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.