Fallout Isotopes Research Articles

Determining sediment deposition dynamics influenced by check dams in a semi‐arid mountainous watershed

AbstractSemi‐arid environments are characterized by infrequent large magnitude rainfalls that produce flash flood events with high sediment concentration. Control structures such as check dams are widely used in this environment for mitigation. However, their impact on the overall sediment balance of watersheds, particularly those severely affected by anthropogenic activity, is sparsely documented. This study used topographic measurements, sediment analysis, and fallout isotope techniques to assess the effectiveness and service life of 18 rock and masonry check dams that were constructed in the 1930s for controlling sediment fluxes in an 11 ha mountainous watershed in southern Arizona. All of the dams are currently filled with sediment resulting in reduction of local channel gradients by 35 to 71% to between 0.04 and 0.28 depending on location within the reach. Sedimentation occurred over multiple decades at a relatively slow average rate of 0.59 t ha−1 y−1 indicating low instantaneous retention efficiency. The smaller headwater dams were filled soon after construction; however, their share of the overall storage capacity was minor. Although evaluation of 137Cs was an effective method for dating sediment, the 210Pb dating method was not satisfactory because of sediment sorting effects and complex deposition patterns. The abundance of similar control structures in the region points to the opportunity to better understand the process impacts of check dams over multiple decades to inform planning and design of their use in future mitigation projects.

Erosion of archaeological sites: Quantifying the threat using optically stimulated luminescence and fallout isotopes

AbstractAlthough visible evidence shows that erosion has damaged many archaeological sites, especially when tilled, there has hitherto been scant attention to its quantitative assessment. Accordingly, the archaeology communities lack insight into whether long‐term threats to the stability and integrity of soils at these sites allow these cultural repositories to be preserved for future human generations. Of the techniques that are available to measure erosion rates, few have been tested on the timescales needed. We selected three archaeological sites with high expected erosion rates. We combined optically stimulated luminescence (OSL) dating with analyses of radioactive fallout isotope distributions to assess erosion patterns and rates. An age–depth representation of OSL single‐aliquot results was developed to determine past erosion, and to identify stable land surfaces on centennial to millennia timescales. Fall‐out isotopes of cesium (Cs) and plutonium (Pu) were suitable for shorter timescales: The 240Pu/239Pu ratios and a correlation between activities of 239+240Pu and 137Cs demonstrated the weapons testing fallout origin of these isotopes in the ~1952–1966 timeframe. Erosion rates in recent decades ranged from 2 to 6 mm/year on the studied sites. Our results indicate that erosion is not only tied to the past, but keeps on threatening archaeological sites.

Fallout isotope chronology of the near-surface sediment record of Lake Bolătău

Fallout isotope (210Pbex,137Cs and 241Am) based dating has been carried out on the near-surface sediment core collected from Lake Bolătău-Feredeu (Bukovina, Romania). The motivation was to improve the chronology of this recent section in connection with significant fluctuations observed in sediment accumulation rates, particle size distribution and primordial radioisotope (i.e. 40K and 232Th) composition. Previously only an extrapolation of a broad-range OxCal age-depth model, which was based on 8 AMS radiocarbon dates from the deeper part of a parallel sediment sequence and tentatively validated for the upper part using the double peaks of the 137Cs activity concentration distribution, was available for the studied section (1–24 cm). Parallel to the previous 137Cs measurement, 210Pb and 226Ra (for a more detailed, 210Pbex-based chronology), 241Am (for an additional time-marker), as well as 40K and 232Th concentrations have also been determined by gamma-spectrometry. In case of the 210Pbex-based chronology, due to a large deviation from a pure exponential distribution, the Constant Flux (CF) model has been used for the calculation of sediment ages and accumulation rates. Although the broad-range OxCal and the CF model were broadly similar down to 22 cm, the 210Pbex-based ages are clearly superior in terms of uncertainty in the uppermost 12 cm, while the broad-range model has smaller uncertainty below 20 cm (>150 years). The CF model gave an average mass accumulation rate of (0.08 ± 0.03) g cm−2 yr−1 for sections 0–11 cm, and (0.03 ± 0.01) g cm−2 yr−1 for sections 12–22 cm, respectively. Significant changes have been observed in the depth distribution of both the particle size distribution and the elemental/isotopic composition of the sediment record, most likely related to the variation observable in the intensity and volume of precipitation in the catchment. The obtained high-resolution records of Lake Bolătău, including multiple radioisotopes, can serve as a regional benchmark for similar studies.

