Comment on "Measurement Emr and Spatial Variability Effects on Characterization ofVolatile Organics in the Subsurface" SIR: The introduction of the recent paper by West et al. ( 1 ) notes that "The accurate characterization of VOC spatial distributions depends on the accuracy of the discrete measurements and the density of their locations at the site". Clearly,this statement could be made for any contaminant, but it poses especially severe problems when the contaminants are volatile organic compounds (VOCs). The usual process of collecting relatively few samples for later analysis off-site introduces major uncertainties with VOCs due to their susceptibility to large and highly variable losses during sampling, storage, subsampling, and analysis (27). West et al. suggest that on-site methods of VOC determination may provide equal or greater accuracy than off-site methods, and we agree that such potential exists. Implicit in the requirement for improved accuracy is the need for acceptable precision. Using data from a former treatment facility for waste oils and solvents, the authors compare on-site and off-site estimates of VOC concentrations using split samples from 30-cm-long sample sections extruded from a core sampler. On-site results were then used to produce three-dimensional models of spatial variability. Unfortunately, both the accuracy comparisons and the spatial variability estimates are seriously flawed because the sampling and analysis protocols exclude any possibility ofvalid separation of analytical and spatial variances. The purpose of this comment is to suggest alternative interpretations of their results and to provide some evidence that may help make future efforts more definitive. Valid estimates of spatialvariabilityrequirethat the entire analyticalsystem be under control and that the uncertainties in concentration estimates be as small as possible. For proper variance differentiation, the analytical error should be no larger (and preferably smaller) than the variations in concentration over short distances. Analytical uncertainties should be included for all steps in the process, which starts with sample collection and ends with calculation of concentrations. In the West et al. paper, analytical uncertainty was claimed to be a very small component of the total variance. However, we disagree with this conclusion, which was based on agreement of duplicate headspace gas samples removed from each of the nine vials and analyzed by gas chromatography. In this case, all concentration variability associated with sampling, subsampling, heating, etc. was excluded from the analytical variance. In other words, most of the analytical imprecision was incorrectly assigned to spatial variability. In contrast to their conclusion, it is possible that short-range spatial variations were actually quite small. Take a further look at the West et al. sample collection and splitting procedures. Samples were obtained with a
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hydraulic probe sampler. Immediately thereafter 30-cmlong by 1.25-cm-diameter portions (about 100 g) were extruded into polyethylene bags ( I , 8). Then 10-20-g subsamples were removed using a subcoring device and placed in containers appropriate for the intended VOC analysis method. Efforts were made to keep the time for this transfer under 2 min, but our experience and that of others show that whenever soil is disaggregatedand exposed in this manner, volatilization losses can exceed 90% (3- 7). Voice and Kolb even found that maintaining the sample at -32 "C was not adequate to stop volatilization losses (9). In fact, there is increasing consensvs that such sampling and subsampling losses are the major contributor to unreliable VOC analyses. In thik study, exposure and disaggregation could have been drastically reduced by periodic insertion of a subcorer of suitable diameter into the tip of the hydraulic probe while the core was extruded. Throughout this paper, West et al. promote a heated headspace technique (HHS) for on-site analyses. The system was calibrated using pure compound headspace standards under the assumption that ... sample heating at 60 "C for at least 30 min had caused the VOCs to entirely volatilize from the mineral soil into the sample headspace". However, no data are offered in support of this contention, and there is considerable reason to doubt its validity. A 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. Furthermore, this over-pressurization would include water vapor contributions, which vary from sample to sample. Even if their gaseous standards had the same amount of water as each sample, these sources of error would not