Rn-222 as a partitioning tracer to detest diesel fuel contamination in aquifers: Laboratory study and field observations

The use of Rn-222, 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 Rn-222 was 40 +/- 2.3, in bottles containing diesel fuel and water at 12 degrees C. In water-saturated quartz sand contaminated with diesel fuel, the Rn-222 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 Rn-222 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 Rn-222 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 Rn-222 activity of groundwater decreased by about 40% between monitoring wells upgradient of the contaminated zone and monitoring wells within the contaminated zone. On the basis of this decrease, the average diesel fuel saturation was estimated using the mathematical model. The calculated diesel fuel saturation was in the range of that found in excavated aquifer material.