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a Centre for Soil and Environmental Research, Jordforsk, N-1432 Aas, Norway
b Dep. of Chemistry and Biotechnology, Isotope Lab., Agricultural Univ. of Norway, N-1432 Aas, Norway
* Corresponding authors (nina.syversen{at}jordforsk.no, lillian.oygarden{at}jordforsk.no)
Received for publication July 5, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: cpm, counts per minute • SS, suspended solids
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Under Nordic soil and weather conditions, runoff and erosion are documented to be highest during winter and especially during the snowmelt periods (Njøs and Hove, 1986; Lundekvam, 1993, 1997; Øygarden, 2000). In these periods, erosion is caused by surface runoff from melting water and not from rainfall and raindrop detachment. As seasonal variations in both runoff and soil losses are high under Nordic climatic conditions, field experiments documenting the processes of erosion and sedimentation occurring during rainfall events and the snowmelt period are essential.
Once particles are entrained by the flow, processes leading to sedimentation begin. Reduction in slope and different kinds of barriers such as vegetative buffer zones may reduce runoff velocity and therefore enhance sedimentation of particles. Size, shape, and density of the particles are factors influencing the settling velocity. Aggregates have a higher settling velocity than primary particles (Haan et al., 1994).
In hillslope and field studies, monitoring stations located by the outlet of the field measure the total runoff and total soil losses that are leaving the field. This method gives no documentation of where erosion and sedimentation processes take place within a field. Documentation of these processes is essential for identifying efficient means that can reduce soil losses and transport of particle-bound components, such as fertilizers and pesticides, from a field.
In buffer zone studies, differences in particle input and output are often measured in situ (Dillaha et al., 1989; Magette et al., 1989). However, information on where the particles are eroded from and where in a buffer zone they is deposited are scarce. This may partly be due to lack of available methods to trace the particle sedimentation. Such information is needed when recommendations with respect to the design of the buffer zones are given.
Tracers have been widely used to trace the transport of sediments in river channels (Hubbel and Sayre, 1964). The use of tracers in hillslope research is, however, relatively scarce. Large, labeled particles have been used by Poesen (1987), Torri and Poesen (1988), and Wainwright and Thornes (1991). Parsons et al. (1993) claimed that these tracers are of limited value because the applied particles were larger than the particle sizes normally being eroded. They have used bulk tracing with magnetite to trace sediment movement in interrill overland flow in a hillslope.
Cesium-137 from fallout has been used as bulk tracer in hillslope studies (McHenry and Ritchie, 1977, p. 26–33; McHenry and Bubenzer, 1985; Spomer et al., 1985; Walling and Bradley, 1988; Walling and Quine, 1991). Cesium is strongly adsorbed on soil particle surfaces, especially to clays and organic particles (Tamura, 1964; Brisbin et al., 1974; Squire and Middleton, 1966; Livens and Rimmer, 1988). Therefore, removal of 137Cs is strongly related to the amount of soil loss (e.g., Ritchie et al., 1974; Martz and DeJong, 1987; Walling et al., 1986). Since the technique is based on the fallout of 137Cs from atmospheric nuclear weapon tests during the 1950s to the 1980s, long-term mean erosion and sedimentation patterns can be followed. However, erosion mechanics during a single event cannot be studied by fallout Cs.
No studies are reported where a whole hillslope is labeled with a tracer. One of the major problems facing such investigations is the lack of existing measurement techniques capable of documenting the deposition, storage, and remobilization of particles within a catchment (Walling et al., 1986; Ritchie and McHenry, 1990).
The purpose of this study is to use 134Cs as a tracer for studying both naturally occurring erosion in a tilled hillslope and sedimentation processes within a buffer zone without disturbing the soil surface. Since it is not present in the soil prior to the study, the changes in 134Cs activity distribution measured in situ can be related to runoff and transport processes occurring after application of the tracer. The tilled area was surface-contaminated by 134Cs2+ ions, which have strong affinity to soil components (especially clay particles) and a short half-life (2.05 yr). By measuring 134Cs activity in situ on the soil surface after major runoff events, information on when and where the particle erosion and sedimentation occurred should be provided. By measuring 134Cs activity in plots, the buffer zone, runoff water, vertical soil cores, and vegetation samples, a 134Cs budget will provide valuable information on erosion and sedimentation processes and the pathways for particle transport.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Buffer Zone
The 5- x 5-m buffer zone was divided into a 0.5- x 1-m grid net, with pins marking every meter. The dominant plants in the buffer zone were tufted hair grass [Deschampsia cespitosa (L.) P. Beauv.], thistle [Cirsium arvense (L.) Scop.], common couch [Elytrigia repens (L.) Desv. ex Nevski], timothy (Phleum pratense L.), and meadow fescue (Festuca pratensis Huds.) (Syversen, 1994). Prior to the experiment, the vegetation was cut to a 5- to 10-cm height to optimize in situ measurements of possibly retained contaminated particles transported from the upper plot.
