Rn-222 as Natural Tracer for LNAPL Recovery in a Crude Oil-Contaminated Aquifer

The objective of this study was to investigate whether 222Rn in groundwater can be used as a tracer for LNAPL quantification at a field site treated by dual-phase LNAPL removal. After the break of a pipeline, five hectares of soil in the nature reserve Coussouls de Crau in southern France were contaminated by 5,100 m3 of crude oil. Part of this oil seeped into the underlying gravel aquifer and formed a floating oil body on > 3 hectares. The remediation consists in plume management by hydraulic groundwater barriers with re-injection of treated waters, and LNAPL extraction in the source zone. 222Rn measurements were performed in 21 wells in and outside the source zone over more than 15 months. In pristine groundwater, the radon activity was relatively constant and remained always > 11 Bq L-1. The radon activities in the groundwater of source zone wells were not more variable, but the mean activities were generally significantly lower than upgradient, due to partitioning of 222Rn into the oil phase. The lowest activities were found in zones with high NAPL recovery efficiency. LNAPL saturations around each recovery well were furthermore calculated during a period of high groundwater level, using a laboratory-determined crude oil-water partitioning coefficient of 38.5 ± 2.9. This yielded an estimated volume of residual crude oil of 465 ± 139 m3 below the capillary fringe. We find that 222Rn is a useful and cheap groundwater tracer for finding zones of good LNAPL recovery in a heavily pumped aquifer, but that quantification of NAPL saturation using Rn is highly uncertain