Houston, TX 77005
9:30 a.m. Thursday, Aug. 22, 2013
Health | Wellness,
1,4-Dioxane (dioxane) contamination of groundwater is a potential carcinogen, widely used as a stabilizer for chlorinated solvents and exhibits very high mobility in groundwater. It is recalcitrant to biodegradation, and its physicochemical properties preclude effective removal by volatilization or adsorption. Through this long-term microcosm study, we will assess the natural attenuation potential of dioxane for multiple sediment and groundwater samples collected from three sites located in Los Angeles, CA. Using biologically active microcosms, we assess whether dioxane degradation is naturally occurring and at what rate it is degrading. Groundwater and sediment samples were taken from three sites in segments of source center, middle and leading edge, totaling 13 wells. All groundwater samples are neutral to slightly acid under aerobic condition with highly positive redox potentials. Aerobic microcosms were prepared for each monitoring well location, containing 50 g of sediment and 150 mL of groundwater. Positive controls were created for sites with the highest initial dioxane concentration (3 wells total) and were inoculated with 30 mL of Pseudonocardia dioxanivorans CB1190, the first dioxane degrader capable of utilizing it as sole carbon and energy source, in Ammonia Mineral Salt media. Negative controls were simultaneously created by autoclaving and HgCl2 poisoning. Dioxane concentrations were measured over time in 4 week increments using GC/MS. Mineralization patterns were also tested by spiking each microcosm uniformly with 14C labeled dioxane. CO2 release from metabolism was captured in a 1 mL NaOH trap and measured for radioactive output using scintillation counter. Dioxane removal is observed in all microcosms prepared with the field samples, but is no observed in negative controls, indicating degradation is taking place. Production of CO2 over time is additionally observed in microcosms experiencing dioxane loss which is consistent with a zero-order metabolic interaction. For site 1, microcosms amended with CB1190 exhibit dioxane metabolisms statistically comparable to those prepared in mineral media, indicating an absence of inhibitory conditions at its source zone. The source center of site 1 has reached 87% dioxane removal at 12 weeks, while site 2 achieved 30% and site 3 achieved 17%. Degradation decreases with increasing distance from the plume source, with the exception of two wells at the source zone of site 3 which are likely impacted by an inhibitory condition. Respiked microcosm triplicates of 1-1D experienced rapid degradation exceeding the previously observed degradation and new replicates of 1-1D have experienced 4% dioxane loss after four weeks of incubation, further indicating biological degradation is taking place at site 1. Overall, these results indicate natural attenuation is occurring and monitored natural attenuation (MNA) should be considered as a remedial response.