Application of Tracer and Field Measurement
Application of Tracer
Radioactive 134Cs was used to label surface soil particles in the upper plot. Tracer solution of 134CsCl (half-life time 2.05 yr) with specific activity of 37 x 103 kBq mg-1 Cs (Amersham Int. plc, Little Chalfont, UK) was diluted with water and spread over the 225-m2 tilled area with two-wheel manual spraying equipment. On 26 Oct. 1993, a total of 19.9 x 103 kBq was sprayed over the harrowed area yielding a mean activity level of 88.4 kBq m-2. The contamination level was within the range observed in Norway for areas affected by the Chernobyl fallout.
In Situ Measurement
To follow changes in the 134Cs distribution within the contaminated tilled soil plot and increases in activity levels in the uncontaminated buffer zone plot during the experimental period, in situ NaI detector measurements were performed at each of the 350 marking pins within the grid net. For each in situ measurement run, a map covering the 350 individual measurements could be made. In total, eight in situ measurement runs were performed from October 1993 until May 1996 to follow changes occurring after major runoff periods due to extensive snowmelt or heavy rainfall events.
A portable 3-in (ca. 76 mm) NaI detector (detector surface area = 56.72 cm2) equipped with a Canberra (Meriden, CT) multichannel analyzer (Serie 10 plus 1004, detector diameter = 8.5 cm) was used for in situ measurements. A plastic cover protected the detector from cross-contamination, and measurements were performed with the detector placed directly on the soil surface close to the marking pins. Measurement time was 30 s. To correct for background interference, measurements were performed for every 5 m outside the plots. Uncertainties in the measurements were estimated from the counting statistics.
Calibrations
Changes in 134Cs activity were used to elucidate erosion and sedimentation processes within the contaminated plot and within the buffer zone. Reduction of 134Cs activity is attributed to erosion, while increased 134Cs activity represents sedimentation of contaminated particles. Calculations were based on changes observed from measurements at individual grid points and from changes in the distribution maps obtained from all grid points within the experimental site. By comparing with the original distribution, the changes in 134Cs activity were calculated after each in situ measurement run. All measurements were decay-corrected according to half-life time using the following formula:
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= decay constant, where = ln2/t1/2 and t1/2 = half-life time (2.05 yr); t = time after application, when measurements are performed; Dt = disintegration rate of 134Cs in Bq (1 Bq = 1 disintegration per second, dps) at time t, when the measurement is performed; D0 = disintegration rate of 134Cs in Bq (dps), at the date of application; and f = counting efficiency for the instrument (cpm Bq-1). The counting efficiency, f, is determined in laboratory experiments where a soil sample from the field was packed within a frame (45 x 60 cm), its surface contaminated by 24.7 kBq, and then measured by the NaI detector. The f factor is found to be 0.015 cpm Bq-1. This factor is compared with a calculated f factor due to in situ measurements (applied amount of tracer in Bq m-2 compared with in situ measurements in cpm). To correct for interferences from activity vertically transported in the soil, 1-, 2-, and 3-cm layers were contaminated with three different Cs concentrations in the laboratory. Contamination from soil layers deeper than 1 cm did not significantly interfere with the surface soil 134Cs signals. It is therefore assumed that soil surface measurement reflects the activity of 134Cs in the upper 0- to 1-cm layer only, with no significant contribution from deeper layers.
