Tracer techniques for site characterization and remediation technology performance assessment: recent developments and applications

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1 Groundwater Quality: Remediation and Protection (Proceedings of the GQ'98 Conference held at Tubingen, Germany, September 1998). IAHS Publ. no. 250, Tracer techniques for site characterization and remediation technology performance assessment: recent developments and applications P. S. C. RAO, M. D. ANNABLE, H. KIM & K. P. SARIPALLI Inter-disciplinary Program in Hydrologie Sciences, University of Florida, Gainesville, Florida , USA Abstract Several innovative tracer techniques have been introduced for an in situ estimation of domain-averaged values and spatial patterns in the nonaqueous phase liquid (NAPL) saturation (S ), the NAPL-water interfacial area (a m ), and the biogeochemical reactivity (k s ) within the target test zone, both before and after implementing some in situ technique for site cleanup for evaluating the effectiveness of remediation achieved. Here, we review the theoretical basis for these tracer methods, present selected examples of recent field applications, and briefly discuss the reliability of these tracer methods. INTRODUCTION Clean-up of sites contaminated with non-aqueous phase liquid (NAPL) wastes requires removal of the "source zone" and remediation of the contaminated dissolved plume(s). No matter what technique (excavation, extraction, stabilization, containment, etc.) is used for source removal, the volume of soil (the term "soil" is used here sensu lato) containing the NAPLs must be accurately delineated. The hydrological and biogeochemical characteristics of the zone targeted for remediation need to be adequately characterized for designing and deploying most in situ cleanup technologies. In recent years, several innovative tracer methods and advanced geophysical methods (e.g. laser-induced fluorescence techniques) have been introduced and evaluated under laboratory and field conditions. In this paper, we will present an overview of new tracer techniques for source zone characterization in support of site management efforts. Tracer techniques are also useful for evaluating the performance of remediation technologies implemented for source zone cleanup, such that site-specific conditions influencing remediation efficacy can be evaluated. This information is crucial for cleanup technology extrapolation to other sites. We will primarily focus on recent field work conducted by our group, with appropriate references to similar work done by others. CHARACTERIZATION OF SOURCE ZONES For characterization of NAPL source zones, four categories of tracers have been studied: (a) non-reactive tracers, used for hydrodynamic characterization; (b) partitioning tracers, that selectively partition into the NAPL; (c) interfacial tracers, that

2 354 P. S. C. Rao et al. only accumulate at the NAPL-water interfaces but do not partition into the NAPL; and (d) biogeochemical tracers that are used to quantify abiotic and biotic reactivity. Note that, in general, tracers may be grouped into two broad classes aqueous or gaseous based on the method of their introduction. Aqueous tracers can be used for both saturated- and vadose-zone investigations, while the gaseous tracers are useful for vadose-zone applications. Since the use of non-reactive tracers (e.g. dyes, inorganic anions, organic acids) is well established in environmental hydrology, we will focus here on recent applications of the other three types of tracers for site characterization and cleanup assessment. Partitioning and interfacial tracers The experimental approach used for both partitioning and interfacial tracers is the same. A pulse of tracers (non-reactive and reactive) is displaced through the test zone, and the retardation of the reactive tracer with respect to a non-reactive, reference tracer is measured. During displacement, the non-reactive and reactive tracers experience the same hydrodynamic conditions, but the average travel time for the reactive tracers is delayed due to specific interactions (phase distribution for partitioning tracers and interfacial accumulation for interfacial tracers) with the NAPL. The ratio of the average travel times for the non-reactive and reactive tracers is defined as the Retardation Factor (R), and is used to calculate S or a nw. It is important to recognize that NAPL dissolution kinetics are strongly influenced by the interfacial area. The mechanism leading to partitioning tracer retardation is fluid-fluid phase distribution due to differential solubility in the two fluid phases, whereas for the interfacial tracers retardation is the result of accumulation at NAPL-water interfaces. A linear isotherm is used for describing the reversible phase distribution of partitioning tracers, and the partition coefficient (K,) is constant over a significant range of tracer concentrations. In contrast, a non-linear isotherm (Gibbs model) is used to describe the reversible accumulation of interfacial tracers, and a piecemeal linear approximation is employed since the adsorption constant (K,) decreases nonlinearly with increasing tracer concentration. These features of the partitioning and interfacial tracers are summarized in Table 1, and further details can be found in a recent series of papers (Jin et al, 1995; Whitley et al., 1996; Nelson & Brusseau, 1996; Wilson & Mackay, 1996; Aimable et al, 1995, 1998a,b; Saripalli et al, 1997a,b; 1998; Kim et al, 1997, 1998a,b,c). Using multi-level samplers tracer travel times can be measured at several locations within a three-dimensional flow domain for determining the spatial distribution of S and a llw (Sillan et al, 1998a,b; Annable et al, 1998b), while data based on samples collected in extraction wells provide volume-averaged values representing the entire flow domain swept by the tracers (Annable et al, 1998a; Rao et al, 1997; Jawitz et al, 1998). Tracer tests have been conducted in various flow configurations employing different patterns of multiple injection and extraction wells under forced-gradient steady fluid (water or gas as the mobile fluid) flow conditions. However, single-well "push-pull" tracer tests have also been used for partitioning tracers (G. A. Pope, 1998, personal communication) and bio-reactive tracers (Istok et al, 1997; Haggerty et al., 1998).

