Soil Sampling for VOCs: Best Practices: An overview of methods and equipment options

By David Kaminski, Senior Vice President, Vice President of Technology, QED Environmental Systems, Inc.

Special to The Digest

In all types of environmental testing, using proper sample collection methods is vitally important, as the data analyzed in the laboratory is only as good as the quality of samples collected in the field. Soil testing and sampling is no different than any other kind of environmental testing in this regard. Collecting soil samples by the correct methods is the only way to get accurate, usable data. When collecting soil samples to test for volatile organic compounds (VOCs), every action performed from the time that the soil cores are collected to the preparation of the samples for shipment can affect the VOC recovery and overall quality of those samples. Soil samples can show significant losses in VOC concentration within only seconds of opening soil cores. Due to this inherent volatility of soil samples, following an exact and careful methodology that covers all parts of the sampling process is of the utmost importance.

Proper Collection and Handling of Soil Cores

One of the primary factors that can affect VOC recovery in soil samples is the time spent handling the soil core after the liner is opened. Other factors that may contribute to the rate at which VOCs are lost from the core include the soil type (consolidated vs unconsolidated), soil moisture content, disturbances of the soil core integrity, ambient temperature, and the amount of heat generated by the soil coring method.

Considering all the possible ways that a soil core can lose VOCs, it is vital to follow the best practices while handling the cores and collecting soil samples. First, use a soil coring tool with a liner. Soil sampling tools without core barrel liners, such as split spoon samplers, make it nearly impossible to retain accurate VOC levels before samples are collected, even when using proper sampling methods and procedures. Acrylic liners are most common and have the advantage of being easily opened along their length for sampling. It’s best to cap the acrylic liner immediately after collection to preserve VOCs. The liner should not be split until immediately before it is going to be sampled. Once the liner is split, the sample must be collected as soon as possible – every minute counts.  If the samples cannot be taken immediately after the cores are drawn, it may be necessary to keep the core liners on ice to regulate the temperature of the cores until they are sampled. Finally, before sampling, screen the cores for high VOC concentrations to better select quality samples. USEPA SW 846, Method 3185 is the current recommended screening method, although it is not mandatory.

Along with following the above practices, it is best to avoid a few common mistakes when sampling with acrylic soil core liners. One common mistake to avoid is sampling from the container used for screening the samples rather than from an undisturbed core section. Another practice to avoid is taking a second soil core for samples after screening the initial boring, as there is no guarantee that samples from different locations will match in terms of their VOC content. Lastly, bulk sampling methods should be avoided in favor of field-preserved or sealed sampling methods.

Following the practices described above is the best way to avoid getting low quality samples from collected soil cores. However, there are three different methods of actually sampling a soil core to send to the lab for analysis, all with their own advantages and limitations to consider. These three methods are bulk sampling, sub-core sampling, and sampling using an En Core sampler.

Bulk Sampling

Bulk sampling is done by essentially cutting or scooping the sample from an unlined soil core, then packing that sample in a wide-mouth jar with no preservation. The lab will then take this jar and prepare a small sub-sample from it to be analyzed. Many factors can affect the accuracy of samples collected using the bulk sampling method, including the type of soil, the microbe content of the soil, the time taken to collect the sample, the air space left in the jar, the ambient temperature, the hold time of the sample in the jar before it is prepped and analyzed, the time spent subsampling in the lab, and, finally, whether the jar was properly sealed after the sample was collected (grit from the soil can often catch on the rim of the jar and prevent it from sealing). Due to all of these possible issues, the accuracy of bulk sampling varies widely, from just 0.01% of true value up to 95%. Studies have shown that bulk sampling, at best, gives an indication of the presence of VOCs but not an accurate estimation of VOC concentration.

Field Preservation Sampling

Collection of field preserved samples is covered by US EPA Method 5035A, which was implemented in 1997 and updated in 2002 to standardize sampling and field preservation and avoid the random error and low bias present in bulk sampling. This method is widely accepted across North America, and is required in many US states and Canadian provinces. Under US EPA Method 5035A, either sub-core sampling equipment or the En Core® sampler can be used.

Field preserved sampling is done by collecting plugs or sub-cores from the soil core that are placed in vials filled with a preservative. Soil sub-core samples are best collected using a soil core sampling tool designed to capture an approximately 5-gram or 10-gram sample of soil to achieve a 1:1 ratio of soil to preservative (weight to volume).  The Terra Core™ sampler and EasyDraw Syringe® are widely used for this reason. Other tools, such as cut-off syringes, often don’t fill properly and don’t collect a sample of the proper weight.

