Laboratory, field and modeling studies of radon-222 as a natural tracer for monitoring NAPL contamination

The recently developed natural radon tracer method has potential as a rapid , low-cost, nondestructive, and noninvasive method for quantifying NAPL con tamination. In the subsurface, radon-222 (radon) is produced by the decay o f naturally occurring radium-226 contained in the mineral fraction of aquif er solids. In groundwater radon occurs as a dissolved gas, with a half-life of 3.83 days. In the absence of NAPL, the radon concentration in groundwat er quickly reaches a maximum value that is determined by the mineral compos ition of the aquifer solids, which controls the rate of radon emanation. In the presence of NAPL, however, the radon concentration in the groundwater is substantially reduced due to the preferential partitioning of radon into the organic NAPL phase. A simple equilibrium model and supporting laborato ry studies show the reduction in radon concentration can be quantitatively correlated with residual NAPL saturation. Thus, by measuring the spatial di stribution in radon it may be possible to identify locations where residual NAPL is present and to quantify the NAPL saturation. When the basic proces ses of partitioning, radon emanation from the aquifer solids, and first-ord er decay are incorporated into an advective/dispersive transport model, goo d agreement is obtained with the results of laboratory and field experiment s. Model sensitivity analyses shows many factors can contribute to the rado n concentration response, including the length of the NAPL zone, NAPL satur ation, groundwater velocity, porosity, and radon emanation. Thus, care must be taken when applying the radon method to locate and quantify NAPL contam ination in the subsurface.