ESTIMATION OF SOIL EROSION IN A RESERVOIR WATERSHED USING ^137CS FALLOUT RADIONUCLIDE

Sedimentation from soil erosion is a critical reservoir watershed management issue. Due to the difficulty of field investigations, empirical formulas are commonly used to estimate the soil erosion rate. However, these estimations are often far from accurate. An effective alternative to estimating soil erosion is to analyze the spatial variation of 137Cs inventory in the soil. 137Cs can be adsorbed by the soil and is widely assumed to change its distribution only when disturbed by rainfall and human activities. Thus, 137Cs distributed in soils can be a useful environmental tracer to estimate soil erosion. In this study, the net soil loss estimate is 108,346 t/yr and the gross erosion and net erosion rates are 10.1 and 9 t/ha yr respectively. The sediment delivery ratio is therefore estimated to be 0.9 based on the two erosion rates. Because of the steep hillsides in the watershed, only 10% of the sediment yield stayed in the deposition sites and 90% was transported to the river as the sediment output. Soil erosion estimates from spatial variations of the 137Cs activity in the Baishi river watershed showed satisfactory accuracy when compared to sediment yield data. Using soil 137Cs concentrations is therefore a feasible method for estimating soil loss or deposition in Taiwan. Data sampling, analysis and result of this approach are given in this paper.     

土地利用/覆盖变化对地表蚀积过程的影响——来自137Cs示踪的证据

 土地利用/覆盖变化不仅影响地表137Cs总量的再分配，而且显著影响土壤137Cs剖面分布。坝上地区各类土地利用/覆盖条件下137Cs总量的再分配，显示该区总体处于风蚀环境，其中农田和撂荒地是风蚀最严重的土地利用类型；137Cs剖面分布则指示了土地利用/覆盖变化过程中的土壤风蚀/堆积过程的转换，蚀积过程的转换证实了风蚀重灾区退耕还林还草，特别是退耕还草，对控制土壤风蚀、改善生态环境的重要意义。分析还认为，利用137Cs示踪方法计算土壤风蚀/堆积速率时，应格外注意样地的土地利用/覆盖变化过程。     

Dating recent colluvial sequences with Pb and Cs along an active fault scarp, the Eliki Fault, Gulf of Corinth, Greece

Reliable dating is an essential element of palaeoseismological studies, yet whilst a suite of geochronological methods can now provide late Quaternary age control it remains very difficult to date modern events (i.e., those occurring within the last 150 years). This is significant because the starting point for many palaeoseismological investigations is a modern surface-rupturing event, whose geological effects need to be disentangled in trench stratigraphies from palaeoseismic ruptures. Two dating methods which, in combination, can provide robust dating control in recently deposited sediments are the ²¹⁰Pb and ¹³⁷Cs dating methods. Here, we test the applicability of using ²¹⁰Pb and ¹³⁷Cs to date colluvial sediments exposed in three trenches excavated across an earthquake fault—the Eliki fault, Gulf of Corinth, Greece—which ruptured in an earthquake in 1861. The ²¹⁰Pb and ¹³⁷Cs profiles observed in these colluvial sequences are relatively erratic due to the mixed nature of the sediments, i.e., their deposition in an environment where the supply of slope sediments is driven by seasonal rainfall, causing non-uniform sediment accretion and sediment reworking. In one trench, however, ²¹⁰Pb dating, corroborated by ¹³⁷Cs dating, indicates that a proposed post-1861 surface colluvial unit has been deposited over the period 1950 AD–present (at a rate of ca. 9 mm/year), and overlies a significantly older unit (>120 years old). The dating control provided here by ²¹⁰Pb and ¹³⁷Cs dating corroborates the published interpretation of the trench stratigraphy, and refines the ¹⁴C-based estimated dates for the upper unit. At two other trenches ²¹⁰Pb and ¹³⁷Cs dating only provided minimum ages (based on the presence or absence of ²¹⁰Pb_excess and ¹³⁷Cs). Such approximate ages, however, may still useful in corroborating interpretations made using the trench stratigraphy, or, at sites which have long earthquake recurrence intervals, determining which earthquake event was responsible for a particular bed offset.     

