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Journal of Environmental Quality 30:1341-1353 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Vadose Zone Processes and Chemical Transport

Radionuclide Transport above a Near-Surface Water Table

III. Soil Migration and Crop Uptake of Three Gamma-Emitting Radionuclides, 1990 to 1993

P. Wadeya, G. Shaw*,a and J.N.B. Bellb

a T.H. Huxley School of Environment, Earth Science and Engineering, Berkshire, SL5 7PY, United Kingdom
b Department of Biology, Imperial College of Science, Technology and Medicine, Silwood Park, Ascot, Berkshire, SL5 7PY, United Kingdom

* Corresponding author (gg.shaw{at}ic.ac.uk)

Received for publication November 30, 1999.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This paper summarizes the vertical distributions of 22Na, 137Cs, and 60Co above controlled water tables in deep and shallow lysimeters during a four-year experiment. The activity concentration profiles were all determined at the time of harvest of a winter wheat (Triticum aestivum L. cv. Pastiche) crop. Activity concentrations in different crop tissues were determined and crop uptake expressed as both an inventory ratio (IR) and a transfer factor (TFw), weighted to account for root and radionuclide distributions within the soil profile. Experimental variates were subjected to analysis of variance to determine the single and combined effects of the soil depth and the year of the experiment on the results obtained. Each radionuclide showed significant variations in activity concentration with soil depth, but the significance of these variations from year to year was dependent on radionuclide. A distinction in the behavior of weakly sorbed (22Na) and more highly sorbed (137Cs and 60Co) radionuclides was observed. The former exhibited significant variations in its distribution in the soil profile from year-to-year whereas the latter did not. Relatively high TFw values for 22Na were maintained throughout the experiment, whereas for 137Cs and 60Co, the highest TFw values were recorded in 1990 followed by a significant decline in 1991, with TFw remaining low in 1992 and 1993. The TFw values were, in general, significantly higher for deep lysimeters than for shallow lysimeters. This is thought to provide evidence of enhanced radionuclide absorption by the relatively small fraction of roots in the vicinity of the deeper water table.

Abbreviations: ANOVA, analysis of variance • ILW, intermediate-level radioactive waste • IR, inventory ratio • LLW, low-level radioactive waste • TFw, weighted soil–plant transfer factor • WMAC, weighted mean soil activity concentration


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE isolation of intermediate-level and certain low-level radioactive waste (ILW and LLW, respectively) in a deep geological repository would result in the great majority of the radionuclides decaying in situ. However, on a timescale of tens of thousands of years, a few mobile, long-lived radionuclides would have the potential to return to the human environment (Nirex, 1997). In this context, the behavior of radionuclides in soils and their uptake by plants are important factors to take into account when assessing repository performance.

In initiating its deep disposal research program in the late 1980s, United Kingdom Nirex Limited recognized that, apart from a study by Sheppard and Evenden (1985), the issue of migration in soils and plant uptake of radionuclides from subsurface sources had not been addressed in a way that satisfied the program's needs. A body of work, of which this study forms a part, was therefore put in place under the Nirex Safety Assessment Research Programme (NSARP). Prior to the initiation of this program, there existed a plethora of published studies on the issue of vertically downward migration and plant uptake of radionuclides and other substances following deposition to the soil surface from the atmosphere (for a statistical review of some of these studies see Frissell, 1989). The applicability of results from these studies to situations in which radionuclides are expected to emerge within the near-surface soil environment following hydrological transport from the geosphere is open to question. It is this scenario, however, which is of particular relevance to safety assessments of ILW and LLW placed in geological repositories. Geosphere migration processes from deep repositories of even poorly sorbed radionuclides are expected to be slow and surface breakthrough of radionuclide plumes would not be expected within 1000 years of facility closure (Nirex, 1997). Long-lived, poorly sorbed radionuclides would, however, eventually be expected to appear within the biosphere and would give rise to some radiological impact on man due to exposure via the foodchain and other pathways.

