Environ. Sei. Techno/. 1995,29, 3067-3069
Response to Comments on “Measurement Error and Spatial Variability Effects on Characterization of Volatile Organics in the Subsurface” SIR: We appreciate Hewitt et al.’s interest in our paper on characterizing subsurface VOC contamination that was recently published in this Journal (1). We were pleased that they too recognized the difficulties in conducting reliable soilVOCmeasurements and the potential for using cost-effective on-site measurements vs off-site (laboratory) analyses to obtain more accurate subsurface VOC spatial distributions. However, we disagree with the following assertions they made regarding our work (1) that our principal objective was to compare the analytical accuracy of the on-site (heated headspace) and off-site (purge-andtraplgas chromatographylmass spectrometry) VOC soil analysis methods; (2) that our sampling methods led to significant analyte losses due to soil disaggregation, thus invalidating any subsequent data that we collected from these samples; (3) that our spatial models are invalid since the heated headspace technique is characterized by uncontrollable analytical errors; and (4) that spatial modeling is only worthwhile when analytical variability can be completely separated from spatial variability. In the following discussions, we address each of these issues and present the reasons why we disagree with Hewitt et al.’s assertions regarding our work. ComparingOn-Siteand Off-SiteVOCAnalysisResults: Confirmatory Analyses vs a Methods Comparison. The principal objective of the characterization described in our paper (1) was to determine the distribution of VOCs underlying the study site prior to evauating and implementing remediation alternatives (2). To this end, we collected and analyzed alarge number of spatiallydispersed soil samples, within a reasonable time frame and cost, through the use of a hydraulic probe sampler and on-site VOC analyses by a heated headspace technique (HHS) (2, 3). We also collected soil samples for off-site, purge-andtrap /gas chromatographylmass spectrometry analysis (PTGCIMS, EPA Method 82601, in accordance with regulatory
requirements to confirm on-site analyses with off-site laboratory methods (4). Contrary to Hewitt et al.’s contention, we were not conducting a methods comparison, nor did we expect the HHS analyses to agree quantitatively with the PT-GUMS procedures. We recognize that a rigorous comparison of analytical methods would require that split samples be identically handled prior to comparative analyses. However, a rigorous methods comparison was never the purpose of our work. Soil Disaggregationduring Sample Acquisition. In the preceding comment on our paper, Hewitt et al. suggest that our analytical data are invalid by stating that our sampling method led to soil disaggregation and significant volatilization losses. Figure 1 shows a photograph of soil cores collected using a hydraulic probe sampler from the same Minford formation that underlies the contaminated site in our study. As illustrated in Figure 1, the cohesiveness and plasticity of the Minford soil did not lead to significant soil disaggregation even after extrusion from the collector
0013-936X/95/0929-3067$09.00/0
0 1995 American Chemical Society
FIGURE 1. Soil cores collected using a hydraulic probe sampler. These were taken from the same Minford formation that underlies the study site.
tubes. The cores from the study site were extruded very rapidly using&e same hydraulic mechanism that was used to push the probe into the ground. Hewitt et ala’ssuggestion for conducting subsampling by the “periodic insertion of a subcorer ...while the core was extruded” would have been impractical and difficult to complete within the 2 min that we were able to achieve by extruding the entire core first prior to subsampling. Throughout the sample collection process, we were keenly aware of the volatile nature of our analytes (5, 6) and had made precautions to minimize volatilization losses during sample collection and handling. For the type of soils underlying the study site, we have reason to believe that we had chosen a sampling method that satisfied our criteria for rapid collection and minimal disturbance. Controlling Errors in the Heated Headspace Technique: Defining theAccuracyofVOCAnalyses. The heated headspace technique (HHS) is a rapid method for on-site VOC analysis that was developed and recommended by the EPA as a “quick turnaround method” (3). Determining the accuracy of the HHS method, or any other VOC analysis technique, is extremely difficult due to the lack of standard reference materials for which “true” levels of VOCs are known. Even the performance evaluation samples that Hewitt et al. prepare usingvapor fortification (7) cannot be used for measuring the accuracy ofVOC analysis methods. Although it appears that reasonably uniformly contaminated batches of soil samples can be prepared (71,the “true” VOC concentrations of these performance evaluation samples are still unknown prior to conducting the analyses. In addition, the appropriateness of using spiked samples for accuracy determinations may be questionable, in light of differences in the kinetics of VOC desorption from artificially spiked and field-contaminated soils that have been equilibrated with contaminants over decades (e.g., ref 8). Without standard reference materials, the accuracy of any VOC analytical method can only be judged by assessing the components of the measurement methodology. In the heated headspace technique for measuring the VOC content of a containerized sample (Le., excludingsample acquisition losses that should be addressed by proper sampling methods), these components are as follows: (1)complete
