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

Abstract

The use of 222 Rn, 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 222 Rn was 40 ( 2.3, in bottles containing diesel fuel and water at 12°C. In water-saturated quartz sand contaminated with diesel fuel, the 222 Rn 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 222 Rn 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 222 Rn 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 222 Rn 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.

Assessment of the Use of Partitioning and Interfacial Tracers To Determine the Content and Mass Removal Rates of Nonaqueous Phase Liquids

“…For alcohol tracers, Knw increases with increasing tracer hydrophobicity and decreases with increasing NAPL hydrophobicity (10). The agreement that was generally observed between results obtained with tracer experiments and data from soil core sampling (5,6,8,9) suggests that the retardation of partitioning tracers indeed is determined by Sn and Knw. However, a systematic study has not been conducted to determine whether this relation holds for NAPLs present in various porous matrices and at different surface area-tovolume ratios.…”

Tracer tests for the investigation of heterogeneous porous media and stochastic modelling of flow and transport—a review of some recent developments

“…The same concept but a different tracer philosophy is the background of the PTT experiments performed by Hunkeler et al (1997) and Semprini et al (2000). While all aforementioned studies used artificial tracers these authors used a naturally occurring radioactive isotope, 222 Rn, that is found in groundwater as a dissolved gas, as partitioning tracer to detect and quantify residual NAPLs in contaminated aquifers.…”

Field validation of radon monitoring as a screening methodology for NAPL-contaminated sites

“…Schubert et al ( , 2007a carried out laboratory experiments to determine the partition coefficients /< N APL/W and /< N APL/AIR for commercial mixtures of petroleum hydrocarbons (gasoline, diesel, kerosene) and demonstrated the ability of NAPL accumulations to act as geochemical traps for Rn. The resulting reduction in Rn activity in the soil air of the vadose zone should therefore indicate the presence of underlying levels with high hydrocarbon saturation (Hunkeler et al, 1997;Semprini et al, 2000;Schubert et al, 2001Schubert et al, , 2005Schubert et al, , 2007b. Field tests and laboratory experiments published by Schubert et al (2007c), Fan et al (2007), Davis et al, (2002Davis et al, ( , 2003 and Semprini et al (2000) suggest that Rn can be used as a non-conservative tracer to estimate LNAPL saturation in the subsurface.…”