Search

Cation exchange resin saturated with H+ and Ca2+ was used to extract 137Cs from peat soil at two sites in Britain affected by l37Cs deposition following the Chernobyl accident. The technique identified three classes of 137Cs, similar to those observed for K+ in soils: ‘Fast’, ‘Intermediate’ and ‘Slow’. These classes are probably related to the selectivity for 137Cs of the cation exchange sites on the organic matter and the clay minerals, and to the structure of the soil. With one exception, most 137Cs was in the ‘Slow’ form and was only very slowly released to the resins, if at all. However, there was enough l37Cs in the ‘Fast’ and ‘Intermediate’ forms to contaminate pasture and thus grazing animals for some years. Based on the resin technique, it is estimated that contamination will persist for several decades in uplands contaminated at these activity concentrations.

When elements are simultaneously added to lakes, experimentally or by accident, their ratios in the water phase and in bottom sediments can change with time due to differential partitioning between solution and suspended particles or sediments. A number of equations are developed to show the change of ratio with time in water and sediments assuming simultaneous pulse inputs followed by a range of combinations of loss processes from solution (i.e. hydraulic losses, sorption to particles followed by settling, and diffusion into the sediments). The pattern of events is discussed both for pulse events with specific limiting assumptions and for combined continuous and pulse inputs. The models show that elemental ratios in sediments are generally less sensitive indicators of differential partitioning than are elemental ratios in water. For lakes with long residence times, the long-term elemental ratio in the sediments does not differ from that in the initial spike to the water column, but for short residence times, it is directly dependent on the ratio of either partition or diffusion coefficients. The models are used to interpret Ru : Cs ratios measured in the water and sediments of Esthwaite Water subsequent to the pulse input of Chernobyl fallout. The ratios can be explained by assuming nuclides were lost either by flushing and sorption or by flushing, sorption, and diffusion. The process combination of flushing and diffusion is incompatible with the observed constant ratios.

Nine pairs (three different depths in each of three different zones) of porous-cup solution samplers have been installed in a saltmarsh in west Cumbria, U.K. The sediments of this marsh are well known to contain elevated levels of transuranium elements (up to 16 000 Bq kg-1239,240 Pu) and it has been used for many years and by many workers as a site for the study of the geochemistry of the transuranium elements. Over a period of approximately 18 months, nine sets of porewater samples were collected. Sediment core samples were also collected from each zone of the marsh. The stable element compositions of the solution phase are generally as expected in an estuarine system, simply reflecting mixing of sea- and freshwaters. The 239,240Pu concentrations in the upper and middle zones of the marsh are low, usually between 2 and 20 mBq l-1, and with mean values of about 5 mBq l-1, whilst those from the seaward zone of the marsh are approximately a factor of 6 higher, probably reflecting a much greater addition of seawater. The Kd values for 239,240Pu in the sediments lie between 104 and 106, comparable to the values found by other workers near this site. Nevertheless, there are considerable fluctuations in Kd over time, and also substantial variations in Kd with depth in the sediment profile, and it may be that the assumption of chemical equilibrium underlying the use of Kd values is not valid in this system.

Measurements of the concentrations of Ru-103 and Ru-106 in the water column and sediments of two productive temperate lakes were made during an eight month period following the Chernobyl accident. Ru-103 accumulation in the sediments began simultaneously with that of Cs-134 and a constant ratio (approx. 2.0 on a decay corrected basis) of Ru-103/Cs-137 was observed in surface waters and sediments throughout the measurement period. The ratio is not statistically different from that reported for atmospheric input and for epilimnetic water. The results suggest that the processes of removal of radioactive ruthenium and caesium from the mixed water column to the sediments are the same. Enhancement of Cs, compared with Ru, in the isolated bottom waters of one of the lakes in the summer months provided evidence for Cs release from the sediments during a period of hypolimnetic anoxia.

Field observations suggest that plutonium and americium in the environment are present in very different chemical forms in the interstitial waters of an intertidal sediment. Themodynamic modelling using the PHREEQE code predicts that plutonium is present entirely in oxidation state (V) as the PuO2CO 3 − ion, whereas americium is present entirely in oxidation state (III), largely as the uncharged Am(OH)CO3 species, but with significant concentrations of the Am3+ and the AmSO 4 + ions. There are, however, differences between these predictions and others published for a very similar system which apparently arise from uncertainties in the thermodynamic data. Field data cannot resolve these differences unambiguously.

