Contributed by Jon Roger PG, Project Manager, PPM Consultants
Let’s face it, soil sampling is ubiquitous in the environmental consulting world. In spite of regulations, safety protocols, and best management practices, releases to the environment do, unfortunately, occur. We can mitigate releases to the environment, and that begins with sampling soil and groundwater to determine if there is a risk to human health and the environment and, if so, develop a remedial action plan. However, soil sampling is not as straight forward as it would seem. Collecting a sample of soil introduces hurdles that need to be addressed in order to make laboratory analytical data usefully accurate. One hurdle is the preservation of volatile organic compounds (VOC). Once the soil is disturbed, constituents begin to volatilize more readily, hence the name, and the effective concentrations may no longer be representative of subsurface conditions. To lessen this, there are considerations to be made prior to sampling. Soil type, sample retrieval methods, and sample collection and containerizing methods will have an effect on the loss of VOC concentrations.
Soils such as sand with a high permeability allow for greater VOC volatilization; whereas, clayey type soils have an adsorptive effect on VOCs. But we cannot choose the soil types to sample. To overcome soil type limitation, sample retrieval techniques need to be evaluated. Sample retrieval utilizing a hand auger is less than ideal (not approved by many regulatory agencies) as the process breaks the cohesiveness of the soil. The churning action of the hand auger aerates the soil which in turn leads to significant VOC loss. Equipment designed to be minimally disruptive to soil conditions should be considered. The typical split-spoon sampler is a better option but not altogether ideal. Direct push technology has introduced the soil sampling world to lined core barrels for sample retrieval. The liners are for the most part inert, not adding unwanted constituents. The process of driving the core barrels into the subsurface introduces very little disturbance to the soils. Once retrieved, the liners can also be capped to prevent any VOC loss from the ends prior to sample containerization. Time and temperature affect the rate at which constituents volatilize. Coring advancement done with direct push technology can be paced by the sampling professional. The sampler can dictate to the operator, the rate at which cores can be can be retrieved from the subsurface. This will reduce the time the cores are exposed. If multiple liners are pulled and the sampler is behind, the liner encased soil core can be kept cool on ice until the sampler can address them. While keeping them cool is an option, time is of the essence while sampling, and time delays are best to be avoided.
Once the soil cores are retrieved, the next consideration, and arguably the more important, is the proper container for submittal to the laboratory. For soil samples collected for VOC analysis, the Louisiana Department of Environmental Quality (LDEQ) has adopted the Environmental Protection Agency (EPA) SW-846 Method 5035A. Method 5035A prescribes a method of containerizing soil samples to insure the highest level of VOC preservation in a sample including the use of field preserved vial kits and EnCore® Sampler devices. Studies show both types demonstrate a minimal loss of VOC content. While each are accepted methods of collection, they are not without their own limitations. Preserved vial sampling kits are an attractive option for sampling as the soil is being preserved in a solution in the field essentially “freezing” VOC content concentrations (along with immediately placing samples on ice). As the samples are preserved, the holding time for analysis is up to 14 days. Logistically, this is beneficial allowing for most investigations to be completed and samples submitted to the laboratory in one shipment. The downside to field preservation is the preservatives themselves. Methanol and sodium bisulfate are the primary liquid preservatives. Methanol preservative should be used where VOC concentrations will be above 200ug/kg, and sodium bisulfate is used when VOC concentrations are below 200 ug/kg. Typically, the concentrations are unknown requiring the use of both preservatives for VOC laboratory analysis. With methanol being a volatile itself, the tare weight of the vials must be verified in the field prior to use. Along with confirming the tare weight of the vial, the weight of the soil must be confirmed prior to introducing it to the preservative. The field preservation kits include a plastic syringe capable of collected a 5-gram sample of soil, but the weight of the sample can vary dramatically. Sodium bisulfate is an acid and calcareous soils should not be introduced as the effervescence can drive off VOCs and can create a pressure issue in the vial itself. On the flip side to the field preservation kit is the EnCore® sampling device. These devices are hermetically sealed capsules with zero headspace and minimal trapped air. In the field, collection times are greatly reduced, there is no contact with hazardous preservatives, and the device itself has a long shelf life (practically no expiration). The hurdle to the EnCore® is the extremely short holding time. As prescribed in Method 5035A, samples collected utilizing EnCore® devices must be preserved within 48 hours of sample collection; therefore, samples must be shipped to the laboratory daily to meet this. This can be a logistical nightmare without prior planning in place. In either case, cost becomes a concern. With every sampling interval having the potential for laboratory submittal, numerous kits or EnCore® devices are consumed with only a select few submitted for analysis.
When I started soil sampling many years ago, field preservation kits were the norm. As time went on, issues like quality control on the part of kit suppliers, the hassle of dealing with a balance in the field and the inefficiency that introduced led me to the use of EnCore® sampling devices. The ease of use has increased productivity which helped to offset the increase in cost. This has also reduced errors in sampling which helps insure a better deliverable on the back end. Accurate, reliable data from soil sampling is essential as it will be used to evaluate exposure, risk to human health and the environment, and will be the target for remedial activities. General in nature, this is a reminder that soil sampling is more than just packing dirt in a jar. Many factors must be considered to insure a satisfactory outcome. If you want to discuss soil sampling methods and the pros and cons please feel free to reach out to me at email@example.com.