Spatial analysis of Carbon-14 dynamics in a wetland ecosystem (Duke Swamp, Chalk River Laboratories, Canada)

Determination of carbon-14 in marine biota using oxidation combustion and gel suspension liquid scintillation counting

In this study, we report a new analytical method for determination of 14C in marine biota utilizing oxidation combustion separation combined with CaCO3 suspension of precipitates for liquid scintillation counting (LSC). The main factors affecting the oxidation combustion efficiency of tube combustion furnace and the counting efficiency of the LSC were systematically investigated and optimized. Under the optimal combustion conditions, the combustion recovery of carbon ranged from 85.6 % to 92.4 % for five marine samples. And the method achieved a detection limit of 0.13 Bq/g for 14C and repeatability of 3.9-9.1 %. The analytical turnover time was 6 h, and up to six samples could be processed simultaneously. Assessment of uncertainty components showed that the uncertainty in counting was the largest contributor, followed by combustion recovery and counting efficiency. The developed method enables reliable measurement of 14C in marine biota, providing valuable analytical tool for risk assessment purposes.

MEASUREMENT OF AMBIENT CARBON-14 BY USING THE GEL SUSPENSION COUNTING METHOD

For monitoring the ambient 14C, the CaCO3 suspension counting method was established in this work. In the preparation of CaCO3 powder, a two-stage sampler with 3 mol L-1 NaOH absorbent was designed to collect the ambient CO2 at a sampling flow rate of 1 L min-1, and then the CaCO3 was precipitated by adding saturated CaCl2 solution. By using 2 g of CaCO3 powder, 4 mL double-distilled water and 14 mL scintillation cocktail, the lower limit detection could reach 20.0 mBq m-3 by using a commercially available low background liquid scintillation counter. Co-comparison experiments showed that the activity concentration of 14C measured by the gel suspension counting method consisted well with the results of other three methods. It indicates that the CaCO3 suspension counting method is also a practical method for routine monitoring of ambient 14C.

Spatial Autocorrelation, Source Water and the Distribution of Total and Viable Microbial Abundances within a Crystalline Formation to a Depth of 800 m

Proposed radioactive waste repositories require long residence times within deep geological settings for which we have little knowledge of local or regional subsurface dynamics that could affect the transport of hazardous species over the period of radioactive decay. Given the role of microbial processes on element speciation and transport, knowledge and understanding of local microbial ecology within geological formations being considered as host formations can aid predictions for long term safety. In this relatively unexplored environment, sampling opportunities are few and opportunistic. We combined the data collected for geochemistry and microbial abundances from multiple sampling opportunities from within a proposed host formation and performed multivariate mixing and mass balance (M3) modeling, spatial analysis and generalized linear modeling to address whether recharge can explain how subsurface communities assemble within fracture water obtained from multiple saturated fractures accessed by boreholes drilled into the crystalline formation underlying the Chalk River Laboratories site (Deep River, ON, Canada). We found that three possible source waters, each of meteoric origin, explained 97% of the samples, these are: modern recharge, recharge from the period of the Laurentide ice sheet retreat (ca. ∼12000 years before present) and a putative saline source assigned as Champlain Sea (also ca. 12000 years before present). The distributed microbial abundances and geochemistry provide a conceptual model of two distinct regions within the subsurface associated with bicarbonate – used as a proxy for modern recharge – and manganese; these regions occur at depths relevant to a proposed repository within the formation. At the scale of sampling, the associated spatial autocorrelation means that abundances linked with geochemistry were not unambiguously discerned, although fine scale Moran’s eigenvector map (MEM) coefficients were correlated with the abundance data and suggest the action of localized processes possibly associated with the manganese and sulfate content of the fracture water.

Predicting exposure of wildlife in radionuclide contaminated wetland ecosystems

Many wetlands support high biodiversity and are protected sites, but some are contaminated with radionuclides from routine or accidental releases from nuclear facilities. This radiation exposure needs to be assessed to demonstrate radiological protection of the environment. Existing biota dose models cover generic terrestrial, freshwater, and marine ecosystems, not wetlands specifically. This paper, which was produced under IAEA's Environmental Modelling for Radiation Safety (EMRAS) II programme, describes an evaluation of how models can be applied to radionuclide contaminated wetlands. Participants used combinations of aquatic and terrestrial model parameters to assess exposure. Results show the importance of occupancy factor and food source (aquatic or terrestrial) included. The influence of soil saturation conditions on external dose rates is also apparent. In general, terrestrial parameters provided acceptable predictions for wetland organisms. However, occasionally predictions varied by three orders of magnitude between assessors. Possible further developments for biota dose models and research needs are identified.