Article pubs.acs.org/est
Transverse Hydrodynamic Dispersion Effects on Isotope Signals in Groundwater Chlorinated Solvents’ Plumes Boris M. Van Breukelen*,† and Massimo Rolle‡,# †
Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, NL-1081 HV Amsterdam, The Netherlands Center for Applied Geosciences, University of Tübingen, Sigwartstrasse 10, D-72076 Tübingen, Germany # Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, 94305 Stanford, California, United States ‡
S Supporting Information *
ABSTRACT: The effects of transverse hydrodynamic dispersion on altering transformation-induced compound-specific isotope analysis (CSIA) signals within groundwater pollution plumes have been assessed with reactive transport modeling accommodating diffusion-induced isotope fractionation (DIF) and implementing different parameterizations of local transverse dispersion. The model reproduced previously published field data showing a negative carbon isotope pattern (−2 ‰) at the fringes of a nondegrading PCE plume. We extended the study to reactive transport scenarios considering vinyl chloride as a model compound and assessing, through a detailed sensitivity analysis, the coupled effects of transverse hydrodynamic dispersion (with and without DIF) and aerobic fringe degradation on the evolution of carbon and chloride isotope ratios. Transformation-induced positive isotope signals were increasingly attenuated with distance from the source and higher degradation rate. The effect of DIF on the overall isotope signal attenuation was greatest near the source and for low values of groundwater flow velocity, transverse dispersion coefficient, molecular weight, rate constant, and isotope fractionation factor, α, of the degradation reaction. Models disregarding DIF underestimate the actual α. The approximately twice larger DIF effect for chlorine than for carbon together with the low α for oxidation resulted in strong chlorine CSIA depletions for VC at the plume fringe.
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INTRODUCTION Fringe-controlled biodegradation plays a pivotal role in determining the fate of groundwater pollution plumes. In particular, for plumes approaching steady-state conditions, transverse hydrodynamic dispersion determines the extent of mixing of the organic pollutants with dissolved oxidants (oxygen, nitrate, and sulfate) in the ambient groundwater and, thereby, is a key factor in plume attenuation, as shown by a number of model-based and field studies (e.g., refs 1−7). Therefore, a correct quantification of the interplay between physical transport processes and (bio)degradation reactions occurring at the plume fringe is of utmost importance for understanding and assessing natural attenuation of pollutants in aquifer systems. The analysis of the spatial and temporal evolution of isotope ratios during contaminant transport has the potential of providing additional and often decisive information on contaminant degradation. Compound-specific stable isotope analysis (CSIA) has been established as a monitoring tool in the past decade and provides a more direct approach than concentration-based monitoring to identify the occurrence of degradation of organic pollutants.8−11 Isotope ratios of widespread organic groundwater contaminants like chlorinated ethenes increase considerably over the course of degradation for most transformation pathways. In particular, for © 2012 American Chemical Society
oxidative processes of chlorinated ethenes at the plume fringe, CSIA can be applied to obtain direct information on biodegradation, and, under ideal circumstances, isotope enrichments can be linked directly to mass destruction as is typically required for monitored natural attenuation to be accepted as remediation technology. In many practical applications at contaminated field sites, the interpretation of isotopic data needs to take into account the effects of depth-averaged sampling from long well screens which attenuate degradation-induced isotope signals.12−14 Moreover, recent studies, showed that CSIA data are more affected than previously thought by physical processes like mass transfer from nonaqueous phase liquids to the aqueous phase,15 sorption,12,14,16,17 volatilization,18 dispersion,14,19,20 and diffusion,20,21 thus calling for comprehensive interpretation approaches taking into account the interplay between physical and reactive processes. In this contribution, we make a detailed, model-assisted analysis of the effects of transverse diffusive/ dispersive processes on carbon and chlorine isotope ratios at Received: Revised: Accepted: Published: 7700