Atmospheric fallout radionuclides in peatland from Southern Poland

Two peat profiles were collected in a peat bog located in Southern Poland and their geochronology were determined using 210Pb, 238,239+240Pu and 137Cs radiometric techniques. The 210Pb chronologies were established using the constant rate of supply model (CRS) and are in good agreement with the Pu isotopes and 137Cs time markers. Maximum activities of Pu isotopes were found at a depth corresponding to the early 1960s, which is the period characterized by the maximum nuclear weapon tests. The results showed that the 210Pb method is the most accurate technique for the determination age and accumulation rate of a peat. The next part of this study calculated linear accumulation rates by analyzing 238,239+240Pu and 137Cs vertical distributions in the profiles. Activities of fallout isotopes were also measured in plants covering the peatland. The highest activities of 137Cs and 210Pb were found in Calluna vulgaris samples, and 239+240Pu were found only in two samples (C. vulgaris and leaves of Oxycoccus quadripelatus).

Cause–effect–response chains linking source identification of eroded sediments, loss of aquatic ecosystem integrity and management options in a steppe river catchment (Kharaa, Mongolia)

Although sparsely populated, the progressive degradation of Mongolia’s rivers, lakes and groundwater, driven by land-use changes, poses a key challenge for the future sustainable development of the country. This paper deciphers the cause–effect–response chain between river bank degradation, changes of the ecological status, declines of ecosystem functions and priority measures with the case of the Kharaa River in Northern Mongolia. The underlying research approach comprised: (1) hydromorphological characterisation of the Kharaa River, (2) water quality assessments, (3) determination of the riverbed composition including hyporheic zone properties, (4) the analysis of riverine biota (macroinvertebrates and primary producers) and (5) the identification of the sources of suspended and settled sediments. The assessment revealed a gradient of spatially heterogeneous river bank erosion due to the degradation of the riparian vegetation caused by overgrazing and wood utilization. As the most prominent ecological response, the biomass of benthic algae decreased and macrozoobenthic community metrics changed continuously along the pressure gradient, accompanied by shifts of habitat related functional traits. At the same time, the hyporheic zone dimensions and functioning were affected by suspended and infiltrated sediments in multiple ways (restricted spatial extent, lowered hydraulic connectivity, lower metabolism, ecologically critical quality of pore water). Geochemical and radionuclide fallout isotope fingerprinting has identified riverbank erosion as the main source of the suspended sediments in the Kharaa River, when compared to gully and land surface erosion. Erosion susceptibility calculations in combination with suspended sediment observations showed a strong seasonal and annual variability of sediment input and instream transport, and a strong connection of erosional behaviour with land-use. Amongst others, the protection of headwaters and the stabilization of the river bank erosion hotspots in the mid-stream sections of the Kharaa River are the priority measures to avoid further degradation of the aquatic ecosystem status and functions.

Beryllium-7 wet deposition variation with storm height, synoptic classification, and tree canopy state in the mid-Atlantic USA

Short-lived fallout isotopes, such as beryllium-7 (7Be), are increasingly used as erosion and sediment tracers in watersheds. 7Be is produced in the atmosphere and delivered to the Earth's surface primarily in precipitation. However, relatively little has been published about the variation in 7Be wet deposition caused by storm type and vegetation cover. Our analysis of precipitation, throughfall, and sediments in two forested, headwater catchments in the mid-Atlantic USA indicates significant variation in isotope deposition with storm type and storm height. Individual summer convective thunderstorms were associated with 7Be activity concentrations up to 5.0 Bq l−1 in precipitation and 4.7 Bq l−1 in throughfall, while single-event wet depositional fluxes reached 168 Bq m−2 in precipitation and 103 Bq m−2 in throughfall. Storms originating from the continental USA were associated with lower 7Be activity concentrations and single-event wet depositional fluxes for precipitation (0.7–1.2 Bq l−1 and 15.8–65.0 Bq m−2) and throughfall (0.1–0.3 Bq l−1 and 13.5–98.9 Bq m−2). Tropical systems had relatively low activity concentrations, 0.2–0.5 Bq l−1 in precipitation and 0.2–1.0 Bq l−1 in throughfall, but relatively high single-event depositional fluxes due to large rainfall volumes, 32.8–67.6 Bq m−2 in precipitation and 25.7–134 Bq m−2 in throughfall. The largest sources of 7Be depositional variation were attributed to storm characteristics including precipitation amount and maximum storm height. 7Be activity associated with fluvial suspended sediments also exhibited the highest concentration and variability in summer (175–1450 Bq kg−1). We conclude the dominant source of variation on event-level 7Be deposition is storm type. Our results illustrate the complex relationships between 7Be deposition in precipitation and throughfall and demonstrate event-scale relationships between the 7Be in precipitation and on suspended sediment. Copyright © 2015 John Wiley & Sons, Ltd.

