Tracer Method to Determine Residence Time in a Permeable Reactive Barrier

A method is presented to evaluate ground water residence time in a zero-valent iron (ZVI) permeable reactive barrier (PRB) using radon-222 (Rn-222) as a radioactive tracer. Residence time is a useful indicator of PRB hydraulic performance, with application to estimating the volumetric rate of ground water flow through a PRB, identifying flow heterogeneity, and characterizing flow conditions over time as a PRB matures. The tracer method relies on monitoring the decay of naturally occurring aqueous Rn-222 as ground water flows through a PRB. Application of the method at a PRB site near Monticello, Utah, shows that after 8 years of operation, residence times in the ZVI range from 80 to 486 h and correlate well with chemical parameters (pH, Ca, SO4, and Fe) that indicate the relative residence time. Residence times in this case study are determined directly from the first-order decay equation because we show no significant emanation of Rn-222 within the PRB and no measurable loss of Rn-222 other than by radioactive decay.