222Rn as a Partitioning Tracer To Detect Diesel Fuel Contamination in Aquifers: Laboratory Study and Field Observations

The use of 222Rn, a naturally occurring radioactive isotope, was investigated as a partitioning tracer to detect and quantify the amount of non-aqueous-phase liquids (NAPLs) in contaminated aquifers. Diesel fuel was chosen as a model NAPL. The diesel fuel−water partition coefficient for 222Rn was 40 ± 2.3, in bottles containing diesel fuel and water at 12 °C. In water-saturated quartz sand contaminated with diesel fuel, the 222Rn emanating from the sand partitioned between diesel fuel and water as expected based on this partition coefficient. In a column containing uncontaminated quartz sand, the 222Rn activity in infiltrated water increased from <0.2 to 4.9 kBq m-3, and in a subsequent column containing diesel fuel-contaminated quartz sand, the 222Rn activity in the water phase decreased to 3.3 kBq m-3. This decrease corresponds to what has been predicted using a mathematical model. At a contaminated field site, the 222Rn activity of groundwater decreased by about 40% between monitoring wells upgradient ...