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Carbon Isotopic Evidence for Biodegradation of Organic Contaminants in the Shallow Vadose Zone of the Radioactive Waste Management Complex

^a Earth Sciences Division, MS 70A-4418, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
^b Earth Sciences Division, MS 90R-1116, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720

View larger version (31K): [in a new window]   Fig. 1. Map of the Radioactive Waste Management Facility (RWMC) site showing the location of the wells sampled for this study and the Subsurface Disposal Area (SDA) and Transuranic Storage Area (TSA). Also plotted are the locations of the waste disposal pits, the soil vault containing the Be reactor blocks, the vapor extraction units, and the A–A' longitudinal cross section in Fig. 4. The inset shows the location of the RWMC within the INEEL and the state of Idaho.

View larger version (19K): [in a new window]   Fig. 2. Carbon dioxide concentrations and 13C vs. versus depth for samples from the Subsurface Disposal Area (SDA) and from background wells. The main depth interval of elevated concentrations is between 10 and 30 m (shown shaded). Slightly elevated values occur down to the 75-m depth. Soils from the TEM1 locality and from farther to the west of that locality have particularly high CO2 values within a few meters of the surface. These values require very high local production rates of CO2 (from root respiration).

View larger version (17K): [in a new window]   Fig. 3. Average CO2 concentrations (in volume %) in pore gas samples collected from different depth intervals in and around the Radioactive Waste Management Facility (RWMC) site.

View larger version (34K): [in a new window]   Fig. 4. Longitudinal cross section A–A' (Fig. 1) passing through the high-CO2 zone in the Radioactive Waste Management Facility, with contours of the labeled average CO2 concentrations measured for pore gas samples collected from different sampling port in wells within 100 m of the section. The approximate extent of the surficial sediments and the sedimentary interbeds are shown as shaded areas on the section.

View larger version (18K): [in a new window]   Fig. 5. Carbon dioxide concentrations measured over time in three vapor sampling ports in Borehole 8801, showing the effect of vapor extraction from Borehole 8901 (located approximately 20 m from 8801). Dashed lines indicate intervals where a sampling period was missed.

View larger version (11K): [in a new window]   Fig. 6. Inverse concentrations of CO2 in pore gas samples from between 10 and 70 m depth in the high-CO2 zone in the Subsurface Disposal Area (SDA). Allowing for a shift of 4.4 for diffusive fractionation (Cerling, 1984), this data suggests that the source for the increased CO2 in the vicinity of the disposal pits of has a 13C value of between –24 and –29.

View larger version (19K): [in a new window]   Fig. 7. Radiocarbon contents (in fraction of modern atmospheric C) in CO2 samples collected for this study. Values in parentheses indicate the depth at which the sample was collected.

View larger version (31K): [in a new window]   Fig. 8. Comparison of model results for a = 10 m and d = 20 m and measured CO2 concentrations vs. depth. This model uses a specific CO2 production rate of 0.0023 m3 CO2 m–3 rock yr–1 in the producing layer between the 10- and 30-m depths (shown shaded) and gives a total CO2 production for a 150 by 500 m area equivalent to the degradation of approximately 1.7 t C yr–1.