MAXIMUM INDOOR RADON ON QUARTZ-MICA SCHIST IN NORTHERN VIRGINIA

A mid-Paleozoic quartz-mica schist underlies the central part of the Piedmont Province in northern Virginia, and has created a silt-sand soil about 10 meters thick with numerous small streams, wide stream valleys and modest hills (relief less than 30 meters). The permeability and the total-gamma radioactivity of the schist is in most areas higher than the surrounding soils. Most of the available land has been used for housing developments, as part of the regional population increase in the rapidly growing communities west of Washington, DC. Almost 10,000 measurements of indoor radon using three-month exposures of alpha-track detectors were made in northern Virginia as part of a home-soil-groundwater study, and in the homes over the schist, more than half exceeded 4 pCi/L, the US-EPA recommended maximum to be applied when buying a home (a recommended maximum of 2 pCi/L is applied when testing a home for long-term occupation). Mitigation studies have shown that sub-slab ventilation, installed using US-EPA protocols, is successful in more than 90% of the homes requiring a reduction in indoor radon.