Variability in soil redistribution in the northern Chihuahuan Desert based on Cesium measurements

A hypothesis for understanding the stability of northern Chihuahuan Desert landscapes is that the distribution of soil resources changes from spatially homogeneous in arid grasslands to spatially heterogeneous in invading shrublands. Since radioactive fallout ¹³⁷Cesium (¹³⁷Cs) was deposited uniformly across the landscape during the 1950s and 1960s and was quickly adsorbed to soil particles, any redistribution of ¹³⁷Cs across the landscape would be due to soil redistribution or instability at either plant-interspaces or on a landscape scale. The concentration of ¹³⁷Cs in soils collected from different vegetation communities (black grama grass, tarbush, tobosa grass, and mesquite) at the USDA-ARS Jornada Experimental Range in the Northern Chihuahuan Desert in New Mexico was determined. At the black grama grass and tobosa grass sites, ¹³⁷Cs was uniformly distributed at the plant interspace scale. At the mesquite sites, ¹³⁷Cs was concentrated in the dune area under mesquite shrubs with little to no ¹³⁷Cs in the interdune areas. ¹³⁷Cs data support the hypothesis that significant soil redistribution has occurred at dune sites created by invading mesquite. In the arid grassland-shrub sites with black grama grass, tobosa grass, and tarbush the ¹³⁷Cs data support the hypothesis of spatially homogeneous distribution of soil resources. High concentrations of ¹³⁷Cs in the biological soil crusts (0–5 mm) at the tarbush sites indicate that biological soil crusts can contribute to the stability of these sites.     

Environmental Change and Atmospheric Contamination on Svalbard: Sediment Chronology

Sediment cores collected from eight lakes along the western coast of Svalbard as part of a project investigating atmospheric pollution and environmental change in Arctic regions were dated radiometrically using natural (²¹⁰Pb) and artificial (¹³⁷Cs and ²⁴¹Am) fallout radionuclides. At four sites the sedimentation rates were relatively uniform, and in consequence the ²¹⁰Pb dates were relatively unambiguous. At the remaining sites there were irregularities in the ²¹⁰Pb activity versus depth profiles, indicating significant variations in the net sediment accumulation rate during the past 120 years. At these sites, there were significant differences between ²¹⁰Pb dates calculated using the two standard simple dating models, constant rate of supply (CRS) and constant initial concentration (CIC). In most cases, stratigraphic dates based on the ¹³⁷Cs and ²⁴¹Am records supported use of the CRS model, though at one site (Ossian Sarsfjellet) the CIC model appeared more appropriate. The irregularities in the ²¹⁰Pb records were mainly caused by episodes of accelerated sedimentation due, for example, to inwash or slump events, though at some sites there appears to have been a systematic increase in sediment accumulation rates in recent decades. Sediment accumulation rates were generally lower at the northern sites, and higher at the more southerly locations. Mean sediment accumulation rates varied by an order of magnitude, ranging from 0.002 to 0.050 g cm⁻² y⁻¹ (0.02–0.10 cm y⁻¹). ²¹⁰Pb fluxes measured from the core inventories were mostly in the range 34–80 Bq m⁻² y⁻¹ typical of Arctic sites. Much higher values, recorded at two sites (Birgervatnet and Daltjørna), may be due to significant inputs from the catchments during spring thaw.     

Sediment and trace element depositional history from the Ajkwa River estuarine mangroves of Irian Jaya (West Papua), Indonesia

Radiotracer ²¹⁰Pb and contaminant copper were used to estimate sediment accumulation rates in 4 cores from the Ajkwa River estuary and mangrove tidal channels in western Irian Jaya. Mass accumulation rates (4.5–13 kg dry wt m⁻² yr⁻¹) were within the envelope of expectations for a region of high rainfall, great river catchment relief, and rapid tectonic uplift of mountains. Copper accumulation rates were enhanced 40 fold in surface sediments, compared to pre-1950 sections of the sediment cores. These recent sediments with enhanced copper concentrations come from Freeport Indonesia mine tailings over the last 27 years. Variations in sediment core profiles of Al, Fe, and organic carbon were small, indicating no great change in bulk sediment composition. Sulfur concentrations decline toward the sediment surface, suggesting a decline in rates of microbial sulfate reduction. Enhanced sediment copper concentrations will be a useful tracer of sediment dispersal from the Ajkwa River estuary along this coast.     