To calculate changes in the original surface contamination (Bq m-2) from in situ measurements (cpm) at individual points in the grid, the following equation was applied:
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t = decay-corrected in situ measurements (cpm); f = counting efficiency; A factor = area factor = 176.3 (converting the detector area [56.7 cm2] to 1 m2); and N factor = neighbor factor, correcting for interference from areas outside the detector surface area. A laboratory experiment was performed to determine the N factor. Measurements were performed with and without shielding the areas outside the detector area. For points surrounded by contaminated areas, about 40% of the signal could be attributed to the neighboring area. For points along the fence, 10% of the signal was from the neighboring area. Measured grid points have therefore been corrected by factors of 0.6 and 0.9.
Soil Samples
To identify the vertical soil transport of 134Cs, 30 soil cores with a 5-cm depth (5-cm diameter) and 12 cores with a 10-cm depth were collected within the contaminated tilled soil plot during spring 1995. The soil cores were sliced into 1-cm layers, dried at 105°C, weighed, and measured with respect to 134Cs activity. The results were decay-corrected.
The 134Cs activity level in the 0- to 1-cm layer were compared with results from in situ measurements at the beginning and at the end of the experiment. The 134Cs activity level in layers deeper than 1 cm is assumed to represent the vertical transport of the tracer since application.
Vegetation Samples
To identify uptake of 134Cs in the vegetation within the buffer zone, 30 samples were collected in autumn 1995 from the 3.5- and 4.5-m-wide buffer zones. Each vegetation sample was divided into three: one unwashed sample containing particles, one rinsed sample free of particles, and one sample from the root system (rinsed sample, free of particles). The samples were dried at 70°C, crushed, and weighed before measuring 134Cs. Results were decay-corrected.
Water Samples
Surface runoff water from the plot was led through inlet pipes to a tipping bucket system inside a measuring station. The tipping number was registered on a data logger and by a manual counter, and discharges were calculated. Volume-proportional mixed samples were taken after every runoff event or as frequently as one or two times a day during the snowmelt period. For every second tip a portion of water in the tipping bucket was led to a 100-L sampling tank. Excess runoff water was led to a 1000-L storage tank outside the measuring station, and 134Cs activity was measured before the water was discharged into a stream. From the 100-L sampling tank, 25 L were pumped into a polyethylene container for laboratory analysis of 134Cs activity (decay-corrected) and amount of particles.
Analytical Laboratory Methods
The activity concentration of 134Cs in soil and vegetation samples was determined by using a 3-in (ca. 76 mm) NaI detector (Miniaxi Auto-Gamma 5000 Series; Packard Instrument Company, Downers Grove, IL). Counting times were 30 min to 1 h.
Suspended solids in water were determined by filtering a sufficient amount of water through a glass fiber filter having a pore size of 1.6 µm (Whatman [Maidstone, UK] GF/A). The filter was dried at 105°C and weighed before and after filtering the water and after drying. The amount of suspended solids in the samples was determined from the filter weight differences (Norwegian Standard 4733, 1991).
To test the association of 134Cs with particles and colloids, selected 1-L water samples were filtered successively through 5-, 2-, and 0.45-µm Millipore (Bedford, MA) filters, respectively. As the tracer was associated with particles having diameters larger than 2 µm, no filtration was performed for the water samples collected during the study period. The activity concentration of 134Cs in water contained in a 1-L Marinelli beaker was measured by using a high-resolution co-axial HP-Ge detector (efficiency 20%, resolution 1.9 keV) (Ortec, EG&G Nuclear Instruments, Oak Ridge, TN) interfaced with a multichannel analyzer. Counting times were 16 to 48 h.
Budget Calculations
Based on in situ measurements at all 350 points within the grid, a map of the 134Cs distribution in the upper 0- to 1-cm soil layers within the experimental site can be attained. By comparing the total amount of the distributed tracer with results from the initial in situ measurements, a total tracer input value is established. By comparing the in situ 134Cs distribution at the end of the experiment with that originally present, zones with 134Cs depletion or enrichment could be identified within the contaminated plot and within the buffer zone. Budget calculations also included vertical transport of tracer into deeper soil layers and uptake in vegetation. These parameters were determined from soil cores and measurements of rinsed vegetation, respectively, collected at the end of the experiment. Runoff transport and retention of particles within the buffer zone were determined by continuously collecting water samples and by in situ measurement in the buffer zone.
