Evaluation of natural attenuation of 1,4-dioxane in groundwater using a ¹⁴C assay

Monitored Natural Attenuation (MNA) is a preferred remedy for sites contaminated with 1,4-dioxane due to its low cost and limited environmental impacts compared to active remediation. Having a robust estimate of the rate at which biodegradation occurs is an essential component of assessing MNA. In this study, an assay was developed using ¹⁴C-labeled 1,4-dioxane to measure rate constants for biodegradation based on accumulation of ¹⁴C products. Purification of the ¹⁴C-1,4-dioxane stock solution lowered the level of ¹⁴C impurities to below 1% of the total ¹⁴C activity. This enabled determination of rate constants in groundwater as low as 0.0021 yr^-1, equating to a half-life greater than 300 years. Of the 54 groundwater samples collected from 10 sites in the US, statistically significant rate constants were determined with the ¹⁴C assay for 24. The median rate constant was 0.0138 yr^-1 (half-life = 50 yr); the maximum rate constant was 0.367 yr^-1 (half-life = 1.9 yr). The results confirmed that biodegradation of 1,4-dioxane is occurring at 9 of the 10 sites sampled, albeit with considerable variability in the level of activity. The specificity of the assay was confirmed using acetylene and the absence of oxygen to inhibit monooxygenases.