Questions that arise are a) how can we be sure that the gas was not being diluted by air, b) how much gas did we remove, and is that an issue, and c) from what depth was the sample being drawn?
a. The gas may be diluted by i) air flowing down the outside of the probe tube to the sampling point, ii) leaks in the tubing from the sampling point to the RAD7, and iii) taking too much gas from a sampling point too close to the surface. Care should be taken to minimize these sources of error.
b. The volume of gas removed depends on the technique used to extract it and the porosity of the soil. In practice, with average soil porosity, the RAD7 internal pump is capable of removing soil gas at the rate of about 0.3 – 0.5 L/min. Using the GRAB protocol, in which the RAD7 pumps for five minutes only, the volume removed will be about 2L. In continuous monitoring mode, the RAD7 pumps for five minutes at the start of every cycle, and then for one minute in every five. So it removes 2L at the beginning of each cycle, and then an average of less than 0.1L per minute after that. A flow meter on the RAD7 outlet would show how fast the RAD7 is pumping gas out of the ground. The vacuum gauge, mounted on the probe, will indicate how hard the RAD7 pump has to work to obtain its sample of soil gas.
c. The actual sampling point position will depend on the probe depth, the volume of air removed and also, perhaps, the technique used and the probe design. After the probe is inserted, the ground should be tamped down around the probe, to prevent air or gas from moving vertically along the outside of the shaft. This not only prevents fresh air from descending from the surface down to the sampling point, but also helps to locate the gas sampling point at the position of the probe point. Assuming that the gas is drawn equally from all directions, and assuming that half the soil space is occupied by gas, the soil volume sampled, for a gas sample of 2L, will be about 4L. This occupies a sphere of radius 8cm, or 3 inches. Taking twice as much gas will increase the sphere radius to10cm, or 4 inches.
CHOICE OF PROBE
The RAD7 may be shipped with any of a number of commercially available soil gas probes. One system, the AMS Original Gas Vapor Probe ( www.ams-samplers.com ) , comes with a 15 inch (38 cm) probe and liner rod, a 3 ft (91 cm) extension with liner rod, a cross handle and adapter, and a barbed hose adapter. This is intended for use in fairly light soils down to a depth of no more than 4ft (1.2m). To reach this depth, it may be necessary to use a slide hammer (available as an optional extra). With this or any probe, care should be exercised during insertion, to avoid the probe being damaged or bent. There is no warranty covering damage to the probe.