Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 5.3
SURFACE SOIL SAMPLE COLLECTION

5.3 SURFACE SOIL SAMPLE COLLECTION

The collection of samples reflects the culmination of significant research and planning prior to initial field activities. It is important that the samples ultimately collected be as technically defensible and representative of site conditions as possible. As discussed in Section 4.2.5.2, the tool(s) selected for sample collection must ensure that soil increments are core-shaped or otherwise not biased with respect to depth and of relatively equal mass, and that mass of individual increments be adequate to collectively meet the target bulk mass for resulting Multi Increment samples.

The collection of Multi Increment samples from exposed surface soil should be relatively straight forward with proper planning and tools. The top two to six inches of soil bgs is generally considered for surface soil DUs, depending on the site-specific DQOs (USEPA, 2011d; CalEPA, 2013). On Hawai‘i sites, the top 0-6 inches or 0-4 inches of soil are commonly selected for surface soil DUs. The same tool used for surface soils can often be used to collect deeper, near-surface soil samples (e.g. 6-12 inches bgs) if that is part of the site investigation objectives, or required to further delineate contamination that is documented at the surface. The collection of Multi Increment samples from DUs layers greater than one to three feet bgs typically requires the use of drills or other equipment (see Subsection 5.5). Although not typically carried out for surface soils, methods for the collection of Multi Increment samples to be tested for VOCs are described in Subsection 5.6.

Various types of sampling equipment are available for the collection of surface soil samples. Soil type, compaction, abundance of rocks, and increment depth typically drive selection of the most appropriate tool for a given site. For example, a simple sampling tube is generally most appropriate for relatively non-compacted, fine grained soils. Sampling tubes with core catchers or using a trowel might be most appropriate for very loose, sandy soil, although care must be taken with the latter to collect increments that are not biased with depth. An electric drill with a bit specially designed to remove cuttings can allow the rapid collection of increments and Multi Increment samples in fine-grained, semi-compact volcanic soils common in the Hawaiian islands. A sample tube with a slide hammer, a mattock, electric hammer, or in some cases even a backhoe may be required to collect samples in very compact or very gravelly soil.

The discussion below is presented in terms of tool options for various soil types and field conditions. These are presented as general guidelines only and based in part on the field experience of HEER Office staff; some tools might be applicable for multiple soil types and site conditions. As previously discussed, an inspection of the site to assess soil conditions by someone experienced in sample collection is imperative. Multiple types of tools should be carried to the field to address unanticipated field conditions and ensure that representative samples can be collected.

Wire flags, marked tape measures or rope, and rolling measures can be used to fully or partially mark the location of individual increments to aid in Multi Increment sample collection, based on the targeted increment spacing, and DU documentation (see Section 4.2.4). Ensure that all sampling devices are of sufficient quality to avoid contamination of the samples being collected with paint, chrome plating, grease or other material. Sampling equipment should be either: (1) easy to decontaminate, or (2) cost-effective enough to be disposable.

5.3.1 SOFT, FINE-TO COURSE-GRAINED SOILS

Figure 5-7. Various Soil Coring Sampling Tools

Source: (USDA-NRCS, 2007)

The most common hand tools used for the collection of surface soil samples are sampling tubes or coring devices ("C" and "E" in Figure 5-7) and screw-type drills ("A" in Figure 5-7). Stainless steel soil coring devices rather than augers are recommended for the collection of Multi Increment samples. Augers tools ("B" and "D" in Figure 5-7) can mix soils throughout the vertical depth penetrated, or penetrate a variable width column through the depth of interest, and therefore may be less reliable. If augers are used, methods used to avoid or address these types of problems should be clearly discussed in the site investigation report. Even using the screw –type drills ("A" in Figure 5-7) requires careful attention to extract the full length or core of soil from the targeted depth, as soil can be mixed through the vertical increment drilled. Coring devices made of other materials (e.g., plastic) might also be appropriate for certain contaminants or soil sampling situations.

Small-diameter (e.g., 0.75 to 1.5 inch) sampling tubes are generally preferable in soft or loose, clayey to sandy soils that are not rocky. The tubes are simple, efficient and effective and can help minimize compaction and disturbance of a site when increments from multiple near-surface layers are being collected. They are a good primary sampling tool for quick sampling events, for very large DUs where considerable walking is required, and for cases when only one person is collecting samples. They also serve as a useful backup or alternative to a drill (see below) because there is no need to wait for batteries to charge. Importantly, small-diameter tubes also allow for the collection of 30- to 50-gram increments from the upper 4 to 6 inches of soil, ideal for the collection of a Multi Increment sample.

