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Vadose Zone Characterization and Monitoring Current Technologies, Applications, and Future Developments

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Vadose Zone Characterization and Monitoring Current Technologies, Applications, and Future Developments 3 Vadose Zone Characterization and Monitoring Current Technologies, Applications, and Future Developments Boris Faybishenko Contributors: M. Bandurraga, M. Conrad, P. Cook, C. Eddy-Dilek, L. Everett, FRx Inc. of Cincinnati, T. Hazen, S. Hubbard, A.R. Hutter, P. Jordan, C. Keller, F.J. Leij, N. Loaiciga, E.L. Majer, L. Murdoch, S. Renehan, B. Riha, J. Rossabi, Y. Rubin, A. Simmons, S. Weeks, C.V. Williams INTRODUCTION NEEDS FOR VADOSE ZONE CHARACTERIZATION AND MONITORING Vadose zone characterization and monitoring are essential for: • Development of a complete and accurate assessment of the inven- tory, distribution, and movement of contaminants in unsaturated- saturated soils and rocks. • Development of improved predictive methods for liquid flow and contaminant transport. • Design of remediation systems (barrier systems, stabilization of buried wastes in situ, cover systems for waste isolation, in situ treat- ment barriers of dispersed contaminant plumes, bioreactive treat- ment methods of organic solvents in sediments and groundwater). • Design of chemical treatment technologies to destroy or immobilize highly concentrated contaminant sources (metals, radionuclides, explosive residues, and solvents) accumulated in the subsurface. 133 134 VADOSE ZONE SCIENCE AND TECHNOLOGY SOLUTIONS Development of appropriate conceptual models of water flow and chemical transport in the vadose zone soil-rock formation is critical for developing adequate predictive modeling methods and designing cost- effective remediation techniques. These conceptual models of unsatu- rated heterogeneous soils must take into account the processes of preferential and fast water seepage and contaminant transport toward the underlying aquifer. Such processes are enhanced under episodic natural precipitation, snowmelt, and extreme chemistry of waste leaks from tanks, cribs, and other surface sources. However, until recently, the effects of episodic infiltration and preferential flow on a field scale have not been taken into account when predicting flow and transport and developing remediation procedures. The pronounced temporal and spa- tial structure of water seepage and contaminant transport, which is dif- ficult to detect, poses unique and difficult problems for characterization, monitoring, modeling, engineering of containment, and remediation of contaminants. Lack of understanding in this area has led to severely erroneous predictions of contaminant transport and incorrect remedia- tion actions. For many years, it was assumed that wastes released or stored in the vadose zone would move slowly, if at all, through the vadose zone. Because of the emerging evidence of waste migration from leaking tanks through the vadose zone to the groundwater, scientists and engi- neers have begun to develop a strategy to investigate the vadose zone, including a comprehensive plan to assess vadose zone conditions. OBJECTIVES The overall objective of this chapter is to describe the current status, applications and future developments of vadose zone characterization and monitoring technologies using case-study data from practicing sci- entists and engineers. Using these data, we will recommend a series of site-characterization and monitoring methods, the development of both expedited and long-term vadose zone characterization and monitoring methods, and future developments for the design, performance, and post-closure of contaminated sites. Because our understanding of a site is derived from field observa- tions, this chapter describes the basic principles, advantages, and limita- tions of existing vadose zone characterization and monitoring methods, CHAPTER 3 – VADOSE ZONE CHARACTERIZATION AND MONITORING 135 using case studies from field experiences as well as the American Soci- ety for Testing and Materials (ASTM) and Environmental Protection Agency (EPA) standards related to vadose zone studies. We will present evidence that the central problem of the vadose zone investigation is the preferential fast-flow phenomena and accelerated deep-contaminant transport toward the groundwater that has been observed at several sites. The methods discussed in this chapter can be used for the following purposes: • Characterization of natural variations of flow and transport processes in vadose zone systems • Characterization of anthropogenic stresses on vadose zone systems (such as induced point and non-point infiltration, and well injection) • Design and selection of experimental methods for field and labo- ratory experiments • Design of vadose zone remediation systems • Project planning and data collection. The methods and efforts required for