The vials used in sub-core sampling are typically supplied by the laboratory as part of a soil sampling kit, and are generally pre-preserved and weighed to allow easy calculation of the soil sample’s weight. Once preserved, the holding time for samples using sub-core field preservation methods is 14 days, as long as the samples are cooled at 4 degrees Celsius.

Field preserved sampling is divided into two sub-methodologies – high level and low level analysis. High level analysis is used when VOC concentrations are greater than 200 µg/kg. Samples requiring high level analysis are preserved in methanol. Low level analysis is used when the VOC concentrations are less than 200 µg/kg or where the analytical detection limits used are lower than a lab can achieve with a 50:1 dilution of methanol in water. Low level samples are preserved in sodium bisulfate solution. In low level analysis, carbonate or calcareous soils can react with the bisulfate solution, so it is important to first check the soil for effervescence prior to collecting these samples to prevent the vial from cracking or bursting. Two vials are collected for low level analysis; the second sample is used for quality control and as a replicate, since the entire volume of the sample is required for analysis, whereas high level samples are diluted for analysis.

There are some limitations to low level analysis. Low level analysis is considered to be only semi-quantitative because the aqueous solution used is a poor solvent for soil VOCs, which causes results to be biased low. In addition, high concentrations of acetone can be formed by the preservative itself in some organic-rich soils.

Despite the limitations of low level analysis, sub-core sampling using field preservation techniques is an effective methodology and is widely accepted by regulators. It is an advantageous method for laboratories because it requires no further preparation of samples by the laboratory doing the analysis, and also doesn’t require any special tools or procedures for the laboratory to process samples. In addition, VOC concentrations in samples collected using field preservation remain stable throughout the 14-day hold time and beyond.

There are disadvantages to sub-core sampling with field preservation. For example, the seals of the vials used to collect samples can leak causing a loss of preservative or VOCs. Another issue is the need for more experienced field staff, as the process used to collect field preserved sub-core samples is more complicated than other methods and can be even more difficult in adverse conditions. Lastly, shipping restrictions may apply to high level samples as they contain methanol (although this typically is not an issue if the samples are properly labeled and have a volume of less than 500 mL).

The En Core® Sampler

The only alternative method to sub-core sampling with field preservation within US EPA Method 5035A is sampling using an En Core sampler. The En Core sampler is the only commercially available sampler listed in Method 5035A, and is both a sampling tool and a sample container. There are three major components to the En Core sampler: the reusable T-handle, which holds the sampler and is used to push it into a soil core, the En Core sampler itself, which is where the soil sample is collected and stored, and the cap, which seals the sampler shut for transport to the lab. Once the sample is collected, excess dirt is cleaned from the sampler upon removing it from the soil, the cap is snapped into place on the sampler, and then the sampler is removed from the T-handle and placed in a pre-labeled VOC-proof bag to be shipped to the lab.

The En Core sampler has several advantages over field preservation and bulk sampling methods. First, because it eliminates transfer of the soil sample into vials and the need to weigh vials in the field, it is faster and easier than other methods, allowing for more sample throughput per day. Additionally, it requires minimal training to use effectively, doesn’t require field staff to handle preservative in the field, and has a long shelf life (no preservative expiration dates to consider). Because there are no preservatives involved in field work using the En Core, there are no sample shipping restrictions, and random error in sample results, which often come from mistakes in field preservation, is much less common. Finally, the En Core samples are shipped to the laboratory in a proprietary VOC-proof bag, which further prevents random error in results. The En Core sampler delivers such consistent results in spite of variations in the experience of the field staff that use it that it is often used to minimize liability concerns.

The En Core sampler is also very reliable – millions of En Cores have been sold without one reported analytical incident. This is in part due to the manufacturing quality standards of the En Core, which are higher than those of sample vials. Every lot of En Core samplers is tested and analyzed for performance and cleanliness, including tests at extremely high and low temperatures. In fact, En Core samples can be stored frozen without any damage.

There are some limitations to the En Core sampler. If the En Core is being used under sampling Method 5035A, the samples must be preserved at the lab within 48 hours. Additionally, En Core samples are reported in dry weight, so an extra jar of soil must be collected along with En Core samplers to measure the moisture content of the soil at the laboratory. Lastly, the per-sample cost of using the En Core sampler is higher than that of the field preserved sub-core method, though some of this extra cost is offset by the time savings created by the En Core sampler.

Summary

To collect quality soil samples that will deliver accurate data, one of two different methods of sampling should be used: field preservation of sub-core samples, or sampling using an En Core sampler. While widely used in the past, studies have shown that the bulk sampling method is unreliable and highly prone to sample bias due to VOC losses. Under either of the two viable methods, it is vital to carefully follow the best practices for handling a soil core and sampling from that soil core in order to ensure a quality sample, and in turn, quality VOC data.