Attenuation mechanisms of N-nitrosodimethylamine at an operating interceptand treat groundwater remediation system

The North Boundary Containment System (NBCS), an intercept-and-treat system , was established at Rocky Mountain Arsenal (RMA), Commerce City, CO, to re move low-level organic contaminants from a groundwater plume exiting RMA to the north and northwest. N-nitrosodimethylamine (NDMA) was detected in gro undwater collected from the dewatering and recharge zones of the NBCS syste m. Concern over the fate of NDMA, in terms of potentially exiting the bound aries of the arsenal, prompted an investigation to evaluate potential atten uation mechanisms for NDMA within the alluvial aquifer system and within th e NBCS itself. Groundwater, soil, and granular activated carbon (GAC) sampl es were taken from key locations in the NBCS system. Soil and GAC samples w ere assayed for sorption kinetics and for adsorption and desorption propert ies using C-14-labeled NDMA. NDMA biodegradation experiments were conducted by following (CO2)-C-14 evolution from C-14-labeled NDMA in soils and GAC samples under aerobic and anaerobic conditions. The sorptive capacity of th e site soils for NDMA was insignificant. Furthermore, the adsorption of the NDMA by the soil was almost completely reversible. Evaluation of the degra dation potential of the native microbial consortia indicated a high level o f NDMA mineralization when measured using bench-scale microcosms. The nativ e consortia had capability to mineralize the NDMA under both aerobic and an aerobic incubations, indicating facultative characteristics. Testing of the local groundwater chemistry revealed that the area of the aquifer of inter est was microaerobic and neutral in pH. These conditions were optimal for N DMA removal. While sorption was insignificant. degradation was a significan t attenuation mechanism, which may be the reason that no NDMA has migrated off-site. This gives rise to the potential or a long-term sink for attenuat ing NDMA within the recharge zone of the treatment system. (C) 2000 Publish ed by Elsevier Science B.V. All rights reserved.