Environ. Sci. Technol. 1997, 31, 67-70
Chemical Preservation of Volatile Organic Compounds in Soil ALAN D. HEWITT* U.S. Army Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, New Hampshire 03755-1290
Laboratory protocols are necessary for assessing the concentration stability of volatile organic compounds (VOCs) in site-specific soil matrices, because refrigeration (4 °C) alone does not prevent microbiological degradation. An experimental approach was developed using hermetically sealed vials for storage and in-vial analysis. This technique avoids volatilization losses while assessing the apparent rate of microbiological degradation and enables an assessment of the effectiveness of chemical preservation. Using this approach, concentrations of most of the chlorinated compounds tested were unaffected by storage temperature and/or chemical preservation over a 28-day holding period. Aromatic compounds, however, were rapidly degraded (>95% loss) when stored at 22 °C over this same period and often showed a substantial reduction (30% or greater loss) when stored at 4 °C. Chemical preservation with either sodium chloride (NaCl) or sodium bisulfate (NaHSO4) suppressed the microbiological degradation of these aromatic compounds in samples held at 22 °C.
Introduction Soil samples collected during site investigations are often sent off site for laboratory analysis for volatile organic compounds (VOCs). To allow for some flexibility between collection and analysis, the U.S. Environmental Protection Agency (1) has recommended that samples be stored at 4 °C and held no more than 14 days in hermetically sealed bottles. This practice continues although it is well-recognized that the microbes in soils can remain active under these conditions, thus continuing to degrade labile compounds. Past efforts attempting to demonstrate the likeliness and magnitude of biological degradation at low temperatures were confounded by volatilization losses (2-4). Only recently have studies been performed in which volatilization was eliminated, thereby solely addressing the microbiological degradation of VOCs in soil samples held at 4 °C (5-7). The objective of this study is to document a simple laboratory procedure for assessing the apparent rate of microbiological degradation of VOCs in soil matrices and to present a promising means of chemical preservation using sodium chloride (NaCl). Experimentally, spiked soil samples were held in sealed glass ampules while exposing them to various storage periods, temperatures, and methods of chemical preservation. Presented here are the results assessing the concentration stability of benzene (Ben), toluene (Tol), ethylbenzene (E-Ben), p-xylene (p-Xyl), o-xylene (o-Xyl), trans-1,2-dichloroethylene (TDCE), cis-1,2-dichloroethylene (CDCE), chloroform (CF), trichloroethylene (TCE), and tetrachloroethylene (PCE). Experiments were performed looking at the effects of temperature and chemical preservation with sodium bisulfate (NaHSO4) and NaCl on analyte concentration in soil samples * Fax: (603) 646-4278; e-mail:
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S0013-936X(96)00095-8 This article not subject to U.S. Copyright. Published 1996 by the American Chemical Society.
held over a 28-day storage period. Sodium bisulfate has previously been used successfully to prevent microbiological degradation of VOCs in aqueous matrices (8), while NaCl has historically been used for the preservation of food. Both of these chemical preservatives have a low human toxicity and are compatible with field sampling operations. The longer storage period was selected to complement efforts to establish longer holding times for VOCs in chemically preserved samples (9). The experimental protocol presented here is consistent with a common pathway of environmental pollution and with the practice of in-vial soil sample storage and analysis. Soil samples were fortified by adding groundwater containing the analytes of interest and held in sealed glass ampules that were transferred intact to volatile organic compound analysis (VOA) vials when preparing for analysis. Currently, two invial methods (Methods 5035 and 5021) are scheduled for inclusion in the third update of the SW-846 (10). With an in-vial approach, a soil sample (1-5 g) obtained during the site investigation for VOC characterization is placed directly into a VOA vial, from which it can be analyzed without ever breaking the hermetic seal created upon capping. The invial approach thus avoids the volatilization losses that occur when a bulk soil sample is transferred to a storage bottle from which a subsample must be removed in preparation for analysis (11).
Experimental Section The silty-sand (40%) in concentration over the 14-day storage period (Table 1). While the general rate of degradation for this soil was fairly slow when samples were refrigerated, studies performed with a soil matrix amended with manure and with sediments from an aquifer having an average temperature of 5 °C have shown much faster rates of degradation of aromatic compounds when stored under refrigerated conditions (7, 19). These studies all have shown that, depending on the soil matrix, refrigeration is not a sufficient means of eliminating microbiological degradation of aromatic VOCs in samples being held for analysis. In contrast, only small losses (