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homogeneous porous medium.36 As shown in the Supporting Information, SI, a two-dimensional steady state transport problem can be reduced to a 1-D transient one by a simple transformation of coordinates. The comparison of both numerical codes with the analytical solution (eq S4 of the SI) showed an excellent agreement over a wide range (i.e., 6 orders of magnitudes) of contaminant concentrations. The numerical simulations performed include: (i) conservative transport to interpret the high-resolution concentrations and carbon isotope vertical profiles of a PCE plume observed in a shallow sandy aquifer, (ii) a reactive transport modeling scenario investigating the effect of transverse hydrodynamic dispersion and fringe biodegradation on C and Cl isotopic signatures, and (iii) a detailed sensitivity analysis to systematically assess the influence of changes in transport and reaction parameters on isotope data and their interpretation to quantify biodegradation. Diffusion and Dispersion Coefficients. The modeling tools used in this study are able to accommodate isotopologuespecific diffusion and dispersion coefficients. Few literature data are available on the effect of different isotopic masses on the diffusion of neutral species in water. To the best of our knowledge, these properties should still be well documented for organic contaminants such as chlorinated ethenes. As a working hypothesis for the present study, we calculated the aqueous diffusion coefficients, Daq, of the different contaminants’ isotopologues according to the empirical correlation proposed by Worch 37 for organic compounds:
the fringes of chlorinated solvents’ plumes and their implications for CSIA data evaluation. Isotopic fractionation by diffusion has been studied mainly in diffusion-dominated aqueous environments for cases such as methane oxidation in marine sediments,22 denitrification in the hypolimnion of a lake,23 and chloride diffusion through clay layers for paleo groundwater studies,24,25 and in unsaturated soils for cases including gaseous CO2 transport,26 and diffusion of volatile petroleum hydrocarbons27,28 and chlorinated ethenes.29 Diffusion-induced isotope fractionation (DIF) has not yet been quantitatively investigated in advection-dominated saturated environments or more specifically groundwater pollution plume studies with the exception of LaBolle et al.21 A few recent contributions inspired us for the present work. LaBolle et al. 21 simulated significant carbon isotope enrichments of organic pollutants in thin aquifers ( 1 implying an overestimation of degradation applying the Rayleigh equation. Since we focused especially on the behavior at the plume fringe we excluded these points from the representation of Figure 3. Figure 3 shows that diffusion-induced isotope fractionation further attenuates the biodegradation-induced isotope signal (lower θ for HDIF versus HD), whereas the new parameterization for hydrodynamic dispersion, NP, gives typically lower θ values than the classical one, CP. Distance from the source and kox largely control θ for HD simulations, whereas θ is nearly insensitive to variations of the other parameters for HD simulations (Figure 3). The two aforementioned parameters show a nearly identical effect to θ suggesting a similar mechanistic explanation (Figure 3A,E). Higher degradation rates decrease θ as discussed before (Figure 3E; SI;14,19). Similarly, at larger distance from the source, mixing becomes progressively more limiting 51 and its effect on the overall transformation is well captured by the computed θ, which decreases indicating more substantial deviation from the 7705
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Chlorine isotope fractionation factors for cDCE and VC oxidation are particularly low,45 whereas the DIF effect is approximately twice as large for chlorine than for carbon and may consequently result in negative isotope patterns in plume fringe areas. However, as the carbon isotope fractionation factors for oxidation of these lower chlorinated ethenes are relatively high, the dual carbon−chlorine isotope approach to detect possible cDCE and VC oxidation33,45 should still be useful. The chlorine versus carbon dual isotope slope for fringedegradation may, however, not show a gentle incline,33 but rather moves into the fourth quadrant on a dual isotope plot (Figure 2F). Underestimation or nondetecting biodegradation at pollution fringes based on CSIA data may occur for pollutants which have low fractionation factors for oxidation depending on the microorganisms involved (e.g., MTBE: εC = −0.2 to −2.5 ‰, ε H ≈ 0 to −100 ‰; 52−54 1,2,4trichlorobenzene: εC ≤ −0.4 ‰;55 toluene: εC = −0.4 to −6.2 ‰, but εH ≫ −2 ‰),56 especially under conditions favoring the HDIF effect. This study also pointed out the key role of diffusion on contaminant transport and degradation. Diffusive processes take place at the small pore scale but they are relevant and manifest themselves at larger macroscopic scales.57 Our results shows that a reactive transport model approach which considers DIF was required to reproduce the negative isotope patterns at the investigated field site. Moreover, a reactive transport model accounting for DIF also allows obtaining better estimates on actual degradation rates occurring within groundwater pollution plumes. Finally, this study calls for further experimental research on diffusive/ dispersive processes and their importance for an improved interpretation of isotopic signatures at contaminated field sites.
