5.4.2 Volatile Organic Compound ISM Samples
ISM samples can be collected for VOC contaminant analyses. ISM increments are placed directly into the appropriate volume of methanol in the field.
ISM samples can also be collected for VOCs contaminant analyses from cores, excavation-pit bottoms and walls, stockpiles, underneath paved areas, etc. USEPA SW-846 Method 5035A Section 8.2.2 (USEPA 2002b) describes the collection of discrete soil samples preserved in the field. The ISM VOC approach is similar to this method and to that described for sampling ISM nonvolatiles in the subsurface, except that numerous soil increments are placed directly into an adjusted volume of extraction solvent in the field (e.g., methanol, shown in Figure 5-11). Individual increment mass should be similar provided the soil density is fairly uniform. Typically, individual increments are not weighed in the field during collection. Similar mass per increment is assumed with similar volume collected.
Figure 5-11. Bottles containing methanol and 44 five-gram plugs of soil.
For exposed soils, such as surface soils or exposed excavation sidewalls/bottom soils, the entire mass of soil collected at a single point represents an increment. These increments are collected using VOC coring devices (see Figure 5-12) and combined in a sample bottle containing a predetermined volume of methanol (see Figure 5-11). Thus, VOC ISM samples of exposed soils are collected and combined in similar fashion as non-VOC ISM samples with the exception that they are field-preserved in methanol.
ISM sampling may also be used for VOCs in the subsurface. As previously discussed in Sections 3 and 18.104.22.168, ideally the entire mass of soil collected in a subsurface core across the targeted DU depth represents the increment for that boring. The entire mass, therefore, would be preserved in methanol and incorporated into the ISM sample for the targeted soil layer. Realistically, this task is impractical, since the volume of methanol required to preserve entire core increments or the combination of increments from multiple cores would be impractically large. Additionally, preserving the entire core would prevent increments for non-VOC contaminants to be collected, if required.
Instead, the core may be subsampled by collecting numerous, small (e.g., 5 g) “plugs” at regularly spaced intervals along the targeted DU depth interval of the subsurface core. As with ISM VOC sampling of exposed soil, the plugs are immediately placed in a sampling bottle containing a predetermined volume of methanol. Figure 5-13 shows an example of this type of ISM VOC sample collection from subsurface cores. Nominal 5 g plugs of soil can be collected across the core using a VOC coring device (see Figure 5-12). The spacing interval of the VOC plugs along the core interval should be determined during the systematic planning process. It is possible to determine the optimal spacing on a site-specific basis, through the collection and analysis of differently spaced plugs along the core interval. However, based on limited field experience to date, plugs should be located no more than 2 inches apart as a starting point. This distance was determined to be adequate to capture the potential heterogeneity of VOC concentrations along the vertical length of the core. It may be necessary to decrease the spacing depending on the site-specific distribution of contaminant concentrations and DQOs. The coring device used to collect the increments should be filled completely so that each increment has the same volume of soil. The complete soil plug must be transferred to the sample container. Additionally, the ISM sampler should be aware of potential volatile loss once the core is opened. ISM VOC increments should be collected and preserved as quickly as possible to minimize potential loss. Potential loss of COPCs due to volatilization during collection of ISM increments is expected to be similar to discrete sample collection by USEPA SW-846 Method 5035A for the same sample density across a subsurface core.
Figure 5-13. Example of sample increments being collected and added to a bottle containing methanol for preserving VOC samples.
In general, the potential to combine a larger mass of soil from multiple plugs from a subsurface core spaced along the entire length of a targeted DU depth results in a VOC sample that is more representative of the soil core. However, the handling and shipping of large volumes of methanol, as well as tracking and combining preserved increments into a single ISM sample, may present logistical issues, as discussed in the following paragraphs.
Soil increments should remain completely submerged in methanol at all times. If increments are combined in the field, it is important to use a volume of methanol large enough to accommodate all of the increments. Project planning must determine the number and size of increments (see Section 3). Laboratory personnel should be consulted during systematic planning so that sample size, methanol volume, and bottle size are determined in advance.