The lysimeter experiment described in this paper was designed to investigate the upward migration of radionuclides in cropped soils above near-surface ground water. The objective in developing the lysimeter facility was to achieve as close to field conditions as possible, using natural soil and ambient hydrological inputs and outputs. The experiment required a winter wheat crop to be sown in the soil such that reasonable root penetration and density were achieved, but with the constraint that the water table was to be controlled at a specified depth in the soil. A detailed description of the experiment was given in the first paper in this series by Burne et al. (1994) and a description of the first two years' results for soil migration and crop uptake of the gamma-emitting radionuclides 22Na, 137Cs, 60Co, and 109Cd was given by Wadey et al. (1994) in the following paper. In the latter paper, it was stressed that, to be of substantial value, lysimeter experiments, such as the one reported here, need to be carried out over periods of several years in order to account for short-term uncertainties associated with annual weather patterns. Accordingly, the results presented in this paper were collected from the lysimeter experiment over a four-year period from 1990 to 1993, inclusive (the experiment was discontinued after the 1993 crop harvest). During this time, the main experimental objectives were to (i) maintain near-constant water table depths of 35 cm (shallow) and 65 cm (deep) below the soil surface and (ii) maintain and monitor well-defined concentrations of the experimental radionuclides below the water table in each lysimeter. The water table depths adopted were constrained by the absolute depths of the field lysimeters. Both water tables are shallow by normal standards, but the experiment was intended to provide information on the upward movement of radionuclides by water flow processes, as well as by possible biological transport mechanisms, resulting in vertical profiles of radionuclide activities developing within the lysimeter soil profiles over the four-year period. In tandem with the vertical up-profile migration of the radionuclides, their uptake by winter wheat roots also occurred.

The three gamma-emitting radionuclides reported in this paper are 22Na, 137Cs, and 60Co (109Cd is not included due to difficulties in data acquisition and analysis). These radionuclides have a range of radioactive half lives from 2.6 years (22Na) to 30 years (137Cs) and are not of specific interest in relation to disposal to a deep geological repository. However, the range of chemistries represented by the elements Cs, Na, and Co is sufficiently diverse to allow their use in a generic investigation of issues arising in relation to subsurface to surface migration and biological uptake. The results obtained from this study are intended to assist in the development of safety assessment methodologies appropriate to the disposal of ILW and LLW in the UK, but they are also likely to be of wider interest and applicability. The interpretation of data presented here focuses on the year-to-year variability of vertical soil activity profiles of each of the radionuclides and on the use of simplified methods to quantify soil-to-plant transfer under circumstances in which radionuclide activity and crop root distributions are heterogeneous and nonidentical.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The establishment, radiochemical dosing, and routine operation of the lysimeter system were described in detail by Burne et al. (1994). A brief summary of the experiment is given here. Four replicate shallow and four replicate deep lysimeters were established and maintained, each with a soil surface area of 182 x 91 cm. Water table depths from the soil surface were either 35 cm (shallow) or 65 cm (deep). The total soil depth in shallow and deep lysimeters was 40 and 70 cm, respectively. The soil type in all lysimeters was a sandy loam (Eutric Cambisol; Tavernier, 1985) obtained from the Silwood Park estate near Ascot in southern England (the underlying geology comprises the Bagshot sands, fluvial glacial deposits that support a range of soil types from cultivated sandy loams to podzols). Table 1 shows the basic physical and chemical properties of this soil. After excavating sufficient soil to fill the lysimeters, it was separated into topsoil (0–20 cm) and subsoil (>20 cm), each of which was homogenized before being placed into individual lysimeters. The subsoil was supported on a layer of geotextile material (Geotex) above an inert substrate of polythene beads into which a mixture of radionuclides could be introduced and circulated using peristaltic pumps. Four successive crops of winter wheat were planted in the autumn of each year from 1989 to 1992 and allowed to grow to maturity before harvesting in the late summers of 1990 to 1993. Dosing of the lysimeter system with the radionuclide cocktail was carried out in April or May of each year (when upward evapotranspiration fluxes due to the developing crop were beginning to exceed downward flushing of soil water, which was prevalent during the winter) and involved injection of radionuclides exclusively into the "ground water": in other words, no radionuclide additions were made directly to the soil surface. Table 2 shows the total activities of 22Na, 137Cs, and 60Co added to the lysimeter system (at the locations described by Burne et al., 1994). Periodic redosing with 22Na and 60Co during the experiment was intended to maintain near-target activity concentrations in the "ground water". Each radionuclide was initially added in the carrier-free chloride form. A considerable effort was made to account for the activity balance in the entire lysimeter system, which consisted of lysimeters, buffer tanks, and a common reservoir, as originally described by Burne et al. (1994). A dedicated system model was developed to determine the gross import–export of radionuclides from the "ground water" reservoirs (buffer tanks and common reservoir) into the lysimeters, which is fully described by Butler and Wheater (1999). In general, the system model and regular measurements of soil and solution samples from the lysimeter system could account for the total added activities of the gamma emitters examined in this paper.


