Advanced Environmental Interface

Review the diagrams in sequence to run through an example of an actual soil gas survey at an industrial facility.

1. General layout 2. Vapor point grid 3. Drilling floor
4. Placing points 5. Cross-section 6. Vapor sampling
7. Vapor pattern 8. Physical situation

1. General Layout:

The orange outline is the potentially affected area beneath the 6-inch concrete floot slab of a large manufacturing building. The area covers approximately 30,000 sq ft, in which manufacturing processes used solvents in significant quantities. The building has a construction grid of 40-ft by 40-ft sections, defined by the columns supporting the roof. The area is suspect because a waste solvent tank connected to the floor drain system was believed to contain less material than it should have contained.

2. Vapor Point Grid:

Hot Topic Archive

A finer-resolution grid of 20-ft by 20-ft sections (green gridlines) is established, and the grid intersections are chalked onto the concrete floor to indicate sampling points. (Sampling points are offset from the columns as needed.)

3. Drilling Floor:

An air drill (similar to a jackhammer, but with compressed air blown through the tip of the bit) was used to punch holes through the 6-inch floor slab at each of the 66 grid points. The holes were continued down into the packed sand under the slab to the full length of the drill bit. This left a neat and clean hole, blown clear of loose sand, 2-inches in diameter and 24-inches deep. The air drill process creates a lot of dust; if that's an issue, it can be controlled pretty well by a high capacity vacuum cleaner. Other coring methods are not as messy, but also not as fast.

4. Placing Points:

After the holes were drilled through the floor slab and into the compacted sand under the slab, a sampling tube taped to a steel rod with a sponge seal was inserted into each hole. The sponge seal was set at approximately a 12-inch depth from the floor surface, leaving a sampling cavity 2-inches by 12-inches, extending from 12-inches to 24-inches below the floor slab. Plaster of Paris was poured on top of the sponge seal up to the middle of the floor slab and allowed to set. This sealed off the sampling cavity tightly from the surface. Note that all materials (tubing, tape, rod, sponge, plaster) have to be carefully chosen so as not to contain any of the constituents being tested for in the soil vapors.

5. Cross-Section:

Note: the four sampling methods shown above (syringe septum for field GC, sorption tube, organic vapor analyzer, Tedlar sampling bag) as hooked up in series are not normally all used at the same time -- typically only one or two of the methods are used on a project.

6. Vapor Sampling:

Sampling of the vapors is done by drawing them out of the sampling cavity into a sample collection device. This could be a simple method, such as an organic vapor meter (giving a gross measure of total volatile constituents without identifying the compounds). Or, it could be more complex, such as a syringe collection of a gas sample for direct injection into a portable gas chromatograph. Both give real-time answers, so that the investigation and placement of holes can be directed by the results as they are determined. Also, charcoal tubes or tedlar bags can be used to collect samples for subsequent analysis in the laboratory, if real-time results are not needed. For investigations such as this, real-time results are useful because they allow the field work to follow the vapors patterns. This is usually the most cost-effective approach.

7. Vapor Pattern:

The actual soil vapor concentrations detected at the 66 sampling points ranged from near-zero to over 10,000 ppm. The pattern showed a double-bullseye, with two lesser concentration areas, one to the north and one to the southwest. The floor drain connections and subsurface piping were not well known at the time of the soil vapor sampling.

8. Physical Situation:

After the soil gas survey was completed, an old blueprint was discovered showing the subsurface piping connections. The pipes were checked by a line tracer, and in most cases were found to be approximately where indicated on the drawing. The origins of the high soil vapor concentrations clearly were pipe junctions, where the gaskets in the bell-and-gasket pipe fittings had been degraded, releasing waste solvents into the soil. Test borings were subsequently installed to collect actual soil samples at these locations, to provide a vertical profile of soil contamination.

See the diagram sequence for subsurface investigation by backhoe trenching.