Rapid migration of heavy metals and <Superscript>137</Superscript>Cs in alluvial sediments,Upper Odra River valley,Poland

The studies presented explore post-depositional changes of zinc, cadmium, lead, manganese and ¹³⁷Cs distribution in alluvial sediments accumulated in the upper Odra River valley in southern Poland. The rate of these changes was estimated by comparing metal and ¹³⁷Cs distributions in four vertical alluvial profiles with a history of river pollution and sediment deposition. The untypical ¹³⁷Cs distribution with peaks in the surface 40–60 cm and lower down, even at a depth of 2.5 m in strata deposited before the beginning of nuclear tests in 1954, indicates rapid post-depositional migration of this isotope from the surface and its retention in lower, less permeable layers. Moreover, the highest concentrations of lead, zinc and cadmium were found at a depth of 4 m in sediments accumulated in the mid-nineteenth century in spite of the growth of industrialization and the pollution of the Odra River with heavy metals until the end of the twentieth century. The post-depositional changes in heavy metals and ¹³⁷Cs are rapid in comparison with the slow element migration usually observed in uninundated soils. This difference is explained by the frequent and easy infiltration of polluted river water into the gravelly and sandy sediments present in the profiles.     

北部湾防城港沿岸土壤137Cs背景值及表层分布特征

在北部湾防城港沿岸采集土壤样品,通过测量样品¹³⁷Cs比活度及有机质含量,确定了¹³⁷Cs背景值,分析表层土壤中¹³⁷Cs分布特征,并探讨样品中¹³⁷Cs活度与有机质含量的关系。结果表明：研究区土壤中¹³⁷Cs背景值为626±15 Bq/m²,自然因素和人为活动对¹³⁷Cs在海岸带表层土壤中的再分配起到重要作用,不同表层样中¹³⁷Cs比活度高低表现为：自然林地>水稻田>旱田>草地>河口海湾。把研究区土壤表层样及P01剖面样中¹³⁷Cs含量与有机质含量进行相关性分析,结果表明：两者相关性显著,相关系数分别为0.414和0.732。通过探讨研究区¹³⁷Cs背景值及沿岸表层土壤¹³⁷Cs分布特征,可为进一步定量研究北部湾沿岸土壤侵蚀和堆积状况以及评价防城港红沙核电站运行后对环境的影响提供科学依据。     

Quantifying sheet wash erosion rates in a mountainous semi‐arid basin using environmental radionuclides and a stream power model

Erosion rates and processes define how mountainous landscapes evolve. This study determines the range of erosion rates in a semi‐arid landscape over decadal time spans and defines the dominant processes controlling variability in erosion rates. The varying topography and climatic regimes of the Xiying Basin (Qilian Shan Mountains, China) enables us to examine the relative roles of sheet wash versus rainsplash and the influence of vegetation on soil erosion and deposition. Soil erosion rates since 1954 were determined using ¹³⁷Cs along 21 transects at four sites with varying gradient, rainfall, and vegetation cover. The mean ¹³⁷Cs derived soil erosion rate ~0.42 mm/a was consistent with the catchment level erosion rate derived from total sediment yield for a 44 year record. However, there is considerable variability in ¹³⁷Cs erosion rates both between transects and along transects, perhaps reflecting variation not only in the effectiveness of individual processes but also in their relative roles. We compare the ¹³⁷Cs‐derived erosion rates with 1‐D models for sediment flux that incorporate sheet wash and rainsplash processes, testing them over a previously untested 60 year timescale. The variability in ¹³⁷Cs erosion rates along transects is best replicated by sheet wash dominated simulations, suggesting that this is the dominant erosion process in this semi‐arid landscape. The functional form of the sheetwash model can also explain our observations that ¹³⁷Cs erosion rates decrease with upslope length (i.e. distance down slope) while its variability increases. However, sparsely vegetated sites, located in slightly drier locations, have higher erosion rates, and are not as accurately modeled as densely vegetated sites, suggesting that patchiness of vegetation introduces fine scale variability in erosion rates on these slopes. Copyright © 2015 John Wiley & Sons, Ltd.