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  		Table 1. Physical and chemical properties of the Silwood Park soil used in the lysimeters in this study. The soil is a sandy loam (Eutric Cambisol) belonging to the Wicks series. The underlying parent material comprises the Bagshot sands, fluvial glacial deposits that support a range of soil types from cultivated sandy loams to podzols.

 

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  		Table 2. Total activities of radionuclides added to the lysimeter system from 1990 to 1993.

 
A complete suite of meteorological measurements was made at 15-minute intervals throughout the four-year experiment, using a dedicated, automated meteorological station. These measurements were intended primarily to facilitate determinations of rainwater inputs to the surfaces of each lysimeter as well as calculations of losses from the lysimeter surfaces due to evaporation. The system of water table control described by Burne et al. (1994) pumped contaminated "ground water" either into or out of the substrate beneath each soil profile, depending on the relative magnitudes of evaporation and precipitation at the soil surface. The water table control system, which maintained the target water table within a tolerance of ±0.5 cm, was designed to regulate the fluxes of water into and out of each individual lysimeter. The volumes of water pumped were automatically logged for each lysimeter and provide a record of the balance of water movement across the base of each lysimeter soil profile. These measurements can therefore be used to determine whether each of the lysimeters was in surplus or deficit, with respect to water use, at any time during the experiment; in other words, whether net water flux was predominantly upward or downward with respect to each lysimeter soil profile (here, flux denotes the net cumulative water movement in millimeters across the lysimeter base at any time during the experiment—see Fig. 1a).



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  		Fig. 1. (a) Cumulative water fluxes across the bases of deep and shallow lysimeters during the period 1990–1993. (b) Rainfall and evapotranspiration from deep and shallow lysimeters during the period 1990–1993 (absolute amounts per quarter).

 
Sampling of Soil and Plant Material
Vertical core samples of soil were taken immediately after harvest of the crop each year in late summer. Core samples, of 3.8-cm diameter, were taken with a commercial tube sampler (Eijkelkamp, Agrisearch Equipment, the Netherlands) down to the Geotex layer, which marked the base of the mineral soil within the lysimeters: all soil depths are therefore recorded as height above Geotex. Having removed these samples from the lysimeter, the remaining holes were packed with uncontaminated soil identical to that within the lysimeters. Cores were taken to the laboratory where they were quickly extruded and cut into depth-wise segments. The lowest 10 cm of cores from both deep (70 cm) and shallow (40 cm) lysimeters was divided into 2-cm segments whereas the remainder of each core was divided into 10-cm segments: this gave 8 depth samples for shallow lysimeters and 11 depth samples for deep lysimeters. Each depth sample was stored in a refrigerator at 4°C while awaiting gamma analysis.

Individual wheat plants were harvested by excision of tillers at a point approximately 2 cm above the soil surface and taken to the laboratory where they were air-dried and divided into leaves, stems, chaff, and grain. From the 1992 harvest onward, the wheat ear was further divided to give rachis and chaff samples. All samples were weighed and stored in a dry condition awaiting gamma analysis.