3 Tracer techniques for site characterization and remediation performance assessment 355 Table 1 Summary of tracer techniques. Attribute Partitioning tracers Interfacial tracers Biogeochemical tracers Tracers used Low molecular-weight alcohols and methyl-substituted alcohols Parameter estimated Applications Mechanisms Isotherm model Retardation model Degradation model NAPL residual saturation (average and spatial patterns) NAPL distribution and remediation effectiveness Phase partitioning C = K C». N/A, S Kn P Kg (1-S ) G, Anionic and cationic surfactants and high molecular-weight alcohols Interfacial area (average and spatial patterns) NAPL "morphology" and interfacial mass transfer Interfacial adsorption 1 (d/ 2RT\dC. pk d R-, = l + ' N/A,a,K,' 2RT\dCJ n>c Biologically labile carbon compounds (benzoate and sugars) Transformation rate constant (average, and spatial patterns) Potential for abiotic and biotic reaction rates Microbial or geochemical reaction C,= KuC /?</ = - ri+4- Zero- or first-order K is the NAPL-water partition coefficient; K d is soil sorption coefficient; p is soil bulk density; Q w is the volumetric water content; S is the NAPL saturation; a, is the NAPL-water interfacial area; K- is the interfacial adsorption coefficient; R t is the retardation factor, with the subscripts pt and ift indicating partitioning and interfacial tracers; R is the gas constant, T\s absolute temperature; y is the NAPL-water interfacial tension; C and C 0 are tracer concentration in the effluent and influent solutions, respectively; co is a dimensionless rate constant; P is Peclet number; R d is retardation factor for a degrading tracer; R mlhd is retardation factor for a non-degrading tracer. The interfacial tracer technique has been used for estimating the air-water interfacial areas in partially water-saturated columns of sand or glass bead packs under one dimensional steady-state unsaturated water flow conditions (Kim et al., 1997', 1998a). The adsorption chemistry of surfactant accumulation at the air-water interface is basically the same as that at the NAPL-water interface. Since some remediation techniques (e.g. soil vapour extraction) rely on the vapour phase mobilization of contaminants, the mass transfer rate of the contaminants from soil water to soil gas is a critical parameter, which is, in turn, a strong function of the specific air-water interfacial area in the system. Biogeochemical tracers Biogeochemical tracers are used to estimate the microbial activities or the kinetics of abiotic reactions. Several labile or reactive compounds (e.g. low-molecular weight alcohols, benzoate, sugars, various electron donors or acceptors) can be used as probes to determine in situ the kinetics of biogeochemical reactions (Chapelle, 1993; Bouma, 1995; Istok et al, 1997; Brusseau et al, 1997; Piatt et al, 1997; see Table 1). The transformation rate constant (k s ) can be estimated by measuring the extent of tracer losses during displacement through a specified domain. The spatial variability in k s within a given flow domain can also be determined using an appropriate spatially distributed monitoring network. Depending upon the nature of

