N-nitrosodimethylamine (NDMA) is formed as a byproduct of propellant production, drinking water chlorination, and sewage treatment for wastewater recycling. It is a potent carcinogen whose action level in drinking water is 0.7 parts per trillion. NDMA moves rapidly in groundwater because it does not sorb strongly and it is slowly degraded in both aerobic and anerobic environments.
Initial studies have shown that NDMA can be degraded below the drinking water action level by the reduction of sediment with dithionate. Dithionite can be injected into an aquifer to reduce sediment and create a permeable reactive barrier. Application of this method to in situ sediment reduction will overcome the depth limitation on the construction of zero-valent iron trenches. However, before the potential of this remediation technology can be realized, the degradation pathway and the products of NDMA reduction must be known. The site-specific effects of the reduction process on the in situ microbial community and the ability of the in situ microbial community to mineralize NDMA and its reduction products also must be assessed.
The overall objective of this project was to understand and optimize the combined effects of abiotic and biotic processes to degrade NDMA to nontoxic products in chemically reduced natural sediment.
Different in situ remediation processes were compared in this study—biostimulation (oxic, anaerobic, iron-reducing, sulfate-reducing), abiotic iron-reducing environment (by dithionite reduction of sediment or zero-valent iron addition), coupled abiotic/biotic remediation (iron-reducing environment), and sequential iron-reducing then oxic biostimulation. The sediment used in experiments was from the Aerojet groundwater aquifer (260-ft depth) in California, with additional sediments from other groundwater aquifers (i.e., not shallow soils at Fort Lewis, Washington, 60-ft depth, and at Puchack, New Jersey, 273-ft depth).
Dithionite- or natural-reduced aquifer sediments under iron-reducing conditions degrade NDMA rapidly, but mineralize NDMA slowly. This reactivity was maintained for 84 pore volumes when experiments ended, and it is anticipated that the reactivity would last longer for the Aerojet sediment. From these findings, it can be assumed that other methods to create a subsurface iron-reducing environment may also yield similar results (i.e., zero-valent iron injection or biostimulation). Although numerous experiments evaluating additions to stimulate in situ microbial activity were largely ineffective in this study, ex situ bioreactors utilizing appropriate monooxygenase isolates have been shown to be successful in other studies. Future studies of NDMA remediation would benefit from focusing on the comparison of in situ abiotic NDMA mineralization (iron-reducing environments) to ex situ biomineralization.
The determined mechanistic and actual laboratory- to field-scale data applying this coupled abiotic/biotic technology to different sites for NDMA degradation will provide a fundamental understanding of this remediation process, as well as information on site-specific factors that degrade the efficacy of the process (e.g., co-contaminants). These determinations will reduce the toxicity, mobility, and volume of NDMA in a variety of subsurface environments and allow for multiple field-scale applications.