Analysis of Soil and Plant Material by Gamma-Ray Spectrometry
The 22Na, 137Cs, and 60Co contents of the soil and plant samples were determined by high resolution gamma-ray spectrometry using a GeLi detector (PerkinElmer, Berkshire, UK) coupled to a multi-channel analyzer with commercial data collection and spectral analysis software (Nuclear Data Systems). Count times were adjusted to give 2{sigma} errors of <10%, although count errors were usually considerably less than this. Samples taken from 1992 onward were counted on the same detectors but with GammaVision (EG&G Instruments) software used for spectral analysis. Detectors were calibrated with standards made up to the same geometry as samples. Standards comprised 22Na, 137Cs, and 60Co in deionized water prepared from noncertified sources, which were cross-calibrated against certified sources of 241Am and 152Eu to give a final precision of ±10% (radionuclides were obtained from either Amersham International Ltd., Buckinghamshire, UK, or DuPont UK, Stevenage, Hertfordshire, UK). No attempt was made to correct for differential self-absorption of {gamma}-rays between standards and samples. All radionuclide activities within soil and plant samples were calculated as Bq kg-1 dry weight (all corrected for radioactive decay to the date of harvest) and are referred to within this paper as activity concentrations.

Root Distributions at Harvest
Root distributions within lysimeters were measured on a regular basis using a rigid fiber-optic endoscope (Borescope; Olympus Industrial, Lake Success, NY) inserted into the Perspex access tubes described by Burne et al. (1994). These measurements have been calibrated against direct determinations of root density in a small-scale validation study (unpublished data, 1992), the result of which gives confidence in the use of our endoscope data, although the measurements do not include root hair densities. Two weekly endoscope measurements gave a dynamic picture of root development throughout the year. Though root uptake is determined by root distribution throughout the growing season, in order to interpret the "snapshot" picture of root uptake of radionuclides at harvest, only the mature root distributions recorded over a 6-wk period before harvest are described here.

Measurement of Oxidation–Reduction (Redox) Potentials in Soil Profiles
Redox potential (Eh) is a physico–chemical parameter that is known to exert a strong influence on both the chemistry and biology of soils and sediments. Over a period of a few days following flooding of soils, Eh can fall from positive to negative values, indicating a rapid change from aerobic to anaerobic conditions as oxygen is consumed by microorganisms faster than it can be resupplied by diffusion through pore water (Armstrong, 1982). Flooding of the lysimeter soils in this study was prevented by active maintenance of fixed water tables. However, water contents varied substantially within the soil profiles, with fully saturated conditions prevailing at and below water tables and a gradient of declining soil water contents from the water table to the surface. This range of soil water contents was expected to result in a range of soil redox conditions. To characterize these, one deep and one shallow lysimeter were instrumented with arrays of platinum electrodes in the summer of 1992. These were arranged depth-wise at 5- or 10-cm intervals, in a shallow and a deep lysimeter, respectively, down to the Geotex, to provide profiles of Eh values down the soil profiles. Readings were taken manually at two-week intervals, using a high impedance millivolt meter and a saturated calomel (mercurous chloride) reference electrode inserted into the soil surface when required. Millivolt readings of Eh were corrected for the potential of the calomel reference electrode (250 mV), but not for pH, as this was more or less constant at all depths within the soil profiles. The satisfactory performance of the redox electrodes in relation to a common redox couple (FeII {leftrightarrow} FeIII + e-) had previously been evaluated in laboratory soil column experiments (Hu, 1998) and the stability of readings over the course of the lysimeter experiment gave confidence in their reliability.

Statistical Analysis
Data were subjected to a series of two- and three-way analyses of variance (ANOVA) based on the fixed treatment effects model (Sokhal and Rohlf, 1969). In carrying out these analyses it was hypothesized a priori that water table depth (deep or shallow), the year of the experiment (1990, 1991, 1992, 1993), and, where appropriate, the type of plant tissue being considered, exerted no significant effect ({alpha} = 0.05) on the following variates:

  1. soil activity concentration depth profiles (kBq kg-1)
  2. weighted mean soil activity concentrations (WMAC, kBq kg-1)
  3. soil–plant inventory ratios (IR, dimensionless)
  4. weighted soil–plant transfer factors (TFw, dimensionless).