be under control. This is because when using HHS at temperatures below 100 "C, the adsorptivity of the matrix affects quantitative interpretations because the VOCs are not entirely volatilized (10, 11). In contrast, at 100 "C the complete volatilization and desorption of VOCs from field samples are expected but are likely to take more than 1 h (12). Clearly, rapid (30 min) moderate temperature (60 "C) HHS analysis is a good screening tool forVOCs,but it should not be expected to agree quantitatively with purge-andtrap gas chromatography mass spectrometry (PT-GC/MS) procedures, even when all other variables are held constant. Briefly then, neither the precision nor the accuracy of HHS has been adequately evaluated. The task of comparing any two soilVOC analysis methods is plagued by both systematic and random errors as long as a single inconsistency exists in the subsample handling procedures (13). This problem has been clearly demonstrated in both field (2- 7 ) and laboratory studies (9, 1416). In Figures 2 and 3 of West et al., we see a large bias in addition to large random errors when comparing onsite HHS with off-site PT-GUMS, EPA Method 8260 (1 7). In part, the systematic bias may be caused by the need to recap vials prior to PT-GUMS analysis and the storage time, whereas HHS was done without recapping or storage. "
0013-936X/95/0929-3064$09 OOIO
E 1995 American Chemical Society
TABLE 1
TABLE 3
Trichloroethylene Concentrations (mg of TCVkg) in Collocated Composite Field=Contaminated Samples (5)
Concentrations (mglkg) of Trichloroethylene Measured by Aqueous Extraction HS/GC Analysis of Subsamples Taken from Four Boreholes
aqueous extraction HS/GC A B C
bore holes
PT- G C/MS
0.31 0.58 25
0.28 0.69 22
depth (m) 0- 1 1-2 3-4 5-6 6-7 7-8 8-9 10-11 11-12 13-14 15-16 16-17 17-18 18-19 20-21 21-22 22-23 23-24 25-26 26-27 28-29 30-31 31 -32 33-34 35-36 36-37 37-38 39-41
TABLE 2
trans-1,2.Dichloroethylene (TDCE),
Trichloroethylene (TCE), Benzene (Ben), and Toluene (Tol) Concentrations in Vapor-Fortified Soil Subsamples, Contained in Sealed Glass Ampules mg of VOCkg aqueous extraction HS/GC (18)
methanol extraction PT-GC/MS (19
Tampa Bay Sediments TDCE TCE Ben To1
8.0 f 0.3a 10 f 0.6 9.1 f 0.3 11 f 0.6
6.39 f 1.80b 8.58 f 0.85 7.24 f 0.71 10.2 f 0.95
TDCE TCE Ben To I
13 f 1.0 16 ic 0.6 15 & 0.0 18 f 1.0
11.0 f 1.9 15.6 f 1.7 14.4 f 1.2 19.4 f 1.8
TDCE TCE Ben To1
Pt. Barrow, Alaska 37 f 0.6 56 f 0.6 38 f 0.6 65 f 1.7
39.5 56.2 39.5 63.5
Rocky Mt. Arsenal
i 5.4 i 7.6
f 3.5 f 7.7
a
A 1.6 5.3 11 6.3 82 74 14 2.0 3.0 2.5 5.0 0.25 0.41
B
C
0.41 0.32 2.0 1.8 0.33 1.1 2.4 1.2 0.15 3.4 0.37 0.22 0.63
D N Da ND ND 0.087
0.041 0.097 0.27 0.48 5.9 0.33 0.32 0.40 7.8 0.41
1.4 0.34 0.18 0.93 3.2 0.49
0.069 0.10
3.9 0.27
0.31
0.23 0.22 0.39
0.29 0.42 0.35
1.o
0.37 0.22 0.60 0.30 0.47 0.47
0.42
ND, not detected (detection limit 0.003 mg of TCElkg)
a n = 3. * n = either 18 or 20, and data were generated in duplicate in either 9 or 10 laboratories.
This reinforces the point made earlier about errors caused when subsample manipulation involvesmultiple exposures. In the comparison of collocated subsamples that were analyzed by HHS with those that were immediately immersed in methanol and analyzed by PT-GCIMS off-site, there was an apparent concentration dependence in the bias. Although possible explanations of this behavior were offered, the truth is that we simply do not have the information required to justify any explanation. Aqueous extraction headspace gas chromatography (HSI GC) analysis is also compatible with on-site determination ofVOCs in soils. This method, which involvesthe dispersion of a soil subsample (2-4 g) into water (20-30 mL), has been compared to the EPA Method 8240 (17)PT-GUMS analysis in studies that eliminated inconsistencies in sample handling (5-7,13, 18). Furthermore, HS/GC, enjoys wide acceptance in western Europe and elsewhere (18). It is apparent from Tables 1 and 2 that acceptable precision and accuracycan be realized for VOC measurements. These tables show examples of method comparison performed with both composite field samples (multiple plugs of soils quicklytransferred to a single vial) (5)andlaboratory-treated soil subsamples (18, 20). To our knowledge, no similar scientific evidence, wide use, or acceptance exists for HHS. The efforts of West et al. (1) to model the distribution of TCE at their site fails because spatial variability is inseparable from procedural (sampling and analysis) uncertainties. However, if the errors associated with
Previous Location of
‘I
1\I )
Parking Lot
flb
I
FIGURE 1. Locations of boreholes relative to the area where a leaky underground trichloroethylene storage tank had been.