Larger diameter tools (e.g. 2 inch and up) collect a proportionately larger amount of soil from a single location and may require extra subsampling of the final bulk MIS sample in the field or by the laboratory. Talk with the laboratory to know the maximum total mass of bulk MI sample they are willing to accept and process using their standard MIS processing protocols (typically in the range of 2.0 to 2.5 kg), and select a sample core diameter that will keep you at or under that total mass for the number of increments that will be collected. Note that larger mass bulk MI samples are generally more representative, so the choice of the sample core diameter is a balance between what is effective to utilize in the field, amount of contaminant heterogeneity expected, and cost for the laboratory to process.

Sampling tubes are utilized with extension rods and T-handle attachments (Figure 5-8).The tube is twisted into the ground to the desired depth, cutting into and retaining the soil in the hollow, open-face core barrel. The tube is then withdrawn to extract the increment from the ground. The increment is then removed and placed into a collection bucket for the DU sample. A flat-edged screwdriver or similar tool is useful for removing an increment of clayey or hard-packed soil.

Figure 5-8. Collection of Increments with a Sampling Tube
(Photos from Bureau Veritas)

Left photo: Use of an open-sided sampling tube to collect surface increments in soft soils.
Right photo: Use a flat-headed screwdriver to remove soil increment from tube.

A foot-assisted coring tool developed by the Army Corps of Engineers can also be useful for the collection of increments in soft but cohesive fine-grained soils (Figure 5-9; USACE, 2007). The core barrel is pushed into the soil and retracted. The increment is extruded into the container with a spring-operated plunger. These tools can allow the very rapid collection of Multi Increment samples in uncompacted soils without gravel.

Figure 5-9. Foot-operated Core Sampling Device Developed by the US Army Corps of Engineers

Sampling tubes and core barrels such as those shown above do not work well in very loose soils that lack sufficient cohesion and will not allow particles to be retained or removed by the tool. Use of core catchers, if available for the coring device in use, may be an effective alternative. Alternatively, scoops with flat bottoms or similar hand tools are generally utilized in these conditions (Figure 5-10). If scoops or trowels are utilized, it is important to remember that the goal is to remove similar-sized core-shaped increments in the DU (increments of uniform diameter through the vertical depth targeted), as well as limit increment mass to that needed to prepare a bulk, 1-2 kg Multi Increment sample. The flat lip of the scoops shown in the figure 5-10 can help to ensure that wedge-shaped increments are not collected (refer to Section 4.2.5.2).

Figure 5-10. Flat Bottom Scoops to Collect Soil Increments from Very Loose Soil.

5.3.2 MODERATELY COMPACT, FINE-TO COURSE-GRAINED SOILS

Figure 5-11. Use of Cordless Drill for Sample Collection

Cordless drill used with one-inch diameter, hollow auger bit to collect surface soil samples. Wear nitrile or latex gloves and change gloves between DUs (not shown in demonstration photos).

Upper left photo: Use of a 28V cordless rotary hammer drill to collect increments (e.g. Milwaukee or Grainger models) Use a paper plate with a pre-cut, one-inch hole placed on top of increment location. The center of the plate must be held down to keep soil from piling up under the plate; keep any tears around the hole pressed together to minimize soil loss.
Upper right photo: Keep drill vertical and advance bit to target depth (marked on bit) as soil piles up on plate. Hold the drill firmly so gravel or hard soil that might cause drill to lurch doesn’t strike the second person.
Bottom left photo: Empty soil into sample container (e.g. decontaminated plastic bucket).
Bottom right photo: Similar drill-bucket sample kit; bucket fits on metal foot plate with drill guide; soil increment extracted directly into the bucket (AMS Compacted Soil Sampler).

A cordless drill used in conjunction with a paper plate can be time and cost-effective for semi-compact to hard-packed soils without significant gravel, but can require two people unless a specially designed, foot plate is used (Figure 5-11). Use a heavy-duty cordless drill (such as a 28V) with a one-inch drill bit and a battery or portable generator. Weaker drills are prone to overheat or quickly drain batteries, especially in clayey or hard-packed soils. These drills can generally be used for up to 100 increments per battery and field chargers are available for vehicles. Drills powered by portable generators can often be rented from local tool rental or hardware stores.

Use a one-inch, hollow auger bit (e.g., Speedbore bit) to improve soil removal from the ground and control the collected soil mass (see Figure 5-11). These bits generally produce 30 to 50 grams of soil per six-inch depth.

Hollow center auger bits typically work better in the field than wide-flight bits (Figure 5-12). Hollow auger bits are designed to more efficiently remove cuttings from a boring without bringing up excess soil. The area where soil is removed from a boring is less easy to control with a wide-flight bit, and the bits can either bring up too much or too little soil with respect to the target increment mass.