4 356 P. S. C. Rao et al. tracers, specific biogeochemical characteristics of the flow system can be elucidated. The zero-th moment (i.e. mass recovery) of a tracer concentration breakthrough curve is evaluated and combined with the normalized first temporal moment (i.e. average travel time) for estimating the k s value. Because some tracer mass is lost due to transformations, both the zero-th and the normalized first temporal moment are smaller in comparison to a non-degrading tracer, but appropriate correction can be made to account for this (see Table 1; Annable et al., 1998a). Estimation of k s is predicated on a knowledge of the appropriate rate expression (zero-th or first order; Monod kinetics, etc.). More reliable estimates of k s can be obtained if one or more products of sequential or simultaneous transformations can be monitored in addition to the parent chemical used as the tracer. This latter version of the tracer technique has been used frequently in packed soil columns to measure agro-chemical degradation rate constants, and has been used only to a limited extent for microbial characterization of source areas at waste disposal sites (Istok et al, 1997; Haggerty et al, 1998). FIELD-SCALE APPLICATIONS During the period, we conducted a series of field tests to evaluate the efficacy of two in situ flushing techniques cosolvent flushing (Rao et al., 1997) and single-phase micro-emulsion flushing (Jawitz et al., 1998) for enhanced remediation of a NAPL source zone located in a shallow aquifer at Hill Air Force Base, Utah. These experiments were a part of a larger, coordinated study to evaluate several in situ flushing techniques in several hydraulically isolated test cells installed in the NAPL-contaminated source zone. As a part of these field experiments, we evaluated the use of partitioning and interfacial tracers to estimate S and a, for both an initial site characterization, and for an assessment after flushing to examine spatial patterns in cleanup effectiveness (Jawitz et al., 1997; Annable et al., 1998a,b; Sillan et al, 1998a,b). The tracer results were then compared with mass removal effectiveness based on soil core analysis and NAPL constituents mass balance calculations. Spatial patterns in NAPL distribution based on tracer tests, conducted before and after cosolvent flushing, are shown in Figs 1(a)-1(e), while a comparison of the depth distribution of NAPL removal effectiveness based on soil cores and tracer tests are shown in Fig. 1(f) (Sillan et al., 1998a). Enclosed within each iso-surface is the volume of the geological formation with S greater than or equal to the indicated value. The highest initial NAPL saturations were found just above the clay confining unit, and S values decreased with height above the clay in the NAPL smear zone. NAPL saturation iso-surfaces based on tracer tests after cosolvent flushing (Fig. 1(d) and 1(e)) indicate that the lowest amount of NAPL removal occurred in the lowpermeability, high-is 1,, zone along the clay layer, but in much of the flushed zone cosolvents effectively dissolved and flushed out the NAPL. Note that the NAPL removal effectiveness estimated by the tracer tests is less than that from soil core analysis (Fig. 1(f)). Similar analysis of the tracer data from the micro-emulsion flushing test is now being completed. The estimated values of NAPL content and interfacial areas from tracer tests conducted in a test cell at Hill AFB site were combined to calculate H N = [a a JS n s]

5 Tracer techniques for site characterization and remediation performance assessment 357 P < 5 w o o (IU) XB[3 3Aoqy Ji Si3H >n >o o *-! -! (ui) ÀB[3 9Aoqv }i(s 3H SP» a

6 358 P. S. C. Rao et al an empirical index of NAPL "morphology". Aimable et al. (1998b) reported that the tracer tests indicated high variability (about an order of magnitude range) in H N. Their data suggest that there was high variability in NAPL distribution both at the Darcy-scale and at the test-cell scale. In the low H N zones (near the clay unit), either the interfacial area is low or the NAPL content is high, while the opposite is true in zones of high H N (above the clay) More efficient remediation by flushing is to be expected in zones of high H N Thus, the tandem use of partitioning and interfacial tracers to determine the spatial patterns in H N allows for a more accurate evaluation of the NAPL distribution and effectiveness in NAPL mass removal. Haggerty et al. (1998) presented a method to estimate first-order reaction rate coefficients (k s ) from tracer breakthrough curves obtained from a single-well, "pushpull" test. Their tracer solution contains a non-reactive, conservative tracer and one or more reactive tracers selected to investigate a particular biogeochemical process. They also present an example application where they used this push-pull tracer method to estimate reaction rate coefficients for de-nitrification in a petroleumcontaminated, unconfined, alluvial aquifer. RELIABILITY OF TRACER METHODS For the tracer techniques to become accepted as tools for site characterization and performance assessment, the results provided by these innovative techniques must be compared with those from more-established methods. Such a comparison must be based not only on agreement between traditional and tracer methods, but also on the Homogeneous Media Heterogeneous Media actual value Homogeneous Media actual value Heterogeneous Media actual value actual value Fig. 2 Reliability of tracer methods for estimating parameter values (S, a w, k s ) in homogeneous media (a), (c) and heterogeneous media (b), (d). Deviations from the ideal 1:1 line of agreement are shown in (a) and (b), while in (c) and (d) the possible error ranges are shown.