Measurements of Fission Products from the Fukushima Daiichi Incident in San Francisco Bay Area Air Filters, Automobile Filters, Rainwater, and Food

A variety of environmental media were analyzed for fallout radionuclides resulting from the Fukushima nuclear accident by the Low Background Facility (LBF) at the Lawrence Berkeley National Laboratory (LBNL) in Berkeley, CA. Monitoring activities in air and rainwater began soon after the onset of the March 11, 2011 tsunami and are reported here through the end of 2012. Observed fallout isotopes include 131I, 132I, 132Te, 134Cs, 136Cs, and 137Cs. Isotopes were measured on environmental air filters, automobile filters, and in rainwater. An additional analysis of rainwater in search of 90Sr is also presented. Last, a series of food measurements conducted in September of 2013 are included due to extended media concerns of 134,137Cs in fish. Similar measurements of fallout from the Chernobyl disaster at LBNL, previously unpublished publicly, are also presented here as a comparison with the Fukushima incident. All measurements presented also include natural radionuclides found in the environment to provide a basis for comparison.

Cryoturbation in the Central Brooks Range, Alaska

Cryoturbation in permafrost-affected soils is important to many Arctic biogeochemical processes and critical to the appropriate classification of Gelisols. Standardized methodologies for describing the soils formed from these processes and knowledge regarding the time scales at which they operate continue to evolve. Twenty-six profiles were described across a transect in the Midas Lake region of the Central Brooks Range of Alaska using a modified version of the standard USDA and Turbel description protocol that is appropriate for work in wilderness areas. Profile descriptions and associated field data were aggregated and generalized to reveal major trends and relationships in horizonation between cryoturbated and noncryoturbated soils on the transect. A conceptual model for the geomorphic relationships of cryoturbated and noncryoturbated soils on this landscape was developed. In addition to a landscape-scale perspective, individual profiles were described on patterned ground features at a location where cryoturbated horizons were strongly expressed. Samples from these profiles were analyzed for cesium-137 (137Cs) activity, and it was found that the surficial activity of this fallout radioisotope varied strongly by material type and position on the patterned ground microtopography. These differences suggest that the use of fallout isotopes, such as 137Cs, when combined with quantitative profile descriptions and a standardized description protocol, may significantly improve our understanding of the spatial distribution and mechanisms of cryoturbation.

Actinides, accelerators and erosion

Fallout isotopes can be used as artificial tracers of soil erosion and sediment accumulation. The most commonly used isotope to date has been 137 Cs. Concentrations of 137 Cs are, however, significantly lower in the Southern Hemisphere, and furthermore have now declined to 35% of original values due to radioactive decay. As a consequence the future utility of 137 Cs is limited in Australia, with many erosion applications becoming untenable within the next 20 years, and there is a need to replace it with another tracer. Plutonium could fill this role, and has the advantages that there were six times as many atoms of Pu as of 137 Cs in fallout, and any loss to decay has been negligible due to the long half-lives of the plutonium isotopes. Uranium-236 is another long-lived fallout isotope with significant potential for exploitation as a tracer of soil and sediment movement. Uranium is expected to be more mobile in soils than plutonium (or caesium), and hence the 236 U/Pu ratio will vary with soil depth, and so could provide an independent measure of the amount of soil loss. In this paper we discuss accelerator based ultra-sensitive measurements of plutonium and 236 U isotopes and their advantages over 137 Cs as tracers of soil erosion and sediment movement.

Fukushima fallout in Northwest German environmental media

Traces of short- and long-lived fallout isotopes ( 131I, 134Cs and 137Cs) were found in environmental samples collected in Northwest Germany (rain water, river sediment, soil, grass and cow milk) from March to May 2011, following the radioactivity releases after the nuclear accident in Fukushima, Japan. The measured concentrations are consistent with reported concentrations in air, amount of rainfall and expected values applying simple radioecological models. The [ 134Cs]/[ 137Cs] ratio reported for air (about 1) allows for discrimination between “recent” and “old” 137Cs. Expected 136Cs values fell below the detection limits of the instrumentation, despite large sample masses and long counting times.