Figure 5-12. Comparison of Auger Bit Designs

Upper photos: One-inch diameter wide-flight auger; can work in loose soil but depth and volume of soil recovered can be difficult to control.
Lower photo: Recommended one-inch diameter hollow-center auger bits provide good recovery for fine-grained semi-compact soil (e.g. Speedbor Ship Auger Bit)

Heavy-duty paper plates work well in the field under dry conditions (see Figure 5-11). Pre-cut holes save field time; several plates might be required per DU if the plate tears excessively during increment collection. Wooden or metal plates might also be useful. Care should be taken not to get fragments of the plate into the sample due to potential interference in laboratory analysis from glues or metals targeted as part of the site investigation.

The drill bit does not need to be decontaminated between increments within a single DU but must be decontaminated between replicate samples and between DUs. Recent sampling kits available from soil testing supply stores include a metal foot plate with a drill guide that attaches to the base of a sampling bucket, with increments directly deposited into the bucket (see Figure 5-11).

Heavier duty drills with portable generators are also an option (Figure 5-13). This setup avoids the need for recharging batteries and is able to drill through more compact soils. These drills should only be used by an experienced person, however. The sudden torque of the drill if a rock or compact object is encountered can cause severe injury to the wrists.

Figure 5-13. High-Powered Drill and Portable Generator

Left photo: Using a high-powered, Hilti drill with a portable generator (photo from Weston Solutions)
Right photo: Using a Hilti drill and paper plate to collect soil increments (photo from Weston Solutions)

A manual, hydraulic, or electric slide hammer can also be used to advance the coring device into shallow soil (Figure 5-14). Slide hammers are effective for collecting harder packed soils but require considerable effort and energy to use in the field. A weighted slide hammer is physically lifted and lowered along a guide rod to drive the attached tool string into the ground to collect shallow soil samples.

Figure 5-14. Use of a Slide Hammer to Collect Soil Increments

Top photo: A slide hammer assembly with rod and split spoon coring tool
Lower photos: Core barrel hammered into soil; removed core placed in increment collection bucket.

5.3.3 VERY COMPACT AND/OR GRAVELLY SOILS

For very hard or gravelly soils, a hammer-action electric drill with a masonry bit or spade bit can be very useful to loosen the soil (Figure 5-15). A hand trowel is then used to collect an increment from the location. The trowel should be used in a manner to allow collection of a core- or slab-shaped increment that collects an equal amount of soil from the entire targeted depth.

Figure 5-15. Use of an Electric Hammer and Spade Bit

Left photos: Electric hammer (14 lb) connected to 4,000 watt, gasoline-powered portable generator used to loosen dense, hard-packed soil.
Right Photo: Trowel used to collect increment from loosened soil.

Generators, drills and bits are usually available for rent from a local hardware store. Gradually push the soil to one side as the bit moves toward the targeted depth, opening up a small gap in the ground (Figure 5-16). Use a trowel to remove a core- or slab-shaped increment from side of opening, being careful to collect an equal amount of soil from all depths. Remove large rocks from the bulk Multi Increment sample as it is being collected. Ensure that an adequate amount of <2mm material is being collected per increment to prepare a bulk sample. Collect a similar mass of finer-grained soil from each increment location to the extent practical.

Figure 5-16. Collection of Increments Soil Loosened by a Spade Bit or Heavy Duty Hand Tool

Other options include the use of a mattock or heavy-duty rock hammer to loosen core-shaped volume of soil from the hard-packed ground (Figure 5-17). A trowel is then used to collect the increment. This avoids the need to carry and rely on an electric drill and generator but adds significantly to the time and effort required to collect samples.

Figure 5-17. Heavy Duty Hand Tools

Left Photo: Narrow spade (root digger), o’o (pry bar) and mattock for collection of increments from hard-packed soil
Right Photo: Breaker bar used to cut through old asphalt surface and collect soil increments

Heavier duty hand tools can also be useful to break through hard surfaces or cut through concrete or asphalt it order to access underlying soil. This can be very labor intensive, however, and can significantly slow down sample collection activities. Chisel or spade bits used with an electric hammer show above or a tunnel bit used with an electric drill (Figure 5-18) can be used to more rapidly remove plugs of asphalt or concrete.

Figure 5-18. Tunnel Bit Use to Cut Through Asphalt or Concrete and Access Soil

(Dewalt 2-3/4 inch x 22" bit shown)

Tunnel bits are not recommended for the collection of increments from hard-packed or gravelly soil, in spite of the ability to extract a core-shaped cutting. The mass of material collected inside a standard, 2 ¾ inch bit exceeds that needed for the collection of a 1-2 kg Multi Increment sample, requiring subsampling of the bulk Multi Increment sample in the field or laboratory to reduce mass for additional processing. Removing soil and large gravel from the bit can also be tedious and time consuming.