Evolving radon diffusion through earthen barriers at uranium waste disposal sites.

Field measurements of Rn-222 fluxes from the tops and bottoms of compacted clay radon barriers were used to calculate effective Rn diffusion coefficients (DRn) at four uranium waste disposal sites in the western United States to assess cover performance after more than 20 years of service. Values of DRn ranged from 7.4 × 10-7 to 6.0 × 10-9 m2/s, averaging 1.42 × 10-7. Water saturation (SW) from soil cores indicated that there was relatively little control of DRn by SW, especially at higher moisture levels, in contrast to estimates from most steady-state diffusion models. This is attributed to preferential pathways intrinsic to construction of the barriers or to natural process that have developed over time including desiccation cracks, root channels, and insect burrows in the engineered earthen barriers. A modification to some models in which fast and slow pathway DRn values are partitioned appears to give a good representation of the data; 4% of the fast pathway was needed to fit the data regression. For locations with high Sw and highest DRn (and fluxes) at each site, the proportion of fast pathway ranged from 1.7% to 34%, but for many locations with lower fluxes, little if any fast pathway was needed.

Radon fluxes at four uranium mill tailings disposal sites after about 20 years of service.

To evaluate the properties of earthen covers over uranium mill tailings disposal cells after about 20 years of service, we measured Rn-222 fluxes and radon barrier properties at the Falls City, TX, Bluewater, NM, Shirley Basin South, WY, and Lakeview, OR disposal sites in western USA. A total of 115 in-service Rn fluxes were obtained at 26 test pit locations from the top surface of the exposed Rn barrier (i.e., after protective layers were removed by excavation) and 24 measurements were obtained from the surface of the underlying waste after excavation through the Rn barrier layer. Rn-222 concentrations were determined in accumulation chambers using a continuously monitoring electronic radon monitor (ERM) equipped with a solid-state alpha particle detector. Effects of surface features on Rn flux including vegetation, seasonal ponding, and animal burrowing were quantified. Comparison of measured fluxes with values that were measured shortly after the Rn barriers were completed (as-built) show that most measurements fell within the range of the as-built fluxes, generally at very low fluxes. At two sites fluxes were measured that were greater than the highest as-built flux. High fluxes are typically caused by a combination of enhanced moisture removal and preferential pathways for Rn transport, often caused by deep-rooted plants. Such localized features result in a spatially heterogeneous distribution of fluxes that can vary substantially over only a meter or two.

241Am, 137Cs, Sr and Pb uptake by tobacco as influenced by application of Fe chelators to soil.

To determine the potential for phytoextraction of 241Am and other contaminants from soil, accumulation of 241Am, 137Cs, Sr, Fe, Al, Pb, and Mg by tobacco was determined for soil applications of two concentrations of ethylenediaminetetraacetic acid (EDTA), citric acid, and ascorbic acid. In tobacco receiving EDTA at 3.1 mmol/kg of soil, 241Am content of plants averaged 15 Bq/kg (ranging up to 26 Bq/kg) while Fe concentrations became constant at 4.5 mmol/kg. Soil treatment with 18.8 mmol/kg EDTA resulted in average 241Am concentrations of 29 Bq/kg (19 times higher than controls). Uptake of Pb was similar to 241Am. In these samples, Fe increased to a maximum of almost 18 mmol/kg and 241Am content increased linearly with both Fe and Al. Plants receiving ascorbic and citric acids took up smaller quantities of 241Am, Pb, and Fe, even though these reagents were able to elute about as much Fe from the soil as EDTA. Synchrotron microbeam X-ray fluorescence (SXRF) was used to determine radial distributions of elements in roots and stems with and without EDTA treatment. SXRF maps indicate differences in behavior between Fe and Pb that are consistent with the bulk plant observations and provide insight into changes in metal content of the roots in the presence of EDTA.

Adsorption of trace metals on glass fiber filters.

Filters, containing glass-fiber (GF) filter material, are commonly used as the primary filter or as the prefilter in sampling natural waters and laboratory experiments with high concentrations of suspended solids. We observed that GF filter material removed substantial quantities of trace metals from solutions of low ionic strength at near neutral and slightly acidic pH. The GF material sorbed essentially all Pb and Ag from 5-mL aliquots of solutions containing 0.054 and 0.093 mM, respectively. Somewhat less Ni was sorbed from a 0.099 mM solution. This material retained about 43 micromol of Ag per gram of GF material (4600 microg/g). The Ag and Ni sorption was highest at low KNO(3) concentrations (as background electrolyte) and decreased to a constant concentration of sorbed metal at approximately 10 mM KNO(3). Glass-fiber filter material should only be used with careful testing for the elements of concern under conditions that closely match expected environmental or experimental conditions.

Uptake and release of cesium-137 by five plant species as influenced by soil amendments in field experiments.

Phytoextraction field experiments were conducted on soil contaminated with 0.39 to 8.7 Bq/g of 137Cs to determine the capacity of five plant species to accumulate 137Cs and the effects of three soil treatments on uptake. The plants tested were redroot pigweed (Amaranthus retroflexus L. var. aureus); a mixture of redroot pigweed and spreading pigweed (A. graecizans L.); purple amaranth (A. cruteus L.) x Powell's amaranth (A. powellii S. Watson), referred to here as the amaranth hybrid; Indian mustard [Brassica juncea (L.) Czern.]; and cabbage (Brassica oleracea L. var. capitata). For control plants, the concentration ratios (CR) of 137Cs were greatest for redroot pigweed and the amaranth hybrid, with average CR values of 1.0 +/- 0.24 and 0.95 +/- 0.14, respectively. The lowest value was for Indian mustard at 0.36 +/- 0.10. The soil treatments included (i) application of NH4NO3 solution to the soil after plants had matured, (ii) addition of composted manure to increase organic matter content of the soil, (iii) combination of the manure and ammonium solution treatments, and (iv) controls. The ammonium solution gave little overall increase in accumulation of 137Cs. The use of composted manure also had little influence, but the combination of the composted manure with application of ammonium solutions had a distinctly negative effect on plant uptake of 137Cs. On average the fraction of 137Cs taken up from the soil was reduced by 57.4 +/- 1.2% compared with controls. This was the result of release of competing ions, primarily Ca, from the manure and was observed across all five plant species tested. The application of ammonium solution took place in the last two weeks before harvest. The reduction of plant 137Cs content, by addition of the ammonium solution, as it interacted with the manure, indicates that substantial quantities 137Cs can be released from the shoots of plants as a result of sudden changes in soil solution chemistry.