sampling and analysis are properly addressed, spatial modeling can indeed prove very useful. Table 3 shows some results we have obtained for TCE in soil subsamples taken from split spoons during an investigation that profled the vadose zone for this contaminant. All of these subsamples were taken from split spoons using a nondisruptive single-transfer collection method and analyzed by aqueous extraction HS/GC (5-7). At our site, boreholes A and B straddled the area where a leaky underground TCE storage tank had been located, while boreholes C and D were located some 15 m to the west (Figure 1). Since the subsample concentrations from boreholes C and D closely mirror each other, perhaps with the exception of the 18-19-m depth, VOL. 29, NO. 12.1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
3065
it is very likely that the modeling would confirm that an adequate number of samples had been taken from this region of the vadose zone. Before leaving the topic of spatial distribution, we must comment on Table 1of West et al. (11,where the entire site is characterized by means and standard deviations of untransformed concentration values. After explaining that the concentrations followed log normal distributions and after using log transforms for paired comparisons, we do not understand why arithmetic means and standard deviations were presented in Table 1. When standard deviations are two to four times as large as means, it is quite apparent that these statistics are inappropriate numerical descriptors of the distribution. We agree with the authors of this paper that one reasonable approach is to collect more data by employing cost-effectiveon-site analysis methods. However, one must recognize that the data can never be better than the method of sample collection and handling. Furthermore, the method chosen for on-site VOC analysis must have been properly validated with respect to precision and accuracy so that an optimal sampling strategy can be designed.
Acknowledgments Funding for this work was provided by the U.S. Army Environmental Center, Martin H. Stutz, Project Monitor. The author thanks Thomas F. Jenkins and Marianne Walsh for critical review of the text. This publication reflects the view of the authors and does not suggest or reflect policy, practices, programs, or doctrine of the U.S. Army or of the Government of the United States.
Literature Cited (1) West, 0. R.; Siegrist, R. L.; Mitchell, T. J.; Jenkins, R. A. Environ.
Sci. Technol. 1995, 29, 647-656. (2) Urban, M. J.; Smith, J. S.; Schultz, E. K.; Dickinson, R. K. 5th Annual Waste Testing & Quality Assurance Symposium; US.
Environmental Protection Agency: Washington, DC, 1989: II87-11-101. (3) Hewitt, A. D. Presented at the National Symposium on Measuring and Interpreting VOCs in Soils: State of the Art and Research Needs, Las Vegas, NV, Jan 12-14, 1993. (4)Illias, A. M. Hydrocarbon Contaminated Soils; Lewis Publishers: Boca Raton, 1993; Vol. 3, pp 147-165.
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29. NO. 1 2 . 1 9 9 5
(5) Hewitt, A. D.; Miyares, P. H.; Sletten, R. S. Hydrocarbon contaminated Soils; Lewis Publishers: Boca Raton, 1993; Vol. 3, pp 135-145. ( 6 ) Hewitt, A. D. 1.Assoc. OR. Anal. Chem. 1994, 77, 458-463. (7) Hewitt, A. D.; Jenkins, T. F.; Grant, C. L. Am. Entiron. Lab. 1995, Jan-Feb. (8) 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 Characterizationand 3 - 0 Spatial Modeling; Oak Ridge National Laboratory: Oak Ridge, TN, 1993; ORNL/TM- 12258. (9) Voice, T. C.; Kolb, B. Environ. Sci. Technol. 1993,27, 709-713. (10) Kolb, B.; Pospisil, P. Angew. Chromatogr. 1985, No. 43. (11) Kolb, B.; Pospisil, P.;Auer, M. Chromatographia 1984,19,113122. (12) Kolb, B.; Pospisil, P.; Auer, M.; Voice, T. M. 1.High Resolut. Chromatogr. 1994, 17, 299-302. (13) Hewitt, A. D.; Miyares, P. H.; Leggett, D. C.: Jenkins,T. F.Ent~iroi7. Sci. Technol. 1992, 26, 1932-1938. (14) Jenkins, T. F.; Schumacher, P. W. SR 87-22. U.S. Army Cold Regions Research and Engineering Laboratory: Hanover, KH, 1987. (15) Siegrist, R. L.; Jenssen, P. D. Environ. Sci. Technol. 1990, 24, 1387- 1392. (16) Lewis, T. E.; Crockett, A. B.; Siegrist, R. L.; Zarrabi, K. EPAi 59014-91/001. Technology Innovation Office, Office of Solid Waste and Emergency Response, U S . EPA: Washington. DC. 1991. (17) Test Methods for Evaluating Solid Waste: 1986; US. Environmental Protection Agency, U.S. Department of Commerce, National Technical Information Service: Washington, DC, 1986: Vol. 1B. (18) Hewitt, A. D. Am. Environ. Lab. 1994, March. (19) Voice, T. C.; Kolb, B. J. Chromatogr. Sci. 1994, 32, 306-311. (20) Hewitt, A. D.; Grant, C. L. Environ. Sci. Technol. 1995,29,769773.
Alan D. Hewitt* and Daniel C. Leggett US. Army Cold Regions Research and Engineering Laboratory Hanover, New Hampshire 03755-1290
Clarence L. Grant University of New Hampshire Durham, New Hampshire 03824 ES950346N
