Long-term Reliability of Collateral Mitigation

During 1997 and 1998, members of the Oak Ridge National Laboratory (ORNL) Radon Team installed 47 single-suction-point active soil depressurization (ASD) systems in 47 multiplex housing units located in 24 buildings in Guam. Subslab vacuum measurements performed within adjacent housing units found sufficient field extension to provide potential radon reduction within 39 additional units. Follow-up radon testing confirmed that mitigation had occurred in these additional units. During the summer of 2000, retesting of the 39 units revealed that mitigation had failed in 12 (31%) of the units that underwent collateral mitigation. Detailed investigations identified degradation of fan performance, fan failure, and the property management's energy policy regarding vacant units as the main causes for mitigation failure. COLLATERAL MITIGATION Simply speaking, in active soil depressurization (ASD), the vacuum field extends concentrically from the suction point to a point where the soil gas is no longer being drawn toward the exhaust point (typically the 4-Pa contour line). This contour, called the lateral field extension (LFE), defines the boundary where effective mitigation can be anticipated (Figure 1). In certain cases, the LFE can be impeded by subslab footings (Figure 2) or in cases of nonhomogenous aggregate conditions, the LFE can be biased or doughnut shaped (Figures 3 and 4). However, if interior footings have good aggregate beneath them, the vacuum field could extend for a considerable distance (Figure 5). If the vacuum field extends under the interior footing of a multi-family building (e.g., duplexes and townhouses), installing an ASD mitigation system in one unit could reduce the radon level in one or more adjacent units. This type of mitigation is called collateral mitigation (CM).