hydrogeology, a result most likely attributed to the high cost of acquiring the necessary data on the spatial

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distribution of critical hydrogeological parameters and the confused state of theoretical analysis (Anderson, 1997). Friedrich Schwille of the Federal Institute of Hydrology in Koblenz (then part of West Germany) established the scientific study of the contamination of alluvial geosystems by both LNAPLs and DNAPLs. Schwille demonstrated the importance of heterogeneities in permeability and/or texture in controlling the migration, trapping, and dissolution of non-wetting phase NAPLs. In his pioneering work, which began to appear in print in German in the 1960s and in English in the 1970s, Schwille (1975, 1981, and 1989) set the stage for the more detailed discussion of the effects of geologic complexity made at the University of Waterloo in the 1980s. In particular, Kueper and others (1989) showed how the migration of perchloroethylene (PCE) DNAPLs was affected by the entry pressures exerted by sand units of differing grain size and permeability in a heterogeneous sandbox. Mackay and Cherry (1989) stressed that remediation of NAPL-contaminated aquifers is constrained by heterogeneities in the permeability field, heterogeneities attributed to the textural differences. Our understanding of NAPLs in alluvium is still in development because of new methods of measurement that throw light on the processes of NAPL migration and trapping. For example, it is becoming increasingly clear that much lower saturations than those reported by Mercer and Cohen (1990) are the rule and that NAPLwetting conditions are common. The partitioning interwell tracer test (PITT) developed by G. A. Pope and colleagues (Jin et al., 1995; Meinardus et al., 2002) at the University of Texas at Austin provided a tool by which to measure NAPL saturations over interwell distances, and these values generally are less than 10%. Therefore, PITTs are analogous to Tolman’s pumping tests in that they measure the average interwell hydraulic conductivity rather than the hydraulic conductivity at points, as soil cores and subsequent permeameter measurements do. Furthermore, it is becoming clear that the longstanding assumption of simple water-wet geosystems, in which the NAPL does not adhere to the aquifer material and is retained in the larger pores, is most likely incorrect. Recent research has shown that mixed- or oil-wet geosystems appear to be the rule (Powers et al., 1996; Harrold et al., 2001; and Dwarakanath et al., 2002), which dictates that NAPLs will tend to occupy the smaller pores under oil-wet conditions. Needless to say, this finding has significant implications for the characterization and remediation of geosystems. Finally, it is necessary to comment on how technology has changed our view of heterogeneity and contamination. In the first place, the 1990s saw the development of some extremely useful direct-push tools for examining sandy materials, having borrowed them from geotechni-

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cal engineering. Rossabi and his colleagues, in this series of articles, describe the utility of these methods in providing fine-scale information. Second, numerical simulators and personal computers (PCs) have become so powerful that it is now possible to model processes that were too demanding for PCs in the mid 1990s. For example, Figure 1 shows the simulation of PCE migration in sand materials as reported by Kueper and others (1989) using the UTCHEM simulator (Delshad et al., 1996). It is evident that, provided engineers and geoscientists can determine the physical and chemical properties of the NAPL and the spatial distribution of the alluvial materials, considerable confidence can be placed in such simulations. THE CONTENTS OF THIS SPECIAL EDITION The series of articles that follows summarizes some of what is understood about the effect of textural and depositional-scale heterogeneities on the characterization and remediation of aqueous and non-aqueous phase contamination. The articles in this series address several issues: the distribution of NAPL contamination in alluvium at sites across the United States; the effect of heterogeneities on the performance of technologies that attempt to remove NAPLs or aqueous contamination; and the measurement of the physical properties of alluvium using direct-push methods. Other articles describe progress in the application of new technologies and in the development of geosystem models to assist our understanding of alluvial aquifers. The series begins with a review article by Sharp, Shi, and Galloway (of the University of Texas at Austin) on the architectural nature of terrigenous clastic sediments that form so many important municipal supply aquifers across North America. They provide a guide to the external geometry and internal compartments that comprise alluvial aquifers within which NAPL zones exist. They draw our attention to the importance of depositional facies as the fundamental units of hydrostratigraphic analysis. This review is followed by an article by Stan Feenstra, an independent consultant in Mississauga, Ontario, regarding the difficulty of drawing quantitative information from soil sampling of NAPLs in heterogeneous granular materials, such as alluvium. Feenstra draws on his own extensive experience at Canadian Forces Base Borden in Ontario, where controlled disposals of DNAPLs have added so much to our understanding of the characterization and remediation of alluvial geosystems. An article on DNAPL contamination in the alluvial sediments of the Atlantic Coastal Plain follows. Rossabi and his colleagues at the Savannah River Technology Center describe the adaptation of direct-push methods using the cone-penetrometer to obtain physical and


