Figure 1: The figure above depicts two types of Soil Gas Probe Designs. The one on the left is a “Single Soil Gas Probe Design” and is a temporary soil gas well used for a shorter period of time. The design on the right is a “Nested Soil Gas Probe Design” and is used for multiple samples over a longer period of time.
What is Soil Gas Sampling?
Vertical soil gas sampling is commonly conducted to determine whether subsurface contamination has occurred from the current or previous use, storage, and / or disposal of hazardous materials on a particular property or the surrounding properties. Soil gas sampling may be recommended in a Phase I Environmental Site Assessment (ESA) and is performed on a variety of properties including residential, commercial, and industrial.
The Process
Let ‘s consider a Phase I ESA conducted on a property finding it was previously used as a gas station and further investigation is needed… now what? Soil gas sampling would be a great option. There are a series of steps to taking soil gas samples, including the following:
1. Develop. Develop a Project Work Plan, including a Health and Safety Plan. The Project Work Plan will state the reason for performing the soil gas sampling and describe the investigation in detail. Also, Safety is the number one priority when in the field. A Health and Safety Plan (HASP) must be developed to inform workers of all hazardous chemicals which may be encountered, and must be reviewed and signed by all workers in the field.
2. Map. Where the soil gas samples will be collected is important because they should be targeted towards locations that may be impacted. Therefore, it is important to map the locations of previous/current hazardous chemical use and where the soil gas wells will be placed. Examples include, locations surrounding a previous underground storage tank (UST), or surrounding a previous dry cleaner.
3. Drill. Soil gas wells or boreholes may be drilled in a variety of methods including direct push, hollow stem auger, or hand auger.
4. Assembe. Assemble and install the soil gas probes, this includes the soil gas probe tip at the bottom, followed by the tubbing (Nylaflow, PEEK, or Teflon), and lastly a gas-tight valve. Start by installing a sand pack inside the borehole. This sand pack is important because it will minimize the disruption of airflow to the sampling tip. Place the tip midway in the sand pack with at least six inches of sand above and below the probe tip. Next, place at least six inches of dry bentonite on top of the sand pack. Lastly, create a seal by hydrating bentonite at the surface and pouring it slowly it into the borehole until it is filled.
Figure 2: Depicted in the figure above is a Gas Tight Valve connected to Teflon tubing. Photo provided by McAlister Geoscience.
5. Wait. Subsurface conditions are disturbed during drilling and probe placement; therefore, it is recommended to allow for the subsurface to equilibrate back to the representative conditions. The equilibration time can typically take anywhere between two to 48 hours depending on the type of drill used or soil type. For soil gas wells installed with a rotosonic or air rotary method, the subsurface equilibrium time can take between a few days and a few weeks.
6. Test. Perform a shut-In test and leak test. A shut-in test is used to evaluate if ambient air is introduced into the soil gas sample during the collection process.
7. Purge. The purpose of purging is to remove stagnant air from the sampling system so the representative samples can be collected from the subsurface.
Figure 3: Depicted in the photo above is the XiTech Model 1060 One Liter Purge Box. The purge box is used to remove stagnant air from the sampling system so that representative
samples can be collected. Photo provided by McAlister Geoscience.
8. Sample. For sampling, the purge box flow rate should not exceed 200 mL/min. Sample containers vary, examples are included in the following table.
|
Container Types |
Notes |
Container Handling |
Holding Times |
| Syringes | Gas-tight glass syringes with Teflon® seals are preferred | These containers should be kept in a cool dark location at all times. | 30 minutes |
| Passivated Stainless Steel Canisters | Need a flow regulator and vacuum gauge when sampling. Field crew should record the pressure upon start
and completion of the sampling |
Passivated stainless steel canisters may be shipped for analysis and have the least problems associated with their handling. Canisters are also the highest cost sampling container. | 30 days |
| Polymer Gas Sampling Bags or Glass Bulbs | Surrogates do not need to be added to polymer gas sampling bags; however they should be added to glass bulbs within 15 minutes of collection. | Polymer Gas Sampling bags should be analyzed within six hours after collection and not exposed to light or extreme temperature and pressure changes. Glass bulbs should be analyzed as soon as possible. | Polymer Gas Sampling
Bags – 6 Hours
Glass Bulbs- 24 Hours |
| Sorbent Tubes | Used with USEPA (1999) Compendium Method TO-17 (Method TO-17).
Sorbent tubes should be capped with Swagelok®-type caps and combined Teflon (PTFE) ferrules, rewrapped in aluminum foil, and placed in the storage container immediately after sampling
|
Sorbet Tubes should be stored at 4°C and analyzed within 30 days. | 30 days |
| Non-Traditional Sample Containers | Mini-Passivated Stainless-Steel Canisters; Evacuated Glass Bottles | Evacuated Glass Bottles have a holding time limited to 48-hour |
9. Sample Analysis. A Chain of Custody document should be written from the time of collection through receipt at the laboratory. Chain of Custody forms should include all samples, sample times, container type, EPA method, relevant problems encountered during sample collection, and starting and ending pressures (for passivated stainless-steel canisters).
10. Report. Lastly, a written document stating the procedures, outcomes, and recommendations for the property.
McAlister Geoscience does Soil Gas Sampling!
McAlister Geoscience offers a variety of services including soil gas sampling. Soil gas sampling is performed during a Phase II Environmental Site Investigation to examine whether the subsurface soil and/or groundwater have been impacted from current or former site activities. McAlister GeoScience also offers vapor sampling for methane in accordance with various development requirements set forth by several City and local development and building safety departments. Let us assist you with your due diligence needs, or development needs.
References:
California Environmental Protection Agency, Department of Toxic Substances
Control, Los Angeles Regional Water Quality Control Board, San Francisco Regional Water Quality Control Board, Advisory Active Soil Gas Investigation. July 2015.