Traces of Fukushima fallout in the environment of Northwest Germany

Traces of short- and long-lived fallout isotopes ( 131 I, 134 Cs and 137 Cs) were found in environmental samples obtained in northwest Germany (river sediment, rainwater, grass and milk) from March to May 2011, following the radioactivity releases after the nuclear accident in Fukushima, Japan. The found concentration values are consistent with reported concentrations in air, amount of rainfall and expected values applying simple radioecological models. The [ 134 Cs]/[ 137 Cs] ratio reported for air (about 1:1) allows for discrimination between “recent” and “old” 137 Cs. Expected 136 Cs values fell below the detection limits of the instrumentation, despite large sample masses and long counting times.

Impacts of Dredging Activities on the Accumulation of Dioxins in Surface Sediments of the Houston Ship Channel, Texas

YEAGER, K.M.; BRINKMEYER, R.; RAKOCINSKI, C.F.; SCHINDLER, K.J., and SANTSCHI, P.H., 2010. Impacts of dredging activities on the accumulation of dioxins in surface sediments of the Houston Ship Channel, Texas. Journal of Coastal Research, 26(4), 743–752. West Palm Beach (Florida), ISSN 0749-0208. The Houston Ship Channel (HSC) and upper Galveston Bay (GB), Texas, are known to be contaminated with dioxins (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans), the majority of which are associated with sediments. Since 1914, dredging operations to establish, sporadically expand, and consistently maintain a navigable channel for large ships has been and remains continuous here. The objectives of this research focus on determining if dredging activities have any significant impact on the quantities of dioxins associated with surface sediments in the HSC and GB. Four transects were sampled, located on the dredged and undredged sides of two dredge-spoil islands. Sediment samples were characterized in terms of their organic carbon contents, grain size fractions, indicator dioxin concentrations (2,3,7,8-tetrachlorinated dibenzo-p-dioxin [TCDD], 2,3,7,8-tetrachlorodibenzofuran [TCDF], and toxic equivalents), and fallout radionuclide activities. The physical and geochemical data were examined using a combination of principal components analysis and one-way analysis of variance. Results of the statistical tests show that (1) sedimentary dioxin concentrations are significantly higher adjacent to the northernmost dredge-spoil island, which is located closer to a recently identified dioxin point source (San Jacinto waste pits); and (2) while mean sedimentary dioxin concentrations were slightly higher for undredged as compared to dredged transect samples as a whole, these differences were not significant (p . 0.05). Mean fallout isotope ratio values ( 7

Plutonium as a tracer of soil and sediment movement in the Herbert River, Australia

Plutonium fallout from atmospheric nuclear-weapons testing in the 1950s and 1960s constitutes an artificial tracer suitable for the study of recent soil erosion and sediment accumulation rates. Traditionally 137Cs has been the fallout isotope of choice for such studies, but the plutonium isotopes confer a number of advantages, which can be realised using the ultra-sensitive detection technique of Accelerator Mass Spectrometry (AMS). As a first application of plutonium to a whole-of-basin study, Pu has been measured in both soil and sediment across the catchment of the Herbert River, which is one of the major rivers draining into Australia’s Great Barrier Reef Lagoon. Its catchment includes undisturbed areas as well as regions of pasture and sugar cultivation. The Pu measurements allow the relative contributions of surface and gully erosion from the different land use areas to be determined, and permit the discharged material to be apportioned between the relevant sources.

Quantitative assessment of effectiveness of soil conservation measures using a combination of 137Cs radioactive tracer and conventional techniques

Abstract Quantitative assessment of soil redistribution in landscapes remains a challenging task. In this study we used radioactive soil redistribution tracer 137Cs together with soil morphological characteristics and empirically-based modeling for quantitative assessment of long-term soil conservation effectiveness. Three pairs of arable slopes were selected, all located within the territory of the Novosil experimental station (the Orel Region, central European Russia). One slope in each pair undergone creation of artificial terraces with forest shelter belts located parallel to topography contour lines and spaced at approximately 100 m from each other. Preliminary results have shown that slopes with soil-protective measures are characterized by a 11–80% reduction of average soil redistribution rates, as shown by soil profile morphology and 137Cs methods. Discrepancy in values obtained can be attributed to differences in temporal resolution of methods as well as possible influence of individual extreme events on results yielded by the 137Cs method. On the other hand, more significant decrease in average soil degradation rates on slopes with soil conservation (62–75% for each pair of slopes) was predicted by the model. The 137Cs method overestimates gross and net soil redistribution rates, as a result of the influence of extreme erosion prior to tillage mixing of a fresh fallout isotope, not accounted for by calibration models used. Another shortcoming of the estimations obtained is that sediment redeposition directly within forest belts was not taken into account. Therefore, net erosion rates obtained for slopes with forest belts should be regarded as overestimation. Nevertheless, it can be generally concluded that the multi-technical approach has allowed acquiring much more detailed information on temporal and spatial variability of soil redistribution rates than single method-based studies.