To calculate retention of particles (suspended solids, SS) in the buffer zone, information on input SS to the zone and output SS from the zone is needed. Measurements of SS (mg L-1) in runoff from a nearby reference plot without a buffer zone is used as an input value, while the output value is determined from measurement of SS (mg L-1) in collected runoff passing the buffer zone. The retention of particles through the buffer zone is calculated according to:
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Erosion and Sedimentation Pattern
The soil in the study plot had a relatively high clay content (44%) and Cs ions will strongly absorb to clay particles (Tamura, 1964; Lomenick and Tamura, 1965). A contact time of 3 wk between the application of Cs to soil surfaces and the first rainfall event is therefore assumed sufficient for irreversible sorption of 134Cs ions to soil particles to occur (Oughton and Salbu, 1994; Salbu, 2001). Hence, a change in the 134Cs distribution within the contaminated plot observed after major rainfall or snowmelt events is attributed to transport or sedimentation of contaminated particles originally present at the soil surface.
Figure 3 illustrates the relative changes (%) in 134Cs activities within the grid net for soil surface measurements between the start of the study (26 Oct. 1993) and the end of the study (13 May 1996; i.e., 930 d after application) for the tilled area. There was a general decrease in the activity levels within the contaminated plot. However, the enrichment that was observed along the borders at 30 m and in the upper buffer zone demonstrated that sedimentation of contaminated particles took place.
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In the tilled area, the mean reduction in the 134Cs activity levels was about 40 to 50%. The erosion pattern obtained from in situ measurements at the end of the study indicated that sheet erosion was the dominant erosion process, as only smaller rills were developed. If concentrated runoff had occurred, greater differences in the 134Cs activity levels associated with the soil surface would be expected. In the present work, all individual measured points in the tilled area showed decreased activity levels 3 wk after application. This phenomenon is attributed to a rainfall of 40 mm and the very first runoff event draining the experimental plot in autumn 1993. In addition, the plot was newly tilled, and such soil is more erodible than more consolidated soil, and the open structure after tillage increased the probability for vertical transport. At the end of the experimental period, some weeds and mosses covered the soil surface, which prevented the detachment of particles both from rainfall and runoff.
Figure 4 illustrates the increase in 134Cs activity in the buffer zone at the end of the experiment relative to the background level. Cesium-134 in the upper part of the buffer zone was contaminated by wind drift during application of the tracer to the tilled area. The contaminated buffer area (length line 4.5 to 3.5 m) is therefore not included in the figure.
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The change in the 134Cs activity levels at some selected individual point with time during the experimental period is illustrated in Fig. 5 and 6. The date 26 Oct. 1993 represents the initial level of 134Cs activity (100%) in the tilled area and the buffer zone (background level = 1). Decrease in the 134Cs activity compared with the initial level indicates erosion and particle transport, while enhanced activities indicate retention and sedimentation of contaminated particles at individual points in the grid net.
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The third measurement was performed on 21 Dec. 1993 (56 d after application) after five runoff events. The first runoff event occurred 7 and 8 December and was caused by rainfall. Then, a cold period with frost and snowfall occurred, followed by a milder period with rainfall on 19 and 20 December. The measurement was performed the day after this runoff event, and frozen soil was observed. The total rainfall and runoff for this period were 65 and 24 mm, respectively (rainfall-runoff coefficient 36%). Because ice covered the soil surfaces, the vertical transport into the soil profiles was limited. The concentrations of suspended solids in runoff water were high (2860–5050 mg L-1) during this period compared with the runoff values (333–757 mg L-1) from rainfall on the unfrozen soil surface in the beginning of December. The highest concentration of 134Cs in runoff water in the study period was also measured during these runoff events. Values of 134Cs ranging from 28.4 to 32.4 Bq L-1 in runoff from the thawing soil and 4.2 to 12.7 Bq L-1 in runoff from rainfall before the frost period were observed. Therefore, the high concentrations of contaminated particles in runoff are attributed to snowmelt combined with heavy rainfall on partly thawed soils.
Nearly all the runoff events following the snowmelt period in March and April 1994 reflected the presence of 134Cs. The concentrations of suspended solids were, however, not particularly high compared with the previous autumn, with a maximum value of 481 mg L-1. The erosion pattern after the winter period did not show any visual signs of enhanced erosion or rills, but was dominated by sheet erosion. Thus, the removed particles were transported as shallow surface flow all over the soil surface. The in situ measurement of the 134Cs activity levels in spring 1994 (201 d after application) showed that the decrease was smaller than that observed the previous autumn. However, larger differences were observed between individual points. For example, point 7-4 (Fig. 5) showed increased activity, probably due to sedimentation of contaminated particles. Following additional runoff periods (spring 1995 and 1996, 570 and 930 d after application, respectively), erosion and reduction in 134Cs activity levels were observed at these points. Thus, in situ measurement at individual points provides the opportunity to follow when soil surface particles are exposed to erosion and sedimentation processes.
