Uptake and release of radionuclides in wetland plants and soils
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Date
2010-08-31
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South Pacific Radioactivity Association
Abstract
Wetland plants and soils can accumulate metals including naturally-occurring and anthropogenic radionuclides, and there has been much research performed on "wetlands as sinks." However, the stability of these wetland sinks, with their elevated concentrations of metals and radionuclides, depends on relatively constant water-supply and inundation conditions. Few studies have been performed to document the release rates of radionuclides from wetlands during hydrologic perturbations including the extended droughts and water diversions occurring in many parts of Australia over the past decade. We performed a scoping study to measure uptake, retention, and release rates of a range of gamma emitters (51Cr, 54Mn, 57Co, 60Co, 59Fe, 65Zn, 75Se, 89Sr, 109Cd, 134Cs) relative to Australian wetland plants and soils. Two types of plants (Phragmites australis and Cyperus exaltus), were grown in ten containers with variation in the root density (i.e., number of root-masses per container), and in the amount of Macquarie Marshes wetland soil mixed with a quartzite-pebble substrate. Constant levels of a tracer-water solution were maintained in all containers over a 30-day growth period, followed by clipping of shoots and draining of the containers. The root-soil combinations were allowed to dry intact at glasshouse ambient temperatures for 7 months to simulate a seasonal drying period, followed by a series of leach tests. Leachate was filtered (μ45) followed by gamma counting and analysed applying gamma-ray spectrometry. Several HPGe detectors with nominal efficiencies ranging from 35% to 50% were used. The results of the first two leach tests (L1 and L2) are available. Tracer activity concentrations in Ieachate samples typically trended lower as the root density in the containers increased. For example, the leachates from containers having four root masses had activity concentrations typically ∼50% lower than those with no roots. This occurred even though the root mass was a small fraction (<0.001) of the mass of the soil-pebble matrix suggesting that roots have a substantial accumulation effect on a per weight basis. The retention of tracers within the root-soil treatments was measured by comparing the ratio of activity concentrations in the "first flush” Ieachate (L1) to that of the second Ieachate (L2). For three tracers (54Mn, 57Co, and 59Fe) the L1/L2 ratios were ∼8-18 fold when roots were absent, but lower (0.4-2.9) when roots were present, suggesting that intact roots increase retention capacity over repeated wet-dry cycles for 54Mn, 57Co, and 59Fe. ln contrast, the L1/L2 ratios of the remaining tracers were relatively constant (1.3-3.9) regardless of the presence or absence of roots, or variation in root density. The increasing amounts (0%. 20%, 35%, and 50%) of organic-rich wetland soils from Macquarie Marshes present in the host soil-quartzite matrix corresponded to up to 100—fold decreases in Ieachate concentrations of 51Cr, 57Co, 60Co, 65Zn, 755e, 89Sr, and 134Cs. ln contrast, 54Mn, 59Fe, and 109Cd concentrations increased 2-12-fold with increasing soil amounts of up to 50%. Further Ieachate results, as well as analysis of concentrations in roots and shoots, are being conducted this year. A larger-scale study is being considered. This scoping study provides radiotracer release rates for wetlands soils subject to a seasonal drying cycle, and suggests that higher tracer release rates occur when plants are not present (no roots in the soil matrix) and that some tracers have order-of-magnitude higher concentrations in the initial leaching as compared with subsequent leaching. The retention of radionuclides by roots has implications of enhancing water quality by maintaining general health of wetland plant communities.
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Keywords
Radionuclide migration, Wetlands, Plants, Soils, New South Wales, Australia, Tracer techniques, Roots, Ecological concentration
Citation
Johansen, M. P., Twining, J. R., & Mokhber-Shahin, L. (2010). Uptake and release of radionuclides in wetland plants and soils. Paper presented to the 11th South Pacific Environmental Radioactivity Association Conference "SPERA 2010", Crowne Plaza, Surfers Paradise, Gold Coast, Australia, 31 August - 3 September 2010.