The objective of this project was to develop a method for the simultaneous in situ measurement of 1,4-dioxane and water flux. Measurement of flux is critical to both the design and evaluation of remedial strategies that rely on injection of stimulants to promote biological or abiotic reactions to degrade 1,4-dioxane. Since 1,4-dioxane is predominantly associated with dilute plumes, which are often detached from the original source zone, measurement of the flux distribution within the aquifer becomes critical to successful and economical implementation of in situ remedial efforts.
The Passive Flux Meter (PFM) was developed at the University of Florida in 2001 to obtain direct measurements of contaminant mass flux and Darcy flux at contaminated sites. A modified PFM approach was developed to quantify contaminant flux of low-partitioning contaminants while simultaneously measuring Darcy flux with an acceptable measurement area. Increased error may occur in this technology when low-partitioning contaminants are involved. Modifications were proposed based on past studies such as the Passive Surface Water Flux Meter (PSFM) and low-density polyethylene (LDPE) passive diffusion samplers. Designs incorporating modified permeability were tested in box aquifer setups. The low partitioning contaminants used in the experiments were 1,4-dioxane and methanol.
A second approach considered in this study was the incorporation of a diffusion domain in a PFM configuration. A diffusion passive sampler was constructed using LDPE and filling it with granular activated carbon (GAC). The diffusion sampler was surrounded by sand to produce a similar configuration to a standard PFM and was tested in a laboratory aquifer model. The results were compared to samplers filled only with deionized water. Contaminants tested in this evaluation included 1,4-dioxane, methylene chloride, 1,1-dichloroethene (1,1-DCE), and cis-1,2-dichloroethene (1,2-DCE). This approach does not work for all types of contaminants due to the diffusion characteristics of the LDPE.
While the modified design produced significant error with the very low partitioning methanol flux measurements, the error in 1,4-dioxane measurements was acceptable at 21% in contrast to 41% for the standard PFM application when contaminant breakthrough occurred. Based on experiments conducted in this study, the modified design may work well for chlorinated contaminants such as methylene chloride, but would require alternate diffusion membranes to function for methanol or 1,4-dioxane.
In this study, only three box aquifer tests were conducted on the modified PFM when 1,4-dioxane breakthrough occurred. To verify the use of the modified design, more box aquifer tests with 1,4-dioxane breakthrough occurring in a standard PFM should be performed. If possible, a test that has complete breakthrough for 1,4-dioxane should be performed. It would also be useful to test for the maximum deployment duration of the modified PFM. Further calibration is necessary for the modified PFM.
Originally it was thought that the diffusion bag could be surrounded by more GAC that is impregnated with tracers to allow for contaminant flux and Darcy flow measurement. However, this may require too long of a deployment for a significant amount of mass to be collected in the diffusion bag. An inert media such as course sand could prove useful in surrounding the diffusion bag in a PFM, as this was used in the box aquifer tests. The PFM could be configured to have alternate layers, with one layer being a standard PFM to measure Darcy flux and the other layer the diffusion bag containing GAC and surrounded by course sand to determine contaminant flux.