Correlation relations for estimating the content of plutonium isotopes in fallout due to the Chernobyl accident

The possibility of determining the total α activity A(239,240Pu) of 239Pu + 240Pu according to the γ radiation of 103Ru and 106Ru is examined. The statistical properties of the experimental ratios of the activities ρ = A(106Ru)/A(103Ru) are investigated. The distribution of the experimental ratios is similar to the theoretical ratios for fuel and close to a Gaussian distribution with average value 〈p〉 = 0.22 and standard deviation Δ ∼ 0.06. This ratio is used to obtain a relation for estimating A(239,240Pu) from the γ activity A(103Ru). A theoretical analysis is made of the experimental data for the southern raions of Gomel oblast. It is shown that given a good statistical sample of measurements of the γ activity of 103Ru it is possible to estimate the average value of the γ activity A(239,240Pu) and with probability 0.95 the interval of the possible values of this activity.

The relative importance of terrestrial versus marine sediment sources to the Nueces-Corpus Christi Estuary, Texas: An isotopic approach

Determining sources of sediment to coastal systems is an important and complex problem that figures prominently in a myriad of geological, geomorphological, geochemical, and biological processes. Lithogenic (226Ra,228Ra,228Th,230Th,232Th) and fallout (137Cs,210Pb) isotopes were employed in conjunction with sedimentological methods to determine rates of sedimentation in the Nueces Delta and Nueces-Corpus Christi Estuary and to assess the relative importance of marine versus terrestrial sediment sources to the estuary. Similarity of lithogenic isotope ratios in surface sediments throughout the system precluded a numeric approach to discerning the importance of each of the two large scale sediment sources (terrestrial and marine). A stepwise, graphical examination of discrete lithogenic isotope activity concentrations shows more promise. Terrestrial, marine, and bay sediment means for226Ra versus232Th,226Ra versus230Th, and228Ra versus232Th show that terrestrial and marine sediment sources have different signatures, despite having similar grain size distributions (sands), and that sediment deposited in Nueces and Corpus Christi Bays are indistinguishable from the terrestrial component. Supporting evidence is provided by thorium isotopes,230Th versus232Th,228Th versus232Th, and228Th versus230Th. Nueces Delta sedimentation (0.09–0.53 g cm−2 yr−1) shows a subtle gradient, with rates generally lower in the west and progressively higher moving east, likely reflecting contrasts in land use and topography. Nueces Bay cores differ from those in Corpus Christi Bay in that sands comprise a larger percentage of their composition, and they are mixed over greater depth, most likely due to geographic and physiographic effects. Sediment accumulation rates consistently decrease over the first 20 km from the Nueces River and become constant after that, implying that the river is the most significant source of sediment to the estuary. The interpretation of sediment supply to this estuary as dominated by terrestrial inputs is based on three complimentary sets of data: sediment grain size distributions, discrete lithogenic isotope data (Ra versus Th and Th versus Th), and sediment accumulation rates for both Nueces and Corpus Christi Bays.

Dating recent peat deposits

Dating recent peat deposits (i.e., past } 300 yrs of peat accumulation) has emerged as an important yet challenging task for estimating rates of organic matter accumulation and atmospheric pollutant deposition in peatlands. Due to their ombrotrophic nature and the tendency for Sphagnum-derived peat to have high cation exchange capacity, peatlands are ideal archives of atmospheric pollution. However, efforts to establish depth-age relationships in peats are complicated by the difficulty of dating deposits reliably. Assumptions underlying the techniques available for dating peat deposits often are poorly understood and generally untested. We outline the approaches used to establish depth-age relationships in peat chronologies, including brief descriptions of the theory, assumptions, methodology, and logistics of each technique. We include both continuous dating methods (i.e., methods based on 14C, 210Pb, constant bulk density, acidinsoluble ash, moss increment, pollen density) and chrono-stratigraphic markers (i.e., fallout isotopes from the Chernobyl accident and nuclear weapons testing, pollen stratigraphies, isothermal remanence magnetism, charcoal particles, spherical carbonaceous particles, PAHs, PCBs, DDT, toxaphene) that can be measured in peat and correlated temporally with known historical events. We also describe the relatively new radiocarbon application of wiggle matching and use hypothetical data to highlight the potential of this developing technique for dating recent peat. Until the uncertainty associated with each of these dating approaches is clarified, we recommend employing multiple techniques to allow for corroboration between different methods.

