Formatted Title

Challenges and Lessons Learned from a Desktop Evaluation of 1,4-Dioxane at a Complex Solvent Site

Background/Objectives

1,4-Dioxane (DX) has emerged as a common groundwater and surface water contaminant due to its use in a variety of industrial and consumer products, particularly its historical use as a stabilizer for the chlorinated solvent 1,1,1-trichloroethane (TCA). Due to the subsequent and combined use of multiple chlorinated solvents in many industrial processes, DX contamination often accompanies mixtures of chlorinated compounds including, tetrachloroethene (PCE), trichloroethene (TCE), as well as lesser-chlorinated ethenes and ethanes produced from environmental transformations. While the extent of DX contamination is well demonstrated and the US EPA has issued a 10^-6 lifetime cancer risk level of 0.35 µg/L in drinking water, a federal maximum contaminant level has not been issued. As such, DX regulation in the US occurs on a state or even more local basis. This has resulted in nonexistent or delayed assessment of DX at contaminated sites.

A complex site in the southwestern US illustrates these challenges of incorporating DX into site assessment. Other contaminants of concern at the site are primarily chlorinated volatile organic compounds (CVOCs). The site has heterogeneous geology, containing areas of fractured bedrock. The site is organized into multiple investigation areas based on structures, former activities, and hydrogeology, and contains more than 100 monitoring locations. A draft Corrective Measures Study (CMS) was completed for the site but it did not assess the extent of DX contamination and was therefore not considered in suggested remedies. However, DX is present in at least one investigation area, necessitating further evaluation as plans for the site are developed.

Approach/Activities

DX evaluation at the site was initially approached as a desktop study. DX data were assessed in conjunction with 1,1,1-TCA, TCE, PCE, and 1,1-dichloroethene (1,1-DCE). To get a spatial sense of DX extent, particularly to identify hot spots and data gaps, maximum DX concentrations for each monitoring location (if assessed) were plotted on a site map. For each location, available DX and CVOC concentrations were plotted versus time, which illustrated the presence of contaminant mixtures, the temporal extent of evaluation at each location, as well as varying analytical detection limits throughout the sampling timeframe. When there was sufficient data, a Mann-Kendall trend was assessed. Data analysis was performed via Python scripts to efficiently generate plots and calculate trends for five contaminants at over 100 locations. Finally, treatments proposed in the CMS for other contaminants were evaluated for their potential effectiveness for DX.

Results/Lessons Learned

Review of the data illustrated that DX was measured at least once at 80% of sampling locations sampled for CVOCs. DX contamination co-occurred most commonly with TCE. However, DX was evaluated in less than 50% of samples assessed for CVOCs. DX has been assessed at the site as early as 1993, though the method detection limit was 500 µg/L until 1997, greater than 1 µg/L until 2003, and not below 0.35 µg/L before 2021. Some locations, particularly relevant given the presence of CVOC co-contaminants, were not reassessed after detection limits were reduced to environmentally and regulatorily relevant concentrations. Available DX data allowed for the calculation of a Mann-Kendall trend at only 10 sampling locations, illustrating a mix of increasing, decreasing, and stable trends at the site. The applicability to DX of remediation approaches proposed in the CMS for CVOCs and other contaminants varied. While anaerobic reduction can be an effective remediation strategy for CVOCs, there is little evidence of either anaerobic chemical or biological reduction of DX. DX has been shown to naturally attenuate at other sites, however there was limited data to assess the appropriateness of monitored natural attenuation for DX in the areas where it was proposed to manage other compounds at this site. Generally, this assessment illustrated the challenge DX may present at many CVOC sites, due to its potentially widespread extent, inconsistent historical analysis, and incompatibility with some common remediation approaches for CVOCs.