Environ. Sci. Technol. 2005, 39, 8543-8544
Response to Comment on “New Evaluation Scheme for Two-Dimensional Isotope Analysis to Decipher Biodegradation Processes: Application to Groundwater Contamination by MTBE” Combining the two-dimensional analysis of measured isotope ratios with a novel concept to evaluate bulk isotope fractionation of organic pollutants, we recently identified the nucleophilic second-order substitution (SN2) mechanism as predominant biotransformation pathway of methyl tertbutyl ether (MTBE) at a contaminated field site (1). A minor aspect of our work was the application of the well-known Rayleigh-streamline approach (2-6) to estimate the extent of biotransformation, B, of MTBE along the plume:
B ) (1 - f) ) 1 -
[
1000 + δ13Cx
1000 + δ13C0
]
1000/
(1)
δ13Cx and δ13C0 are measured contaminant carbon isotopic signatures at well x and at the contamination source, respectively. For the carbon bulk enrichment factor we applied a value of ) -9‰ which has been reported for the identified transformation process. Because most of the measured MTBE concentrations in the plume were significantly lower than those predicted by this approach, we speculated on the presence of additional removal processes such as evaporation which do not lead to significant isotope fractionation of MTBE. The letter of Kopinke et al. (7) focuses on this particular side aspect of our study. The authors show that volatilization of MTBE from groundwater is unlikely but a certain flow regime in the aquifer, i.e., “segregation of macrofluids” may explain the unexpectedly low MTBE concentrations in the plume. When different water parcels start off from the same source, convert the substrate to various extents along their separated flow paths, and mix again at a later point, the overall extent of biodegradation generally is underestimated
if isotope ratios are measured after the mixing of the water parcel. So far we fully agree with Kopinke et al. It is, however, important to realize that both the concept of segregation of macrofluids itself as well as the specific boundary conditions chosen by Kopinke et al. to exemplify this concept (see Table 1 in ref 7) represent rather extreme cases. First, the time scale for the separation of macrofluids must be comparable to that of the reaction rate of the transformation process. Separation and mixing of fluids at relatively short time scales results in effective mixing due to (macro) dispersion, a well-known phenomenon in aquifers. Especially for slow reactions such as the anaerobic biodegradation of MTBE, very long time scales of separation must be postulated in order to be significant in the sense discussed here. In an alternative and probably more realistic scenario, where separation of macrofluids occurs at time scales leading only to moderate concentration differences of the water parcels at the time of mixing, the effect on measured δ-values would be moderate or even insignificant (see scenario b in Table 1). Also with regard to the second aspect discussed by Kopinke et al.sthe influence of a parallel nonfractionating removal process (evaporation) on observed δ-valuessthe specific assumptions chosen by Kopinke et al. (7) represent extreme cases. If we follow the argumentation of Kopinke et al., evaporation of MTBE from groundwaters should be low or even insignificant. Thus, a relative importance of removal rates as assumed in Table 1 of ref 7 of 3/8 to 5/8 for evaporation and biodegradation, respectively, considers a scenario which allows demonstration of the principle effects of such a constellation but does not give a realistic picture of the effects on the δ-values that can be expected in the field. If one considers a less extreme but still significant contribution of evaporation to the overall removal of MTBE (1/8 rather than 3/8 (cf scenario c in Table 1), the significance of evaporation for the calculated extents of biodegradation becomes almost negligible (0.75 rather than 0.7 for “real” biodegradation). What are the implications for practical applications of isotope studies in contaminated aquifers? As emphasized by Kopinke et al. (7, 9) non- (or little) fractionating attenuation processes in the field such as mixing, sorption, or evaporation
TABLE 1. Comparison of Different Scenarios of Groundwater Flow Regimes and Elimination Processes with Regard to Their Effects on Isotope Fractionation and Calculated Extents of Biodegradationa
∆ ∂13C calculated extent of biodegradation (eq 1) “real” extent of biodegradation fraction of MTBE removed from aquifer
scenario ab
scenario bc
scenario cd
-9.0 +14.6 0.80 0.80 0.80
-9.0 +14.2 0.79 0.80 0.80
-7.85 +12.7 0.75 0.70 0.80
a All scenarios assume constant groundwater flow, 80% removal of the contaminant between the two points of observation, and homogeneous mixing within the two observation wells. b Scenario a: Plug flow with biodegradation (base scenario, referred to as “streamline-Rayleigh approach”. c Scenario b: 2 × segregation and re-mixing of water parcels; biodegradation in the upper streamlines is 3 × faster than in the lower streamlines; in each of the 4 consecutive segments (I-IV) the same overall amount of biodegradation (20% of the initial mass) takes place. d Scenario c: Plug flow with simultaneous biodegradation (70% removal rate) and evaporation (10% removal rate).