7 Tracer techniques for site characterization and remediation performance assessment 359 uncertainty in the estimated parameter values for both types of methods. Figure 2 presents a schematic means to evaluate the reliability of the tracer methods. In these plots, the 1:1 line represents a perfect agreement between the "actual" parameter values and those estimated (S n, a nw, k s ) using tracer methods. In general, the magnitude of deviations from the ideal line as well as the uncertainties are likely to be small for homogeneous media with uniform NAPL distribution. Also, constraints to hydrodynamic accessibility of tracers to NAPLs and non-equilibrium mass transfer of tracers is expected to result in an underestimation of the actual values (Kim et al., 1998b). This can be a particularly vexing problem for tracers tests in: (a) hydrologically homogeneous media with sparse NAPL distribution (Fig. 2(a)), or (b) hydrologically heterogeneous media with non-uniform NAPL distribution (Fig. 2(b)). Hydrodynamic accessibility and mass-transfer constraints can also be significant sources of underestimation and error when partitioning and interfacial tracer techniques are used in zone with high NAPL saturation (e.g. "pools"), which could lead to underestimation of the actual values (Figs 2(c) and 2(d)). Jin et al. (1997) used UTCHEM model simulations of partitioning tracer displacement in heterogeneous media with non-uniform NAPL distribution to evaluate the sensitivity of the partitioning tracer methods to detect NAPLs present either as pools or at residual saturation or both. They concluded that for low S and sparse spatial distribution, the estimation errors can be minimized by optimizing K n by selecting an appropriate suite of tracers. For both pre- and post-remediation tracer tests, the interference of organic matter and mineral components may be quite small (Jin et al, 1997). However, for some post-remediation tracer tests, S may be significantly over-estimated (Rao et al., 1997; Jawitz et al., 1998; Dai, 1997). Removal of NAPL via in situ flushing may "unmask" sorptive domains that were otherwise inaccessible to the tracer (Dai, 1997). Conversely, a recalcitrant, insoluble fraction of the NAPL (referred to as the "pitch" component) may be left behind as thin coatings on soil surfaces after remediation, and tracer partitioning into this hydrophobic carbon matrix may yield a false-positive signal for presence of NAPL (Rao et al., 1997; Jawitz et al., 1998). Acknowledgements This study was supported by grants from the R. S. Kerr Research Laboratory, US EPA, and the Tyndall Air Force Base, Florida. REFERENCES Annable, M. D., Rao, P. S. C, Hatfield, K. & Graham, M. D. (1995) Use of partitioning tracers for measuring NAPL distribution in a contaminated aquifer: preliminary results from a field-scale test. In: Proc. 2nd Annual Tracers Workshop (Univ. of Texas, Austin, Texas, USA). Annable, M. D., Rao, P. S. C, Hatfield, K., Graham, W. D., Wood, A. L. & Enfield, C. G. (1998a) Use of partitioning tracers for measuring residual NAPL: results from a field-scale test. J. Environ. Engng 124(6), Annable, M. D., Jawitz, J. W., Rao, P. S. C., Dai, D. P., Kim, H. & Wood, A. L. (1998b) Field evaluation of interfacial and partitioning tracers for characterization of effective NAPL-water contact areas. Ground Water 36(3), Bouma, S. L. (1995) M.E. Thesis, University of Florida, Gainesville, Florida, USA. Brusseau, M. L., Piatt, J., Hu, M. Q. & Wang, J. M. (1997) The use of biotracers for measuring biodégradation

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