The Effect of Heterogeneities

Figure 1. Comparison of simulated versus observed perchloroethylene (PCE) migration through a water-saturated heterogeneous sand, 310 seconds following PCE release.

chemical data related to DNAPL distribution in the vadose zone at the Savannah River Site in South Carolina. The effect of heterogeneities on remediation is presented in the next three articles. Seol and others describe the effect of heterogeneities on the success of oxidant floods. Ochs and colleagues present design simulations showing a planned steam flood in layered sediments at the Savannah River Site. Peterson and Murray demonstrate how texture and stratification affect injected air in alluvium contaminated with dissolvedphase volatile organic contaminants. Finally, Hoffman, Blake, and others at the Lawrence Livermore National Laboratory in California describe how the characterization of alluvium beneath Livermore, California, has permitted not only the interpretation of solute transport but also the design of pump-and-treat operations. These seven articles, therefore, present one view of the state of practice of contaminant hydrogeology and remediation design in alluvial materials approximately 40 years after Theis identified the dominant role of heterogeneity in contaminant migration within alluvial geosystems. REFERENCES ANDERSON, M. P., 1990, Aquifer heterogeneity—A geological perspective. In Hitchon, B. (Editor), Proceedings of the Fifth Canadian/American Conference on Hydrogeology, Banff, Alber-

ta, Canada: National Ground Water Association, Dublin, OH, pp. 3–22. ANDERSON, M. P., 1997, Characterization of geological heterogeneity. In Dagan, G. and Neuman, S. P. (Editors), Subsurface Flow and Transport: A Stochastic Approach: Cambridge University Press, Cambridge, pp. 23–43. D ELSHAD , M.; P OPE, G. A.; AND S EPEHRNOORI , K., 1996, A compositional simulator for modeling surfactant-enhanced aquifer remediation: 1. Formulation: Journal Contaminant Hydrology, Vol. 23, pp. 303–327. DWARAKANATH, V.; JACKSON, R. E.; AND POPE, G. A., 2002, Influence of wettability on the recovery of NAPLs from alluvium: Environmental Science Technology, Vol. 36, No. 2, pp. 227–231. GELHAR, L. W.; MANTOGLOU, A.; WELTY, C.; AND REHFELDT, K. R., 1985, A Review of Field-Scale Physical Solute Transport Processes in Saturated and Unsaturated Porous Media: Electric Power Research Institute, Palo Alto, CA, EA-4190. HARROLD, G.; GOODDY, D. C.; LERNER, D. N.; AND LEHARNE, S. A., 2001, Wettability changes in trichloroethylene-contaminated sandstone: Environmental Science Technology, Vol. 35, No. 7, pp. 1504–1510. JIN, M.; DELSHAD, M.; MCKINNEY, D. C.; POPE, G. A.; SEPEHRNOORI, K.; TILBURG, C. E.; AND JACKSON, R. E., 1995, Partitioning tracer test for detection, estimation and remediation performance assessment of subsurface nonaqueous phase liquids: Water Resources Research, Vol. 31, No. 5, pp. 1201–1211. KRUMBEIN, W. C. AND MONK, G. D., 1943, Permeability as a function of the size parameters of unconsolidated sand: AIME Transactions, Vol. 151, pp. 153–163. KUEPER, B. H.; ABBOTT, W.; AND FARQUHAR, G., 1989, Experimental observations of multiphase flow in heterogeneous porous media: Journal Contaminant Hydrology, Vol. 5, pp. 83–95.