10.1021/es0580183 CCC: $30.25 Published on Web 09/28/2005
2005 American Chemical Society
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might confound a precise quantification of fractionating degradation processes by a simple streamline-Rayleigh approach. Sorption may become significant only at the leading edge of a plume (8, 9). Hydrodynamic processes including segregation of macrofluids may, as exemplified above, lead to a moderate underestimation of degradation. In the case of evaporation, the extent of biodegradation might be somewhat overestimated. Here, the commonly used streamline-Rayleigh approach would, nevertheless, give a conservative estimate (i.e., an underestimate as shown in the last line of Table 1 in ref 7 and the last column in Table 1 herein) of the overall extent of removal of the contaminant from the aquifer since evaporation from groundwater is an additional removal process. Therefore, one important overall conclusion with regard to the assessment of net attenuation processes of contaminants in the groundwater is that even though measured isotope ratios maysunder special circumstancessoverestimate the extent of degradation, they will not overestimate the extent of net removal from the aquifer. If the groundwater flow patterns at a particular field site are expected to be dominated by segregation of flow lines, we recommend sampling and analysis of groundwater depth profiles. This approach will allow applying a simple Rayleigh approach for individual streamlines (10). Finally, as pointed out by Kopinke et al. (7) the presence of nonfractionating processes will, of course, not affect the identification of contaminant degradation pathways using the two-dimensional isotope evaluation scheme presented in our study (1).
Literature Cited (1) Zwank, L.; Berg, M.; Elsner, M.; Schmidt, T. C.; Schwarzenbach, R. P.; Haderlein, S. B. A new evaluation scheme for twodimensional isotope analysis to decipher biodegradation processes: Application to groundwater contamination by MTBE. Environ. Sci. Technol. 2005, 39, 1018-1029. (2) Sherwood Lollar, B.; Slater, G. F.; Sleep, B.; Witt, M.; Klecka, G. M.; Harkness, M.; Spivack, J. Stable carbon isotope evidence for intrinsic bioremediation of tetrachloroethene and trichloroethene at Area 6, Dover Air Force Base. Environ. Sci. Technol. 2001, 35, 261-269. (3) Mancini, S. A.; Lacrampe-Couloume, G.; Jonker, H.; Van Breukelen, B. M.; Groen, J.; Volkering, F.; Sherwood Lollar, B. Hydrogen isotopic enrichment: An indicator of biodegradation at a petroleum hydrocarbon contaminated field site. Environ. Sci. Technol. 2002, 36, 2464-2470. (4) Richnow, H. H.; Annweiler, E.; Michaelis, W.; Meckenstock, R. U. Microbial in situ degradation of aromatic hydrocarbons in a contaminated aquifer monitored by carbon isotope fractionation. J. Contam. Hydrol. 2003, 65, 101-120. (5) Griebler, C.; Safinowski, M.; Vieth, A.; Richnow, H. H.; Meckenstock, R. U. Combined application of stable carbon isotope analysis and specific metabolites determination for assessing in situ degradation of aromatic hydrocarbons in a tar oil-contaminated aquifer. Environ. Sci. Technol. 2004, 38, 617-631.
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(6) Peter, A.; Steinbach, A.; Liedl, R.; Ptak, T.; Michaelis, W.; Teutsch, G. Assessing microbial degradation of o-xylene at field-scale from the reduction in mass flow rate combined with compound-specific isotope analyses. J. Contam. Hydrol. 2004, 71, 127-154. (7) Kopinke, F.-D.; Georgi, A.; Richnow, H. H. Comment on “New evaluation scheme for two-dimensional isotope analysis to decipher biodegradation processes: Application to groundwater contamination by MTBE”. Environ. Sci. Technol. 2005, 39 (20), 8541-8542. (8) Van Breukelen, B. M.; Hunkeler, D.; Volkering, F. Quantification of sequential chlorinated ethene degradation by use of a reactive transport model incorporating isotope fractionation. Environ. Sci. Technol. 2005, 39, 4189-4197. (9) Kopinke, F.-D.; Georgi, A.; Voskamp, M.; Richnow, H. H. Carbon isotope fractionation of organic contaminants due to retardation of humic substances: Implications for natural attenuation studies in aquifers. Environ. Sci. Technol. 2005, 39 (16), 60526062. (10) Hunkeler, D.; Chollet, N.; Pittet, X.; Aravena, R.; Cherry, J. A.; Parker, B. L. Effect of source variability and transport processes on carbon isotope ratios of TCE and PCE in two sandy aquifers. J. Contam. Hydrol. 2004, 74, 265-282.
Stefan B. Haderlein and Torsten C. Schmidt Center for Applied Geosciences Eberhard-Karls University of Tuebingen Wilhelmstrasse 56 D-72074 Tuebingen, Germany
Martin Elsner Stable Isotope Laboratory Department of Geology University of Toronto 22 Russell Street Toronto, Ontario M5S 3B1, Canada
Luc Zwank CRP Henri Tudor Centre de Ressources des Technologies pour l’Environnement 66 rue de Luxembourg B. P. 144 L-4002 Esch-sur-Alzette, Luxembourg
Michael Berg and Rene P. Schwarzenbach Swiss Federal Institute of Aquatic Science and Technology (EAWAG) and Swiss Federal Institute of Technology Zurich (ETHZ) Ueberlandstrasse 133 CH-Duebendorf, Switzerland ES0580183