Large soil cores (23.5 cm diameter), containing elevated concentrations of plutonium and americium derived from fuel reprocessing effluents (typically 2000–3000 but up to 10000 Bq kg−1 239,240Pu and 3000–5000 but up to 15 000 Bq kg−1 241Am), were collected from three zones of a salt marsh in west Cumbria. The standing vegetation was removed, the cores placed in a greenhouse and the vegetation allowed to regrow. After 2 months, the regrowth was harvested and the amount of plutonium and americium taken up determined. These data, combined with data from earlier, in-situ sampling of soil pore waters and from analysis of soil cores, allow estimation of soil-plant transfers. If these are expressed simply as the ratio of the activity concentrations in the plant tissues to those in the soil, the values lie in the range 10−5−10−6, comparable to those found by many other workers. However, the ratio of concentrations in plant tissues to those in the soil solution is much higher; 40–110 l kg−1 for 239,240Pu and 210–640 l kg−1 for 241Am. There appears to be a consistent relationship between soil-plant transfer and the in-situ solid-solution distribution coefficient (Rd).

Cs-137 activity levels in Devoke Water Cumbria (UK) remained high for much longer than was expected after the Chernobyl accident. This is shown to be due to the release of dissolved Cs-137 from the catchment to the lake. Cs-137 inventories in different soil types suggest that fibrous peat soils loose Cs rapidly. Rankers and amorphous peats loose smaller amounts and podzols the least. The concentration of Cs-137 at any time in the streams is given by either: log A = −0.001064 time(days) + 0.302003 log flow(m3 s−1) + constant or: log A = −0.001064 time(days) + constant depending on the size of the constant, where the constant is given by: constant = (0.0473% fibrous peat + 4.383) x deposition (kBq/m2)/9.8. Possible release mechanisms are discussed.

Inductively coupled plasma mass spectrometry (ICP-MS) and neutron activation analysis (NAA) have been investigated as alternatives to alpha-spectrometry for the low-level determination of237Np and other actinides in environmental matrices. ICP-MS in particular, has been shown here to offer suitable sensitivity, precision and accuracy compared to the other techniques, with considerably faster sample throughput relative to radiometric and activation approaches. Added advantages of ICP-MS are found to include the abilities to determine other long-lived actinides simultaneously and to quantify239Pu:240Pu ratios. The neutron activation analysis approach was found to be particularly prone to interference especially from uranium nuclides.

The construction and installation of porous cup soil solution samplers and their operation in a saltmarsh is described. Levels of transuranic contamination in the saltmarsh sediments are relatively high (up to 930 kBq m−2 for 241Am and 500 kBq m−2 for 239,240Pu), but levels in the interstitial, or sediment-pore, waters are low, generally in the range 1 to 50 mBqL−1. Both 241Am/239,240Pu ratios and concentrations of dissolved transuranic elements are affected by the degree of inundation of the sampling site. These parameters also vary over time in an apparently unpredictable manner. The Kd values (solid/solution distribution coefficients) for Pu and Am are comparable with those measured by other workers at this site.

Samples of cloud and rain water, soil and vegetation were collected from sites at up to 847 m altitude on Great Dun Fell in northern England during the period 2–6 May 1986. Wet deposition is the dominant depositional process for 103Ru and the caesium isotopes, whilst dry deposition is much more important for 131I. The observed deposition velocity for 131I is 3·4 mm s−1. The patchy distribution of activity is related to the convective nature of the rainfall, and there is evidence of direct deposition of contaminated orographic cloud water at the highest site.

The distribution and behavior of Cs in the soil-plant systems at some upland sites in Northeastern Italy, Scotland, and Norway have been investigated. From the limited range of samples taken, there appears to be no dominant physicochemical control on the plant availability of Cs. The presence of micaceous minerals or illitic clays does not significantly inhibit Cs uptake, either because of recycling in the organic surface horizons or because of clay-organic complex formation. Lower plants (bryophytes and lichens) show the highest Cs accumulation. Of the higher plants, ericaceous species take up Cs more than the others.