Rayleigh equation which assumes a well-mixed system. The additional effect of HDIF becomes dominant when degradation is sluggish or has progressed to limited extent when the point of observation is near the source area. For low extents of degradation, negative isotope patterns produced by HDIF are only marginally turned toward positive isotope values indicative of biodegradation. At high rates of degradation and with large travel distance all methods converge to low θ (Figure 3A,E). Variations in the degree of heterogeneity of the aquifer (σ2lnK) and v also show comparable control on θ which follows from the influence of these parameters on transverse hydrodynamic dispersion. The importance of pore diffusion over mechanical dispersion is higher at low flow velocities and in mildly heterogeneous media causing strong reductions in θ for HDIF simulations. For higher values of σ2lnK and v, CP gradually increases to the invariant θ of HD, whereas θ remains low for NP. Low ε values produce a positive but small isotope signal which easily becomes reduced due to HDIF (Figure 3F). For the base case settings, CP and NP inhibit the development of positive isotope signals for εC below roughly −2 and −4 ‰, respectively (Figure 3F; SI). Figure 3D shows that the molecular weight has no effect on HD, whereas θ becomes lower with lower MW for CP and NP due to enhanced HDIF. In summary, ongoing degradation processes at plume fringes may be strongly underestimated by the Rayleigh equation if the distance from the source zone to the monitoring well is long, degradation rates are fast, and the groundwater flow velocity, heterogeneity of the hydraulic conductivity field, and ε are low. Diffusion-induced isotope fractionation effects have the most impact at low values of the following factors: distance from the source, groundwater flow velocity, heterogeneity of the hydraulic conductivity field, degradation rate constant, kinetic isotope enrichment factor, and molecular weight. Implications for CSIA-Based Assessment and Modeling of Biodegradation. The field site model demonstration showed that diffusion-induced isotope fractionation (DIF) is a relevant process at plume fringes and could be satisfactorily simulated with the developed model, which could capture the diffusive/dispersive effects on the measured δ13C signals. Neither of the two parameterizations of hydrodynamic dispersion, CP and NP, always gave a good fit for the three investigated field locations but they allowed an explanation of the observed negative isotope patterns at the PCE plume fringe and their combination indicated the potential range of effects. The sensitivity analysis indicates that the DIF effect needs to be considered for CSIA data interpretation under a wide range of conditions. CSIA as a tool to assess degradation is limited to low molecular weight organic pollutants, which are inherently sensitive to DIF. Beside chlorine CSIA, oxygen (δ18O−NO3, δ18O −SO4), sulfur (δ34S−SO4), and bromine CSIA are more sensitive to DIF because of the isotope mass difference of two compared to one for carbon, hydrogen, and nitrogen. The DIF effect may have gone unnoticed at contaminated sites because of the lack of high-resolution monitoring devices necessary to capture the steep vertical diffusive/dispersive gradients and, where degradation does occur, as biodegradation-induced positive isotope patterns have only become reduced to less positive values. Field calibration of models which disregard DIF results in underestimation of actual isotope enrichment factors operative at field sites. This tendency becomes more important for low actual ε values.
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ASSOCIATED CONTENT
S Supporting Information *
Model validation to analytical solution; outline of the fluxrelated moment analysis; parameters and results of all simulations performed for the sensitivity analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +31-20-5987393; fax: +31-20-5989940; e-mail: b.m.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Both authors, B.M.V.B. and M.R., contributed equally to this work. B.M.V.B. was financially supported by the European Union under the 7th Framework Programme (project acronym CSI:ENVIRONMENT, contract number PITN-GA-2010264329), ESTCP project ER-201029, and Direct Funding. M.R. acknowledges the support of the Marie Curie International Outgoing Fellowship (DILREACT project) within the 7th European Community Framework Programme. We thank three anonymous reviewers for their helpful and constructive comments.
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