Experimental and Theoretical Assessment of the Lifetime of a Gaseous-Reduced Vadose Zone Permeable Reactive Barrier
The feasibility of using gaseous reduction to establish a vadose zone permeable reactive barrier was evaluated through a combination of laboratory testing activities and consideration of fundamental vadose zone transport concepts. For the experimental evaluation, a series of laboratory column tests were conducted in which sediment was first treated with diluted hydrogen 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. At lower flow rates, the reductive capacity approached the maximum value predicted on the basis of the treatment reaction. Thus, laboratory treatment tests should reliably predict the reductive capacity of the barrier under field conditions. A theoretical approach was undertaken to estimate the lifetime of the vadose zone barrier. An initial model assumed that the barrier lifetime is determined by the reoxidation of the barrier owing to the transport of oxygen through a vadose zone interval in which all sediment is unsaturated. The results of this evaluation suggest that barrier reoxidation is primarily related to diffusion of oxygen through the gas-filled portion of the sediment pore space. If so, the barrier lifetime could be fairly short (several years). However, the presence of finer grained strata with higher moisture content could potentially increase the barrier lifetime to 100 years or more owing to a decrease in the effective diffusion coefficient for oxygen. Thus, detailed stratagraphic characterization and modeling is needed to provide an accurate assessment of barrier lifetime at specific sites.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 921412
- Report Number(s):
- PNNL-SA-52525; KP1302000; TRN: US200804%%737
- Journal Information:
- Vadose Zone Journal, 6(4):1050-1056, Vol. 6, Issue 4
- Country of Publication:
- United States
- Language:
- English
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