Increased cadmium tolerance and accumulation by plants expressing bacterial arsenate reductase.

• Cadmium (Cd) is a major environmental pollutant that poses a serious threat to natural ecosystems. However, most initial attempts to engineer phytoremediation of Cd have not succeeded in developing sufficient Cd tolerance for vigorous plant growth. • We found that the bacterial arsenate reductase gene (arsC) provided Cd(II) resistance to Escherichia coli. When ArsC is overexpressed in tobacco (Nicotiana tabacum) and Arabidopsis thaliana, both transgenic plant species showed significantly greater Cd tolerance than wild-type controls. • At 50, 75, and 100 µm concentrations of Cd (II), the ArsC expressing transgenic lines grew bigger with broader leaves and longer roots than wild-type controls, which were stunted, turned yellow, flowered early, and often died. At the various Cd(II) concentrations, ArsC transgenic plants attained f. wt 2-3-fold higher than the wild-type plants and had roots significantly longer than wild-type plants. These transgenic plants also contained 30-50% higher Cd concentrations than wild-type plants. • It is likely that the arsC gene directs Cd tolerance via the electrochemical reduction of Cd(II) to Cd(0).

Uptake of cesium-137 and strontium-90 from contaminated soil by three plant species; application to phytoremediation.

A field test was conducted to determine the ability of three plant species to extract 137Cs and 90Sr from contaminated soil. Redroot pigweed (Amaranthus retroflexus L.), Indian mustard [Brassica juncea (L.) Czern.], and tepary bean (Phaseolus acutifolius A. Gray) were planted in a series of spatially randomized cells in soil that was contaminated in the 1950s and 1960s. We examined the potential for phytoextraction of 90Sr and 137Cs by these three species. Concentration ratios (CR) for 137Cs for redroot pigweed, Indian mustard, and tepary bean were 2.58, 0.46, and 0.17, respectively. For 90Sr they were substantially higher: 6.5, 8.2, and 15.2, respectively. The greatest accumulation of both radionuclides was obtained with redroot pigweed, even though its CR for 90Sr was the lowest, because of its relatively large biomass. There was a linear relationship between the 137Cs concentration in plants and its concentration in soil only for redroot pigweed. Uptake of 90Sr exhibits no relationship to 90Sr concentrations in the soil. Estimates of time required for removal of 50% of the two contaminants, assuming two crops of redroot pigweed per year, are 7 yr for 90Sr and 18 yr for 137Cs.

Calcium compartments in brain.

Excellent progress has been made toward understanding the physiology and pharmacology of specific calcium-related cellular processes of the brain, but few studies have provided an integrated view of brain calcium kinetics. To further the knowledge of the size and binding properties of brain calcium compartments, the authors have conducted a series of experiments in hippocampal brain slices exposed to high and low extracellular calcium. Slices were incubated in buffers containing 0.001 to 4.5 mmol/L calcium for up to 75 minutes. Slice calcium content was analyzed by three methods: exchange equilibrium with 45Ca, synchrotron-radiation-induced x-ray emission, and inductively coupled plasma. Data were analyzed using a model based on a Langmuir isotherm for two independent sites, with additional extracellular and bound compartments. In parallel experiments, altered low calcium had no effect on slice histology and only mild effects on slice adenylates. When combined with prior 45Ca and fluorescent probe binding experiments, these results suggest that there are at least five kinetically distinct calcium compartments: (1) free extracellular (approximately 10%); (2) loosely associated extracellular plasma membrane (approximately 55%); (3) intracellular compartment with moderate avidity (approximately 17%); (4) tightly bound, nonexchangeable intracellular compartment ( approximately 15%); and (5) free cytoplasmic (<0.01%). If only the third compartment is considered a potential calcium buffer, then the buffering ratio is calculated to be approximately 2,700:1, but if the second compartment is also included, then the buffering ratio would be approximately 13,000:1. This may explain the wide range of estimates observed by fluorescent probe studies.

Arsenic distribution and speciation in the fronds of the hyperaccumulator Pteris vittata.

• Pteris vittata is the first plant reported to be a hyperaccumulator of arsenic (As), and little is known about the mechanisms of As hyperaccumulation in this plant. • Arsenic distribution at the whole plant (fronds) and cellular level was investigated using chemical analyses and energy dispersive X-ray microanalyses (EDXA). Speciation of As in the fronds was determined using X-ray absorption near edge spectroscopy (XANES) analyses. • The majority of As was found in the pinnae (96% of total As). The concentration of As in pinnae decreased from the base to the apex of the fronds. Arsenic concentrations in spores and midribs were much lower than in the pinnae. EDXA analyses revealed that As was compartmentalized mainly in the upper and lower epidermal cells, probably in the vacuoles. The distribution pattern of potassium was similar to As, whereas other elements (Ca, Cl, K, Mg, P and S) were distributed differently. • XANES analyses showed that approximately 75% of the As in fronds was present in the As(III) oxidation state and the remaining as As(V).