Each of these variates was found to be approximately log-normally distributed and the data were, therefore, ln-transformed prior to analysis. This transformation had the effect of producing approximately normally distributed data sets with homogeneous variances, two fundamental prerequisites for ANOVA (Sokhal and Rohlf, 1969). Post hoc comparisons of individual mean values of variates were facilitated by calculating least significant differences (LSD) at p = 0.05: where appropriate, these LSD values are plotted directly on graphs of mean values.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Summary of Rainfall and Lysimeter Water Balances during the Experiment
Meteorological data were collected at 15-min intervals during the four-year experimental period but, for the purposes of this paper, are summarized on both seasonal and annual bases. Figure 1 shows rainfall and evapotranspiration data for the Silwood Park lysimeter system from spring 1990 to spring 1993, summarized graphically. Table 3 shows summary rainfall and water balance statistics for the spring and summer seasons, combined, from 1990 to 1993: these statistics are of particular interest from the point of view of radionuclide uptake by crops, as these are the seasons of peak crop growth. It can be seen from this figure and table that rainfall in the first spring and summer of the experiment (1990) was unusually low, with less than 40% of average rainfall recorded during the four-year experiment falling in this first cropping period. Evapotranspiration (estimated from the overall lysimeter water balance) was fairly constant in both deep and shallow lysimeters during spring and summer periods from 1990 to 1993. However, water fluxes across the lysimeter bases were quite different in 1990 from the subsequent three years. In Fig. 1a and Table 3 a positive water flux indicates a net water movement from the saturated zone below the water table, upward through the unsaturated zone toward the soil surface (i.e., evapotranspiration exceeds precipitation). Thus, in the spring and summer of 1990 there was a clear upward water flux through both deep and shallow lysimeters, with this flux evidently greatest in shallow lysimeters. Throughout the remaining spring and summer seasons of the experiment (1991 to 1993, inclusive), a negative water balance indicates net downward movement of water through the soil profiles, with the exception of an approximately zero water balance during the summer of 1993. As water fluxes are expected to provide the primary driving force for radionuclide migration in soils these data suggest that the potential for upward movement of radionuclides was at its greatest in the first spring and summer season of the experiment, shortly after the initial dosing of the lysimeter system with radionuclides in 1990. Other mechanisms that may effect radionuclide migration in soils include uptake and translocation in plant roots as well as bioturbation involving soil fauna. The relative contribution that each of these mechanisms is likely to have made to the observed radionuclide distributions in this study is considered in the Discussion section, below.


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  		Table 3. Summary rainfall and water balance statistics for spring and summer seasons, combined, during the lysimeter experiment.

 
Vertical Radionuclide Distributions within Soil Profiles
The vertical distributions of 22Na, 137Cs, and 60Co in deep and shallow soil profiles are shown in Fig. 2a–c, respectively. These profiles were determined at the time of wheat harvest over the four-year experimental period (1990 to 1993). The highest activity concentrations of each radionuclide were found in the bottom 10 cm of the soil profile. This was in the region of the water table, maintained at a height of 5 cm (±0.5 cm) above the Geotex. Migration of each radionuclide up the soil profile to the soil surface was observed, although concentrations near the soil surface were invariably much lower than those found at the base of the soil profile: abscissae in Fig. 2a–c are therefore presented on logarithmic scales. Analysis of variance (ANOVA; Table 4) showed that there was a highly significant (p < 0.001) reduction in soil activity concentration with height above the Geotex for each radionuclide. However, the distributions of 137Cs and 60Co within the deep lysimeters was characterized by a distinct increase in activity concentration at the soil surface. Of the three gamma-emitting radionuclides present, 22Na was the most mobile, reaching much higher activity concentrations at the soil surface than the other radionuclides. In the previous paper in this series (Wadey et al., 1994) only the 22Na soil profile showed a significant shift in distribution from 1990 to 1991. With the inclusion of a further two years' data, a significant change in the vertical distributions of each radionuclide occurred from year to year, with the exception of 137Cs in the shallow lysimeters, which showed considerable variability throughout the soil profile. Only for 22Na, however, was there a clear statistical interaction between depth distribution and year for both deep and shallow soils, indicating that only the 22Na profiles altered significantly during the period of the experiment. There was a clear year-on-year reduction in the activity concentrations of 22Na in the lowermost region of soil profiles from 1990 to 1993, though in the uppermost part of the soil profiles an increase in 22Na activity concentrations occurred between 1990 and 1991, followed by a systematic year-on-year decrease until 1993. A highly significant (p < 0.001) interaction between soil depth distribution and year was also seen for 60Co in the deep soil, in which increased activity concentrations were detected in 1993 compared with the previous years.