conceptualization, characteriza- tion, and quantification of vadose zone systems for each application will vary with site conditions, objectives of the investigation, and investigator experience. We would like to note that this chapter is not intended to sub- stitute for the thousands of excellent papers and a number of books on vadose zone problems (for example, Everett et al. 1984; Jury et al. 1991; Kutilek and Nielsen 1994; Wilson et al. 1995; Stephens 1996; Selker et al. 1999), but rather to present the main directions, advantages, and limita- tions of vadose zone characterization and monitoring methods. CONCEPTUALIZATION OF VADOSE ZONE SYSTEMS Conceptualization of vadose zone systems is needed for the inte- grated qualitative and quantitative characterization of unsaturated flow and transport processes affected by natural behavior and man-induced changes. Conceptualization is provided for any scale of investigation, including site-specific, subregional, and regional applications. Concep- tualization involves a step-wise, iterative process of developing multiple 136 VADOSE ZONE SCIENCE AND TECHNOLOGY SOLUTIONS working hypotheses for flow and transport process characterization. These hypotheses are then used in selecting a proper combination of monitoring methods, interpretation, and analysis for refinement of flow and transport conceptual models. A conceptual model is an interpretation or description of the physical system’s characteristics and dynamics. The development of a conceptual model is an important step for site characterization because an incorrect model can lead to significant errors in the development of mathematical and numerical models, thus adversely affecting predictions and planning of remediation efforts. The development of a conceptual model is based on the analysis and simplification of data collected during field-moni- toring and laboratory experiments, simplification of hydrologic systems, and the representation of hydrogeologic parameters in models (Bould- ing 1995). In general, conceptual models that describe water flow include a description of the hydrologic components of the system and how mass is transferred between these components. Without a conceptual model, we do not know what tests to conduct, what parameters to measure, where to place probes, or what probes to use. Conversely, without such data, we cannot develop a conceptual model. This situation requires an iterative approach, in which we con- duct a series of observations and tests and, concurrently, develop a con- ceptual model of water flow to refine our tests. The development of a conceptual model for water flow in the heterogeneous soil and fractured rock of the vadose zone is particularly difficult for three main reasons: (1) The contrasts in permeability of soils and rocks at different parts of the system may be extreme and localized. (2) The geometry of water flow depends strongly on the intercon- nection or connectivity of a preferential-flow-zone network. In a given vadose zone system, many probes may be located within nonconductive zones, which have no significant role in flow. In fractured rocks, fractures may be nonconductive because aper- tures are closed under the ambient stress state or by mineral pre- cipitation. Additionally, soil and rock hydraulic conductivity may decrease during an infiltration event because of clogging, sealing, or air entrapment. (3) The design of borehole tests and the interpretation of data in het- erogeneous soils and fractured rocks are complicated because the CHAPTER 3 – VADOSE ZONE CHARACTERIZATION AND MONITORING 137 response in a monitoring well may only be from a single zone of preferential flow or a fracture (Long 1996). Therefore, “point” measurements in heterogeneous soils and fractured rocks cannot reveal complex processes that result from the interaction of fea- tures at many different scales. Conceptualization begins with a theoretical understanding of the entire groundwater-vadose zone-atmosphere system, followed by data collection and the refinement of that understanding. Additional data col- lection and analysis, as well as the refinement of the groundwater sys- tem conceptual model, occur during the entire process of conceptualization and characterization, and during groundwater model development and use (Figure 3-1). Numerical modeling (forward and inverse) as a means of developing a flow-and-transport-process conceptual model allows us to obtain a better understanding of the level of detail and features needed to improve site-characterization design and monitoring methods. WATER FLOW AND CHEMICAL TRANSPORT PROCESSES IN DEEP AND SHALLOW VADOSE ZONES SPATIAL AND TEMPORAL SCALES OF VADOSE ZONE INVESTIGATIONS AND SCALING Spatial Scales Heterogeneity of hydraulic processes in soils and sediments occurs in a hierarchy of spatial and temporal scales (Cushman 1986; Faybishenko 1986; Wagenet et al. 1994;
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