Implications of matrix diffusion on 1,4-dioxane persistence at contaminated groundwater sites.
Management of groundwater sites impacted by 1,4-dioxane can be challenging due to its migration potential and perceived recalcitrance. This study examined the extent to which 1,4-dioxane's persistence was subject to diffusion of mass into and out of lower-permeability zones relative to co-released chlorinated solvents. Two different release scenarios were evaluated within a two-layer aquifer system using an analytical modeling approach. The first scenario simulated a 1,4-dioxane and 1,1,1-TCA source zone where spent solvent was released. The period when 1,4-dioxane was actively loading the low-permeability layer within the source zone was estimated to be <3years due to its high effective solubility. While this was approximately an order-of-magnitude shorter than the loading period for 1,1,1-TCA, the mass of 1,4-dioxane stored within the low-permeability zone at the end of the simulation period (26kg) was larger than that predicted for 1,1,1-TCA (17kg). Even 80years after release, the aqueous 1,4-dioxane concentration was still several orders-of-magnitude higher than potentially-applicable criteria. Within the downgradient plume, diffusion contributed to higher concentrations and enhanced penetration of 1,4-dioxane into the low-permeability zones relative to 1,1,1-TCA. In the second scenario, elevated 1,4-dioxane concentrations were predicted at a site impacted by migration of a weak source from an upgradient site. Plume cutoff was beneficial because it could be implemented in time to prevent further loading of the low-permeability zone at the downgradient site. Overall, this study documented that 1,4-dioxane within transmissive portions of the source zone is quickly depleted due to characteristics that favor both diffusion-based storage and groundwater transport, leaving little mass to treat using conventional means. Furthermore, the results highlight the differences between 1,4-dioxane and chlorinated solvent source zones, suggesting that back diffusion of 1,4-dioxane mass may be serving as the dominant long-term "secondary source" at many contaminated sites that must be managed using alternative approaches.
GSI Environmental Inc., 2211 Norfolk, Suite 1000, Houston, TX 77098, United States. Electronic address: dtadamson@gsi-net.com.
David T
DT
Adamson
GSI Environmental Inc., Houston, TX 77098, United States. Electronic address: pcdeblanc@gsi-net.com.
Phillip C
PC
de Blanc
GSI Environmental Inc., Houston, TX 77098, United States. Electronic address: skfarhat@gsi-net.com.
Shahla K
SK
Farhat
GSI Environmental Inc., Houston, TX 77098, United States. Electronic address: cjnewell@gsi-net.com.
Charles J
CJ
Newell
D004058:Diffusion; D004146:Dioxanes / Q000032:analysis* / Q000737:chemistry; D004784:Environmental Monitoring; D060587:Groundwater / Q000737:chemistry*; D054879:Halogenation; D012995:Solubility; D012997:Solvents; D014874:Water Pollutants, Chemical / Q000032:analysis* / Q000737:chemistry
D004146:Dioxanes; D012997:Solvents; D014874:Water Pollutants, Chemical; C025223:1,4-dioxane
1,1,1-Trichloroethane; Emerging contaminants; Low-permeability zones; Source zone characterization