Radon Soil Survey Protocols
Radon in Soil Gas
Description - The heart of the Alpha Track radon in soil gas monitor consists of an indoor radon monitor, which has been modified. In order to be mounted inside an inverted plastic cup, the tester should apply a patch of Velcro tape on its underside. A matching patch should be mounted in the center of the bottom of the plastic cup. After the monitor is removed from its protective pouch it is easily fastened to the bottom of the cup.
The cup has three functions. It acts as a trap for the soil gas. It also serves as a "diving bell" and in case of flooding of the site, the air bubble inside the cup keeps the monitor dry. Lastly, the cup serves as the support for the cover that is placed over the monitor to prevent interference from atmospheric air currents.
Survey Layout - Radon seeps out of the Earth's crust as a decay product of uranium, found in trace amounts throughout the ground. Structural features such as fault zones or joints affect the outgassing of radon. The porosity of the soil also influences the transport of the gas.
It is clear that no two survey sites are the same. A first guide is the local topography and geology. Flat, undisturbed plains areas may suggest fewer monitors than in tilted, faulted, disturbed geological formations. In a half-acre to acre lot, one should place one or two detectors in the center location of the proposed building, and a monitor in each quadrant of the property. If there are any known or recognizable geological features one should increase the number of monitors accordingly. The same general rule should be followed for larger building sites, such as apartment or condominium lots.
In some case, patterns for radon sampling are provided by other exploration requirements. For example, drill holes for pilings may be used. With proper precautions discussed below, radon measurements in such holes can be useful.
Empirical Rule - Historically it has been found that about 1% of the soil gas radon will enter a house that has been built conventionally and without extra radon protective measures. Thus if a house is to be built on 400+ pCi/l soil, it may be prudent to use polyethylene barriers, SGC matting or other measures in the construction.
Methods - At each location dig a hole at least one foot deep with vertical walls. An inverted AT-100 inside a plastic cup is placed in the bottom. A rigid, waterproof tile or plywood board is placed above the cup and the edges filled in with rags or paper. The hole is filled in with soil and a waterproof cover may be placed. Mark the location with a flag or other marker. If a drill hole is used, the detector may be suspended from a string. It is important that the hole be closed off from surface air. Record the date, time and location on the supplied field data sheet. The detectors should be left in place for one week.