Experimental and Theoretical Assessment of the Lifetime of a Gaseous‐Reduced Vadose Zone Permeable Reactive Barrier

The feasibility of using in situ gaseous reduction to establish a vadose zone permeable reactive barrier was evaluated through a combination of laboratory testing and consideration of fundamental vadose zone transport concepts. For the experimental evaluation, a series of laboratory column tests were conducted in which Hanford formation sediment from the USDOE Hanford Site in Richland, WA, was first treated with a diluted hydrogen sulfide gas mixture to reduce sediment iron oxide to ferrous sulfide. Water containing dissolved oxygen was then pumped through the columns at different flow rates to determine the reoxidation rate and the reductive capacity of the treated sediment. The results indicated that the treated sediment has a significant reductive capacity consistent with the basic reactions associated with the treatment and reoxidation processes. The observed reductive capacity was found to be dependent on the flow rate of water during the reoxidation phase of the tests. The reductive capacity approached the maximum value predicted on the basis of the treatment reaction as the flow rate was decreased. Thus, laboratory treatment tests provide a means for predicting the reductive capacity of the barrier under field conditions. In the theoretical assessment, oxygen diffusion was identified as the dominant mechanism leading to reoxidation of the barrier. Depending on vadose zone characterisitics, the predicted barrier lifetime varies from several years to more than 100 years.