Thyroid carcinoma after Chernobyl latent period, morphology and aggressiveness.

The large numbers of papillary thyroid carcinomas that have occurred in those exposed to high levels of short-lived isotopes in fallout after Chernobyl provide a unique opportunity to correlate latency and tumour biology. We show that short latency is associated with tumours with a phenotype that is significantly less structurally differentiated, shows significantly less peritumour fibrosis, and significantly more invasive spread when compared to tumours with a longer latent period. In contrast, the type of differentiation (papillary or follicular architecture) is associated with age at exposure. These findings suggest that the initial mutation at the time of exposure played a major role in tumour latency and aggressiveness. We and others have shown that RET-PTC3 rearrangements are associated with the solid morphology seen in these short latency tumours, while classical papillary carcinomas more often show RET-PTC1 rearrangements. Studies in transgenic mice show similar findings, and in vitro studies show that RET-PTC3 induces more rapid growth than RET-PTC1. We therefore suggest that the solid morphology, high frequency of RET-PTC3 rearrangements and aggressive behaviour noted in early investigations of post-Chernobyl tumours were characteristic of short latency rather than the nature of the mutagen, and that successive overlapping waves of papillary carcinoma with differing latency, differing patterns of mutations and differing clinical behaviour are occurring in those exposed to Chernobyl fallout.

The fallout isotope 207Bi in a Delaware salt marsh: a comparison with 210Pb and 137Cs as a geochronological tool

We report the presence of the fallout isotope 207Bi (half-life = 38 years) in salt marsh sediments collected from Wolf Glade Salt Marsh, Lewes, Delaware and evaluate its usefulness as a transient tracer for geochronology. The commonly used geochronological tracers of salt marshes. 210Pb and 137Cs, were also measured to corroborate the results. The average sedimentation rate based on a constant excess 210Pb flux model in three cores (2 plus 1 duplicate) is estimated to be 3.2 ± 0.1 mm/year. Based on 137Cs and 207Bi data and an assumption that the fallout activities of these nuclides peaked during 1963, average rates are 3.5 ± 0.7 and 3.7 ± 0.2 mm/year, respectively. The scatter on the down-core 210Pb activity is relatively large which may be ascribed to variations in the atmospheric deposition rate of 210Pb, core compaction, or sedimentation. The 137Cs is present in sediments older than 50 years and suggests that these sedimentation rates may be compromised by the post-depositional mobility of 137Cs in salt marsh environments. The presence of 207Bi is detected at similar depths in all three cores which is consistent with a pulsed input of this nuclide into the environment from nuclear detonations and a post-depositional immobility. A higher degree of reliability in sedimentation rate estimates may thus be obtained using 207Bi. Based on the measured flux of 207Bi into salt marsh and an assumed inventory ratio of 207 Bi 137 Cs , the total production of 207Bi from nuclear weapon testings is estimated to be on the order of 10 15 Bq.

The Distribution of Caesium-137 in Rangeland Soils at Three Sites in Western Australia.

Soil sampling for the fallout isotope caesium-137 (137Cs) was canied out on three pastoral properties in Western Australia to assess its suitability for estimating soil erosion status. The sites were situated east and north-east of Geraldton in a region receiving an annual rainfall of approximately 200 rnm. It was hypothesised that 137Cs levels would be lower outside Department of Agriculture exclosures, established in the early 1950s, because of higher rates of soil erosion due to pastoral activities. The exclosures are areas of fenced-off rangeland which have the purpose of excludin'g in11 herbivores. It was further hypothesised that 137Cs levels would be related to microtopography in this rangeland-plains landscape. Soil erosion and deposition have given rise to scalds and soil mounds under shrubs, respectively, and amounts of 137Cs at these sites could reflect these processes. The Mann-Whitney U test showed that there were no significant differences in 137Cs levels between samples collected inside and outside the exclosures. At one property there was a significant relationship between 137Cs and microtopography (U test: sig. 0.025 level), suggesting that soil erosion was more severe on open and scald sites than under shrubs. No detectable 137Cs was found at 23% of all sites, but there was evidence that localized deposition of sediments had occurred. Because the total number of samples used in this survey was small, further work will be required to confirm the utility of the 13'Cs technique for measuring erosion status in arid Australia.