Shipment of solvent to and from the sampling activity can be problematic. When possible, methanol should be transported to the field via a surface transport to avoid or mitigate volume limitations common in air transport. Guidelines for the transportation of a solvent such as methanol can be found in 49 CFR §172, “Hazardous Materials Table, Special Provisions, Hazardous Materials Communications, Emergency Response Information, Training Requirements, and Security Plans.” Shipments via air transport may also be required to adhere to International Air Transport Association Dangerous Goods Regulations (IATA DGR, IATA 2011).
If the larger volume of methanol presents logistical problems for shipping which cannot be satisfactorily addressed, alternatives can be considered in consultation with the laboratory. With procedures and protocols in place ahead of time, these alternatives may include the following:
- The larger volume of methanol could be subsampled in the field, prior to shipment to the laboratory. With this option, the complete ISM methanol-preserved sample is disaggregated/ extracted in the field by shaking periodically for at least 24 hours, allowing the solids to settle, decanting or pipetting 20–30 mL of methanol into a vial, and shipping this aliquot to the laboratory for analysis. The total mass of the ISM soil sample, as well as the total volume of methanol, must be recorded and provided to the laboratory.
- Increments for VOC analysis could be collected and preserved with methanol individually (e.g., 5 g soil in 5 mL methanol in volatile organic analysis vials per USEPA SW-846 Method 5035A) and submitted to the laboratory for combination of methanol aliquots before analysis. The laboratory would remove equal aliquots of methanol from all individual increment vials and combine them in a single vial to represent the complete ISM VOC sample, using the methanol handling techniques described in USEPA SW-846 Method 5035A (see Figure 5-14). This option also allows for analysis of individual increments or alternate combinations of increment groups, if required. Additionally, this option allows flexibility for varying the number of increments without having a large variety of large volume ISM sample bottles. Disadvantages include increased supplies, labor costs, and sample tracking logistics.
Figure 5-14. Example of methanol aliquots from individual 5 g field-preserved increments being combined in the laboratory.
- Individual increments could be collected in separate sampling devices that have vapor-tight seals and are designed for zero headspace (e.g., Core N’ One™, EnCore, or equivalent type sampler), and submitted to the laboratory at the appropriate temperature and within appropriate time frames (typically 24–48 hours) for combined placement in methanol before analysis.
- To fall under the small-quantity exemption of the shipping regulations, ISM volatile “subsets” could be collected, preserved with methanol in the field, and submitted to the laboratory for combining before analysis. For example, six increments of 5 g each would be collected in an appropriate container containing 30 mL of methanol. Five of these volatile subsets would be collected for a 30-increment ISM sample and submitted to the laboratory. The laboratory would then combine equal methanol aliquots from the five subsets for analysis.
ISM VOC sampling procedures should minimize soil disturbance and possible VOC loss due to volatilization. For this reason bottles that have a narrow neck or other means of restricting volatilization losses and containing the volume of appropriate solvent should be prepared prior to the sampling activity. Typically, the bottle and solvent are prepared and preweighed at the laboratory prior to shipment to the field. This method allows for laboratory calculation of the final ISM soil mass. The volume of solvent should at least equal the mass of soil that will be introduced. The headspace to preserved sample ratio (methanol + sample) should be less than or equal to that commonly achieved with discrete methanol VOC preserved samples (e.g., ~32 mL headspace to 8 mL preserved sample). Details should be specified in the SAP, and any alterations due to unforeseen field conditions should be recorded in field logs. When target analytes require immersion in a solvent, trip and field blanks (no sample added) should be included, depending on DQOs. For example, when sampling for VOCs, if samples are immersed in methanol in the field, then trip blanks and field handling blanks, that is, bottles containing this solvent, should travel to and from the field and the field blank bottle(s) should be opened in the field under the same conditions and for the same amount of time as the sample bottles.
