Establishing the prevalence and relative rates of 1,4-dioxane biodegradation in groundwater to improve remedy evaluations.

Options for remediating 1,4-dioxane at groundwater sites are limited due to the physical-chemical properties of this compound. The relevance of natural attenuation processes for 1,4-dioxane was investigated through data from field, lab, and modeling efforts. The objectives were to use multiple lines of evidence for 1,4-dioxane biodegradation to understand the prevalence of this activity and evaluate convergence between lines of evidence. A 14C-1,4-dioxane assay confirmed 1,4-dioxane biodegradation at 9 of 10 sites (median rate constant of 0.0105 yr-1 across wells). Site-wide rate constants were established using a calibrated fate and transport model at 8 sites (median = 0.075 yr-1). The 14C assay constants are likely more conservative, and variability in rates suggested that biodegradation at sites may be localized. Stable isotope fractionation was observed at 7 of 10 sites and served as another direct line of evidence of in situ biodegradation of 1,4-dioxane. This includes sites where indirect lines of evidence, including geochemical conditions or genetic biomarkers for degradation, would not necessarily have been supportive. This highlights the importance of collecting multiple lines of evidence to document 1,4-dioxane natural attenuation, and the widespread prevalence of biodegradation suggests that this process should be part of long-term management decisions.