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desorption of the analytes into the sample headspace and (2) reliable sampling and consistent quantitation of the analytes in the headspace by the GC. The latter was assessed through duplicate headspace sample analyses. Acceptable precision of these duplicate headspace samples indicated that random errors in sampling and analyzing the VOC content ofthe heated headspace were not significant.Hewitt et al. speculate that “moist soil heated to 60 “C inside a closed vessel would experience a pressure increase of more than 20% over atmospheric, causing a significant amount of headspace gas to be lost from the syringe during a manual transfer”. Granted that this off-gassing of the headspace samples occurred, these were still not sufficient to produce overwhelming negatively biased (Le.,lower) VOC levels in the HHS samples when compared to the PT-GC/MS analyses of the non-methanol immersed samples. Hewitt et al. also suggest that water vapor contributions could affect the analytical variability of the heated headspace method. Based on our experience with the soil at the study site, the moisture content does not vary significantlywithin 30 cm. Therefore, variation in headspace water vapor was probably not the primary factor behind the short-range p30-cm) spatial variability we observed in the HHS data. Complete desorption of the analytes into the headspace was assumed in calculating the VOC concentrations in the heated headspace samples. Hewitt et al. take issue with this assumption, interpreting it as a contention that requires proof. For the relatively low organic content in the soil ( lower) HHSVOC levels. This bias, if it had existed, was still insufficient to offset the other factors contributing to the negative bias in the off-site, non-methanol immersed samples. Incidentally, the 1-h desorption time at 100 “C cited by Hewitt et al. was for desorption from fuller’s earth (9). This type of soil has a very strong adsorptivity and was considered by the authors of the latter paper as the “worst case” (9). In addition, Hewitt et al.’s statement regarding very slow desorption of the VOCs to the headspace at 60 “C appears to be inconsistent with their previous assertion thatVOC losses from exposed disaggregatedsoil “can exceed 90%”. This remark was made when they were concerned that significant VOC losses had occurred within the 2 min that we were subsampling from the 30-cm cores. Hewitt and others have authored several papers on using the aqueous headspace gas chromatography (HS/GC) method for on-site analysis (e.g., ref 7). In their commentary, they state that HSiGC analysis “enjoys wide acceptance in western Europe and elsewhere”. The latter statement, which is supported by a reference to ref 10, is misleading since the “European”headspace method described in this reference includes heating of the soil samples to as high as 80 “C (IO). The popularity of the European headspace method was unjustifiably applied to Hewitt et al.’s HS/GC methods, given that the latter is conducted at room temperature. In addition, the criticisms that Hewitt et al. fielded against the heated headspace technique we used can very well be applied to the European headspace method which Hewitt et al. describe as having “wide use and acceptance”. SpatialModeling Spatialvs AnalyticalVariance. Shortrange spatial variability of VOC contamination at the study 3068
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site was demonstrated by some large discrepancies between heated headspace analyses of soil samples obtained from the same 30-cm core. Hewitt et al. contend that these discrepancies may be due to (1) random VOC losses that occurred during extrusion of the core and resulting soil disaggregation and (2) analytical variances originating from sources other than the headspace sampling. We have already demonstrated that we did not observe soil disaggregation during core collection. Nonuniform desorptiodextraction of VOCs from collocated soil samples is another possible source of analytical variance that cannot be addressed by the duplicate headspace analysis from the same vial. We believe that analytical variance is extremely difficult to separate from spatial variance in field-contaminated samples. Analytical variance obtained from uniformly spiked samples (e.g.,throughvapor fortification) cannot be validly applied to analysis of field-contaminated soil samples. Reasons for this include the following: (1) different desorption behavior observed in spiked and fieldcontaminated soil samples (e.g.,ref 8) and (2) subsurface heterogeneity in soil properties (e.