During the summer and autumn measurements in 1994, only minor changes in 134Cs activity levels could be observed at most of the measured points in the grid net. Only one runoff sample (September) contained 134Cs activity. There were several runoff events during November and December, but the activity concentration of 134Cs was lower than the detection limit. Most of the runoff occurred on partly frozen soil giving low particle concentrations, and the potential for erosion was limited.
During the snowmelt period in 1995, 134Cs activity was only registered during the middle of the snowmelt period when the particle concentrations had increased. During the early stages of snowmelt, suspended solids but no 134Cs activity were measured in the runoff samples. During continued melting, more of the topsoil thawed and 134Cs associated with particles was observed in the runoff. By the end of the snowmelt, with less runoff, no 134Cs was measured in the water although the concentrations of suspended solids were still high. This indicated that snowmelt and runoff events that occurred during the previous autumn (1993) and winter period (1994) had removed much of the surface layer labeled with 134Cs and that uncontaminated particles on the soil surface were exposed to erosion. Therefore, during runoff periods after the snowmelt in 1994, the runoff water transported labeled and unlabeled particles.
After the snowmelt period in 1996 there was a new general decrease in the 134Cs activity levels measured within the grid net (930 d after application), as illustrated in Fig. 5 for selected points. As for the snowmelt period in 1995, 134Cs was only measured in surface runoff during the middle of the snowmelt period. After 2.5 yr without tillage, weeds and mosses covered the soil surface and prevented the detachment of particles both from rainfall and runoff. In addition, soils with a high clay content are cohesive, form stable aggregates, and may turn hard during dry periods. They are susceptible to cracking and are less erodible than, for example, the less cohesive silty and sandy soils. The soil was therefore less erodible at the end of the experimental period than if normal annual tillage had been performed. The concentrations of suspended solids were also generally lower during these snowmelt periods. The visible inspection of the soil surfaces also supported the hypothesis of a compacted soil with cracks and that sheet flow had been the dominating flow pattern. Only minor rills, not deeper than 3 cm, were developed.
As illustrated in Fig. 6, the total increase in the 134Cs activity level within the buffer zone during the experimental period was a factor of 1.3 to 3.8 times higher than the background level. This indicates that sedimentation of contaminated particles varied within the buffer zone. Although all points (0.5-1, 2.0-4, 3.5-1, 3.5-4) in Fig. 6 have the same curve development, sedimentation was highest in the upper and right part of the buffer zone (Fig. 4). At point 0.5-1, showing only minor variation with time, the retention was less efficient.
The 134Cs activity level was close to the background level after the first runoff event (23 d after application), which indicated that no sedimentation of contaminated particles occurred. Due to high infiltration during this runoff, most of the contaminated particles at the tilled plot were probably transported vertically into the soil. In addition, some contaminated particles were transported by runoff water passing the buffer zone during this event.
The highest increase in 134Cs activity levels occurred during rainfall events and/or snowmelt in December 1993. As illustrated in Fig. 8, the peak event on 19 December resulted in runoff water with high concentrations of suspended solids and 134Cs. Results from a comparable study plot in Norway showed higher retention of particles in the buffer zone during wintertime (from November to the end of the snowmelt period) than during the summer (Syversen, 1997). Higher retention during wintertime and snowmelt was probably due to higher surface runoff and erosion, which can cause selective erosion of coarser particles. Trapping efficiency in buffer zones increased with increasing particle size. A high concentration of particles in runoff water may also enhance aggregation of finer particles into larger ones. These processes may also have occurred in December 1993 and contributed to sedimentation in the buffer zone.