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  		Fig. 2a. Vertical distributions of activity concentrations of 22Na in deep and shallow lysimeters during the period 1990–1993.

 


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  		Fig. 2c. Vertical distributions of activity concentrations of 60Co in deep and shallow lysimeters during the period 1990–1993.

 


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  		Fig. 2b. Vertical distributions of activity concentrations of 137Cs in deep and shallow lysimeters during the period 1990–1993.

 

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  		Table 4. Results of two-factor (fixed effects) analysis of variance (ANOVA) for data relating to vertical distributions of gamma-emitting radionuclides in deep and shallow lysimeters from 1990–1993.

 
Soil–Plant Inventory Ratios
One of the simplest measures to quantify the degree of soil–plant transfer of a radionuclide, and thereby gauge its bioavailability, is to calculate the fraction of the total soil inventory of the radionuclide that is incorporated in the crop at the time of sampling. Gilbert and Simpson (1983) described this inventory ratio (IR) as:

The total radionuclide content of the soil and in all wheat collected was determined for each lysimeter and used to calculate inventory ratios for each radionuclide in both deep and shallow lysimeters (Fig. 3). Results of analyses of variance on these inventory ratios are given in Table 5. In the case of each radionuclide, water table depth (deep or shallow) had no significant effect on the inventory ratio, though the IR for each radionuclide did vary significantly from year to year. The highest IR values were recorded for 137Cs in both deep and shallow lysimeters in 1990, although these values were still less than 0.01, indicating that less than 1% of the total soil inventory of 137Cs had been transferred to the crop tissues at harvest during this year. Figure 3 indicates that there was a systematic decline in the soil–plant inventory ratio of 137Cs with time. The lysimeters were initially dosed with 137Cs in 1990 but no additional 137Cs was added in the succeeding years; therefore, this apparent decline may reflect time-dependent adsorption of 137Cs onto clay particles in the soil matrix and a reduced availability for plant uptake.



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  		Fig. 3. Soil–plant inventory ratios of (a) 22Na, (b) 137Cs, and (c) 60Co in deep and shallow lysimeters from 1990 to 1993.

 

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  		Table 5. Results of two-factor (fixed effects) analysis of variance (ANOVA) for data relating to inventory ratios of gamma-emitting radionuclides in deep and shallow lysimeters from 1990–1993.

 
Root Density Distributions
Mean root density distributions in deep and shallow lysimeters over the 6 wk preceding harvest in 1990 to 1993 are shown in Fig. 4. In all years of the experiment, root densities declined from a maximum at the soil surface to a minimum at the depth of the Geotex. Analysis of variance (Table 6) indicated that height above the Geotex and year of the experiment both had a highly significant (p < 0.001) effect on the root distributions in both deep and shallow lysimeters and there was a highly significant (p < 0.001) interaction between these two factors.



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  		Fig. 4. Vertical distributions of winter wheat root densities in deep and shallow lysimeters averaged over a period of 6 wk prior to harvest in 1990–1993.

 

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  		Table 6. Results of two-factor (fixed effects) analysis of variance (ANOVA) for data relating to root density distributions at harvest in deep and shallow lysimeters from 1990–1993.