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Jackson MACKAY, D. M. AND CHERRY, J. A., 1989, Groundwater contamination: Pump-and-treat remediation: Environmental Science Technology, Vol. 23, No. 6, pp. 630–636. MASCH, F. D. AND DENNY, K. J., 1966, Grain size distribution and its effect on the permeability of unconsolidated sand: Water Resources Research, Vol. 2, No. 4, pp. 665–677. MEINARDUS, H. W.; DWARAKANATH, V.; EWING, J. E.; HIRSAKI, G. J.; JACKSON, R. E.; JIN, M.; GINN, J. S.; LONDERGAN, J. T.; MILLER, C. A.; AND POPE, G. A., 2002, Performance assessment of NAPL remediation in heterogeneous alluvium: Journal Contaminant Hydrology, Vol. 54, pp. 173–193. MEINZER, O. E., 1923, Outline of ground-water hydrology with definitions: U.S. Geological Survey Water-Supply Paper 494, U.S. Government Printing Office, Washington, DC, 71 p. MERCER, J. W. AND COHEN, R. W., 1990, A review of immiscible fluids in the subsurface: Properties, models, characterization and remediation. Journal Contaminant Hydrology, Vol. 6, pp. 107– 163. NEUMAN, S. P., 1990, Universal scaling of hydraulic conductivities and dispersivities in geologic media: Water Resources Research, Vol. 26, No. 8, pp. 1749–1758. OGATA, A., 1970, Theory of dispersion in a granular medium: U.S. Geological Survey Professional Paper 411-I, U.S. Government Printing Office, Washington, DC, 34 p. OGATA, A. AND BANKS, R. B., 1961, A solution of the differential equation of longitudinal dispersion in porous media: U.S. Geological Survey Professional Paper 411-A, U.S. Government Printing Office, Washington, DC, 7 p. PICKENS, J. F. AND GRISAK, G. E., 1981, Scale-dependent dispersion in

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a stratified granular aquifer: Water Resources Research, Vol. 17, No. 4, pp. 1191–1211. POWERS, S. E.; ANCKNER, W. H.; AND SEACORD, T. F., 1996, Wettability of NAPL-contaminated sands: Journal Environmental Engineering, Vol. 122, No. 10, pp. 889–896. SCHWILLE, F., 1975, Groundwater pollution by mineral oil products, in Groundwater Pollution Symposium, Proceedings of the Moscow Symposium, August 1971, IAHS-AISH Publ. No. 103, pp. 226– 240. SCHWILLE, F., 1981, Groundwater pollution in porous media by fluids immiscible with water: The Science of the Total Environment, Vol. 21, pp. 173–185. SCHWILLE, F., 1989, Dense Chlorinated Solvents in Porous and Fractured Media, Model Experiments, Translated by James F. Pankow: Lewis Publishers, Inc., Chelsea, MI, 146 p. SIMPSON, E. S., 1962, Transverse dispersion in liquid flow through porous media: U.S. Geological Survey Professional Paper 411-C, U.S. Government Printing Office, Washington, DC, 29 p. SKIBITZKE, H. E. AND ROBINSON, G. M., 1963, Dispersion in ground water flowing through heterogeneous materials: U.S. Geological Survey Professional Paper 411-B, U.S. Government Printing Office, Washington, DC, 3 p. THEIS, C. V., 1963, Hydrologic phenomena affecting the use of tracers in timing groundwater flow, in Radioisotopes in Hydrology, Proceedings Series, Tokyo: International Atomic Energy Agency, Vienna, Austria, pp. 193–205. TOLMAN, C. F., 1937, Ground Water: McGraw-Hill Book Company, Inc., New York.