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Vertical Transport of Cesium in the Soil
The mean 134Cs concentration density (kBq m-2) in soils (n = 42), ranges, and the relative distribution (%) of 134Cs in different layers of the soil profiles are given in Table 1. About 52% of the 134Cs activity in the soil profile were observed below the 0- to 1-cm layer, about 30% below the 1- to 2-cm layer, and 19% below the 2- to 3-cm layer. The coefficient of variation was higher in the deeper layers, indicating greater variations between the soil cores. Figure 7 shows a typical vertical distribution of the 134Cs activity, where most of the activity was retained in the two upper layers (Cylinder no. 40-4). None of the 10-cm-deep soil cores showed any significant activities below 5 cm, while some of the 5-cm-deep soil cores showed significant activities within the 4- to 5-cm layer (Fig. 7).
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A vertical transport of Cs in soils just after application or deposition from fallout is also reported by Bunzl et al. (1989). Thirteen days after the Chernobyl fallout, Cs isotopes were found below 5 cm depth in the soil, probably due to high precipitation during the fallout event and macropores in the soil. Preferential transport in macropores and cracks have been documented to occur on leveled clay soil in the area of this study (Øygarden et al., 1997; Sveistrup et al., 1999).
Cesium-134 Uptake in Vegetation
No uptake of 134Cs in the vegetative part of the plant took place during the study period, neither for washed nor unwashed samples. Only two samples collected within the 3.5-m-wide buffer zone contained small amounts of 134Cs activity in the root system: 329 ± 139 Bq kg-1 and 744 ± 93 Bq kg-1. Thus, neither surface contamination nor root uptake were of any importance.
The 134Cs labeling of the soil took place in October, and surface runoff containing a high load of 134Cs associated with particles occurred during winter 1993–1994, before the growing season in spring. The 134Cs associated with particles seems, therefore, relatively inert, not attached or sorbed to surfaces of the vegetation and not easily available for plant uptake. Root uptake will depend on the contamination depth in the soil and the distribution and length of roots. Most of the 134Cs activity is located in the upper soil profile, while tufted hair grass, one of the dominant plant species in the buffer zone, has a deep root system. Thus, root uptake is expected to be minimal.
Cesium-134 and Particles in Surface Runoff Water
Correlation between Cesium-134 and Particles
During the experimental period from autumn 1993 to spring 1996, 58 runoff events occurred. Fifty-one of the runoff events occurred during the snowmelt period (usually December–April), while four runoff events occurred during the autumn period (usually September–December). Only two runoff events occurred during summer period (April–September). The snowmelt periods usually represent frozen soil with no infiltration and a great amount of runoff water. It is expected that the efficiency of the vegetated buffer zone in removing particles from the runoff water is lower during the snowmelt period than during the rest of the year.
Annual runoff of suspended particles through the buffer zone varied from 4.2 to 6.0 kg ha-1, with the highest runoff in 1993 (October to December). The runoff peaked in December 1993, which led to a high loss of particles from the plot (Fig. 8). Despite the few runoff events, 1996 also had a high total amount of particles (4.2 kg ha-1). This is due to a high concentration of particles in an early snowmelt event.
Multivariate regression analysis was performed to test the relationship between the 134Cs concentration and the particle transport (SS) in runoff water after passing the buffer zone. Plausible differences between years were included in the regression analysis by means of two dummy variables. The SS and 134Cs variables were log-transformed due to the heterogeneity of dates (i.e., unequal variances) and non-Gaussian distribution. Runoff events with no 134Cs activity but with particles in the runoff water were not included in the test. These unlabeled particles originate possibly from deeper soil layers than the surface soil where 134Cs was applied. The multivariate model was statistically significant (p < 0.0001 and R2 = 0.76). In addition, the results showed significant differences between 1993 and 1995–1996, but no significant differences between 1993 and 1994. More precisely, the results showed that at a given SS concentration, the 134Cs level was significantly lower in 1995–1996 than in 1993–1994 (p < 0.05). The years 1995 and 1996 were merged because there was only one runoff event with 134Cs associated with particles in 1996. The results show that the particle transport by runoff during the first period of the project could be attributed to erosion of soil surface particles labeled with 134Cs, which indicates sheet erosion. Runoff from the last part of the project contained unlabeled particles from deeper soil layers.