& total organic carbon content), which can affect both VOC contamination and analyte extraction in field-contaminated soil samples. Of the three methods used for developing threedimensional models of the on-site data, only the kriging technique can account for analytical variability and various scales of spatial variability. The parameter quantifymg analytical variability was estimated from duplicate headspace samples; we used this approach since quantification of the headspace GC analysis was the only component of the analytical variance that we could reliably assess. The rest of the analytical variance (Le., due to changes in extraction efficiency)was combined with spatial variability. Incorporation of the analytical and spatial variances, although not completely separated as one would desire under ideal conditions, enabled us to measure the uncertainty in the predictive capability of the kriging model. We have predicted the VOC distribution underlqing the study site, recognizing that uncertainties in these models are impossible to completely characterize and validate under the constraints and difficulties of measuring VOCs in soils. Hewitt et al. believe that the tabulation of sample averages and standard deviations in Table 2 of our paper (1) was inappropriate and inconsistent with our observation that the sample distribution for the totalVOC concentration was log normal. Our intent in presenting these sample statistics was to give the reader a general sense for the range and magnitudes of the measured levels of the individual analytes in the heated headspace samples. The sample average and standard deviation are mathematically-defined descriptive sample statistics. It is true that the underlying distribution determines the use of these sample statistics to estimate statistical distribution parameters (e.g.,sample average as an estimator for the mean). However, our tabulation of sample statistics was not for this purpose. One final observation pertains to Table 3 of Hewitt et al.’s commentary, in which field-contaminated samples were analyzed for trichloroethene (TCE)using their aqueous HSiGC method. Table 1 of this response shows a calculation of the ratio between the higher and lower concentrations in depth-matched samples from boreholes C and D. These ratios ranged all the way from 1.03to 4.2 to an extreme value of 56.52. Hewitt et al. describe data from boreholes C and D as “mirroring each other”,yet Table 1 shows ratios that are comparable to the ratios we calculated for our
TABLE 1
Calculated Ratios between Depth=Matched Samples from Boreholes C and D Shown in Table 3 of Hewitt et al.‘s Cornmentaw depth (m)
5-6 8-9 10-11 11-12 13-14 15-16 16-17 18-19 20-21 23-24 39-41
TCE (mg/kg) in borehole C borehole D
0.097 5.9 0.33 0.32 0.40 7.8 0.41 0.069 0.10 0.31 0.42
0.087 1.4 0.34 0.18 0.93 3.2 0.49 3.9 0.27 0.22 0.47
depth-matched TCE ratio
1.11 4.2 1.03 I .7a 2.33 2.44 1.20 56.52 2.70 1.41 1.12
a Only depths with samples at both boreholes C and D are shown. Ratio is calculated as the higher concentration divided by the lower concentration in a pair.
collocated samples (see Table 2 of ref 1). It is unclear to us how one can say that “an adequate number of samples had been taken” by simply looking at a table of numbers without doing interpolations, let alone without a clear definition of the domain being characterized by the samples. In fact, it is hardly worth performing any kind of threedimensional modeling on this data given the small number of points. Closure. Measurement error, as used in the title of our paper, encompasses errors from degradation/loss of VOCs during sample handling and storage, inefficient extraction of VOCs from the sample matrix, and incorrect quantification by the instrument (which can either be an instrument error or a calibration error). In conducting the characterization work at the land treatment facility, we were cognizant of the volatile nature of our target analytes. We had selected what we thought to be the best available methods that would minimize volatilization losses and analytical uncertainties, while giving us a relatively large number of dispersed point VOC measurements throughout the contaminated site.
In seeking to publish our work in this Journal,we wanted to share our experience in measuring and modeling VOC distributions as well as to provoke insightful thought regarding the process of characterizing VOC soil contamination. We hope that this response to Hewitt et al.’s comments has furthered the fulfillment of our publishing objectives.
Acknowledgments We would like to thank Drs. Charles Bayne, Max Morris, and Bruce James for providing critical reviews of our response.
Literature Cited (1) West, 0. R.; Siegrist, R. L.; Mitchell, T. 7.; Jenkins, R.A. Environ. Sci. Technol. 1995, 29, 647-656. (2) West, 0. R.; Siegrist, R. L.; Mitchell, T. J.; Pickering, D.A.; Muhr, C. A.; Green, D. W.; Jenkins, R. A. X-231B Technology Demonstration for In Situ Treatment of Contaminated Soil: Contaminant Characterization and 3-0Spatial Modeling Oak Ridge National Laboratory: Oak Ridge, TN, 1993; ORNL/TM- 12258. (3) Draft statement of work for quick turnaround analysis; US. Environmental Protection Agency, Analytical Operations Branch: Washington, DC, 1993. (4) Data Quality Objectives Process for Superfund (Interim Final Guidance); U.S. Environmental Protection Agency, Office of Emeraencv and Remedial Resoonse: Washington, DC, 1993; EPA/