Increments should be collected using tools that minimize the loss of VOCs during sample collection and allow the collection of at least a 5 g mass of soil. Special coring tools should be used for the collection of sample increments to be analyzed for VOCs, and increments should be quickly transferred to bottles containing methanol or another appropriate solvent (Hewitt et al. 2008). Syringe-type devices that can be pushed directly into the soil are preferable (e.g., Core N’ One™, Terra Core Sampler, Easy Draw Syringe®, and PowerStop Handle®, etc.). Examples of VOC coring tools are depicted in Figure 5-12. These types of devices, which are available in different sizes, can also be used for the collection of samples to be tested for nonvolatile chemicals. The device is pushed into the soil and retracted, and the increment collected is immediately extruded into a container with a premeasured volume of solvent (e.g., methanol). This procedure is repeated with each increment. Sampling devices can be used within a DU without decontamination but should be decontaminated or disposed of between DUs.
Additionally, a separate, unpreserved soil sample for percent moisture determination should be collected if necessary to report the ISM VOC results on a dry-weight basis. Typically, the unpreserved soil sample should be collected in the same manner as the ISM VOC samples, with a second increment collected at each ISM increment location and placed in an unpreserved container (4 ounces or larger) and submitted to the laboratory.
A minimum of a 1:1 ratio of solvent volume to sample soil mass (i.e., 1 mL of methanol to 1 g of soil) is recommended. This procedure is a conservative recommendation, since a 5 g plug of soil typically has a volume of around 3 mL. Soil increments should remain completely submerged at all times. Additional solvent may be required to ensure that the sample mass is completely submerged by the solvent. This requirement should be discussed with the laboratory. Select the sample container based on the total mass of soil to be collected and solvent required (e.g., 30 increments of 5 grams, approximately 3 mL volume of solid material per increment). For 30 increments a minimum of 150 mL solvent is recommended (see Figure 5-11). Use a container that is large enough to accommodate additional solvent (if needed) and to prevent loss of solvent through splashing as soil increments are dropped into the container. The headspace to preserved sample ratio (methanol + sample) should be less than or equal to that commonly achieved with discrete methanol-preserved VOC samples. Potential headspace loss in ISM VOC samples is expected to be comparable to conventional discrete methanol preserved VOC soil samples (refer to USEPA SW-846 Method 5035A). Note: An unpublished study from Hawaii using a large bottle with methanol-preserved VOCs was stored in the sun and repeatedly opened over the course of the day to simulate increment additions. VOC recovery was better than 80% for all analytes except dichlorodifluoromethane.
Typically, a 24-hour period is a long enough period to extract VOCs from most soils. Tight clays are an exception and may take several days (Hewitt et al. 1992). Therefore, caution should be taken if the plugs of soil do not readily disperse when submersed in methanol. Soils should be completely disaggregated or dispersed in the solvent to ensure efficient extraction.
The analytical laboratory should be consulted prior to sample collection to discuss sample containers, sample handling, solvent type and volume, shipping of samples in methanol, anticipated analytical detection limits, etc.
Guidance on using ISM for the collection and handling of samples for the analysis of VOCs has been published by the State of Alaska (ADEC 2009). The Alaska guidance recommends that consultants provide a sampling and analysis work plan to the overseeing regulatory agency for review and comment prior to collecting any ISM samples. The analytical laboratory should also be consulted prior to sample collection to discuss sample containers, sample handling, solvent type and volume, shipping of samples in methanol, anticipated analytical detection limits, etc. A potential drawback of ISM for VOCs is that the methanol preservation (high-concentration method) approach does result in lower sensitivity. The methanol dilution step causes elevated analytical detection limits, method detection limits (MDLs), reporting limits (RLs), practical quantitation limits (PQLs), etc., as compared to the direct soil purge-and-trap, low-concentration method techniques. Analytical detection limits could be elevated above relevant screening levels for certain targeted contaminants (see Section 6.3.2). If the analytical detection limits (or other issues) present difficulties in using ISM for VOCs, this issue should be discussed with the laboratory and the overseeing regulatory agency prior to sample collection. If the projected analytical detection limits are too high to be of use or some other issue restrains the use of these methods at a specific site, then alternative approaches may need to be used. Options may include alternate analytical methods/techniques, such as selective ion monitoring (SIM), to achieve lower detection limits or select discrete sampling via USEPA SW-846 Method 5035A low-level VOC sampling. Research to improve detection limits from ISM VOC samples is ongoing and expected to improve in the near future. Consult with the laboratory for the latest detection limit capabilities.