The total amount of 134Cs in surface runoff water through the buffer zone was 0.42 MBq (±5.7%), which represents only 2% of the total amount of 134Cs applied (Fig. 10). Figure 8 shows the variation in 134Cs activity and amount of particles (suspended solids) in runoff waters during the experimental period. The highest 134Cs activity was observed during the first runoff period after application of 134Cs, probably due to erosion of 134Cs-labeled particles. The 134Cs activity was also high during snowmelt 1994, but decreased significantly after this period. Figure 8 also shows the correlation between 134Cs activity and suspended particles. In most cases the amount of particles in runoff water followed the curve for 134Cs activity. The results showed that 134Cs-labeled particles in runoff water were washed out during autumn 1993 and spring 1994. The plot was tilled and harrowed prior to the experiment, and, over time the particles would probably be more difficult to erode. As the surface soil with labeled particles eroded during the first phase of the experiment, soil particles from below the topsoil layer may be eroded at a later stage. This may explain the lack in correlation between 1993 and 1995–1996, and the significant correlation between 1993 and 1994. During snowmelt 1996, no significant amount of 134Cs could be observed even though the amount of suspended particles was high. This supports the hypothesis that unlabeled particles from deeper soil layers were eroded during the last part of the experimental period.
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Cesium-134 Budget
An overview of the different elements in the Cs budget is given in Fig. 10. After the application of the tracer, 103 ± 1% of the applied tracer was measured on the soil surface within the tilled area and in the upper part of the buffer zone. At the end of the study, 116% of the tracer was gained from in situ measurements, water samples, soil core samples, and vegetation samples. Measurements are performed in the field, in the laboratory, and with different equipment (uncertainty ±1% for soil surface and buffer zone, ±3.5% for vertically distributed Cs, and ±5.7% for Cs in runoff water). Furthermore, it is assumed that the field points and samples collected are representative for the plot. Taking representativity into account, the obtained results should be sufficiently good to allow 134Cs budget calculations to be performed, to estimate the importance of different pathways for 134Cs labeled particle transport.
At the end of the study, 54% of the 134Cs activity remained at the soil surface in the tilled plot and 46% was removed. The erosion pattern at the soil surface showed that sheet erosion from the entire surface was the dominating process and only minor rilling was observed. No parts of the soil surface showed especially high erosion. The vertical distribution of 134Cs in soil cores indicated that most of the removed 134Cs was transported vertically into the soil profile (Table 1). Comparison of the 134Cs concentration density (Bq m-2) obtained from in situ soil surface measurement (0–1 cm) at the end of the study with the mean activity value (Bq m-2) for the 0- to 1-cm layer from the soil cores gave about the same results (54%). Therefore, vertical transport of 134Cs is the dominant transport pathway at this study site. The 134Cs activity in the 1- to 2-cm layer represented 24% of the applied amount and the 2- to 3-cm layer represented 15% of the applied 134Cs amount.
The 134Cs in surface runoff amounted to about 2%, and about 2% was found in the buffer zone. When the contribution from the drifting of 134Cs into the upper part of the buffer zone was excluded, the net input to the buffer zone was about 1%. There was no detectable uptake of 134Cs in vegetation.
The Potential of Cesium-134 for Tracing Particle Transport
This study demonstrated that:
Limitations
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In situ measurements of 134Cs associated with soil surface particles documented (on an event basis) when and from where particles at a tilled plot were eroded, and where in a buffer zone the particles sedimented. Most of the particles were vertically transported into the soil, probably during the first runoff event after application of the tracer on newly tilled soil. Sheet erosion was the dominating erosion form in this field, which had a relatively high clay content. In the buffer zone, most of the sedimentation occurred in the upper part of the zone. Uptake of 134Cs by the vegetation was negligible. The highest losses of 134Cs associated with particles from the plot were measured during the first autumn and snowmelt period. During the two last years of snowmelt both particle concentrations and 134Cs content in the runoff were lowered, probably due to a less erodible soil surface by the end of the study. There was a good correlation between the amount of 134Cs and suspended solids in runoff water during the first 1 to 2 yr, indicating that labeled surface soil particles were eroded. After this period, however, the correlation between 134Cs and suspended solids in runoff water was poor, indicating that unlabeled particles from initially deeper soil layers were eroded.
The 134Cs budget calculation showed that about 54% of 134Cs was still retained at the surface of the plot after 2.5 yr, most of the tracer was transported into the soil profile, about 1% was retained in the buffer zone, and about 2% of the added tracer was transported out of the field by runoff water.
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ACKNOWLEDGMENTS |
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