 
The ability of roots to exploit the most highly contaminated regions of the soil profiles, adjacent to the water table, is important in the context of the question of radionuclide absorption by roots. In deep lysimeters, root penetration into the 10 cm immediately above the Geotex only occurred in 1990, the driest year of the experiment. In shallow lysimeters, root penetration into this region of the soil profile occurred in all years except 1993, which was not the wettest year (Table 3). It must be stressed that a Borescope recording of the presence of roots in the deepest 10-cm segment of the soil profile does not imply that roots penetrated to, or below, the water table itself. Indeed, careful examination of 2-cm segments of soil cores taken from the water table regions of the lysimeters showed that root penetration did not occur below the water table and rarely within 2 cm above the water table. This is as expected because one of the major controls on the presence and density of roots in this region of the soil profile is low or very low oxygen partial pressure, reflected in a redox potential that is usually very low in the region of the water table (see below).

Weighted Mean Activity Concentrations of Soil Profiles
The ultimate aim of the lysimeter experiment was to obtain information on the degree of incorporation of radionuclides in crops following migration up an initially uncontaminated soil profile from a subsurface reservoir of radioactivity. Butler and Wheater (1993) have described how the effective interface for absorption of radionuclides by crop roots in this situation is the overlapping region between a soil activity concentration profile and a crop root density profile. The former profile generally has a gradient that declines from the bottom to the top of the soil (see Fig. 2a– c), whereas the opposite is generally true for plant roots (see Fig. 4). In order to calculate soil-to-plant transfer factors for the lysimeter experiment it was, therefore, necessary to estimate an effective average soil activity concentration weighted according to the relative soil depth profiles of both radionuclide and root density. The weighted mean activity concentration (WMAC) is defined as:

in which [R]i (Bq kg-1) is the radionuclide activity concentration in the ith soil layer and fi (dimensionless) is the fractional abundance of crop roots in the ith soil layer (i.e., root length in layer i divided by the total root length in all soil layers sampled). Using this equation, each activity concentration profile shown in Fig. 2a–c can be reduced to a single average value: this can be used to compare the relative effectiveness of crop root uptake from year to year and from lysimeter to lysimeter. The WMAC values for 22Na, 137Cs, and 60Co are shown in Fig. 5a–c. From the analyses of variance performed on these data (Table 7) it appears that, for each radionuclide, WMAC values were significantly lower (p < 0.001) in the deep soil profiles compared with those in the shallow soil profiles. In the case of each radionuclide there was also a significant (p < 0.05) general decline in WMAC in deep and shallow lysimeters.



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  		Fig. 5. Weighted mean activity concentrations for (a) 22Na, (b) 137Cs, and (c) 60Co in deep and shallow lysimeters during the period 1990–1993.

 

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  		Table 7. Results of two-factor (fixed effects) analysis of variance (ANOVA) for data relating to weighted mean activity concentrations (WMAC) of gamma-emitting radionuclides in deep and shallow lysimeters from 1990–1993.

 
Weighted Soil–Plant Transfer Factors
The usual approach to quantifying soil–plant transfer of radionuclides for the purposes of radiological assessment modeling is the soil–plant transfer factor (International Union of Radioecologists, 1989):

in which the radionuclide activity concentration in only the upper 15 to 20 cm of the soil is considered. As described by Wadey et al. (1994), the different distributions of radionuclide activity concentrations and plant roots in the lysimeter soils in this study made the calculation of a soil–plant TF using a weighted mean soil activity concentration necessary. The WMAC, defined above, was used to calculate a weighted soil–plant transfer factor (TFw), using:

where [R]plant is the activity concentration of a radionuclide within a specific plant tissue determined at harvest. For the wheat in this study, specific tissues sampled and analyzed were grain, chaff, leaf, stem, and rachis (the latter only analyzed in 1992 and 1993).

The use of WMAC in the TFw calculation allows an evaluation to be made of the availability of the radionuclides for uptake into plant tissues. The year-on-year TFw values for 22Na, 137Cs, and 60Co are shown in Table 8. A three-way analysis of variance was completed to examine, simultaneously, differences and interactions between soil depth, year, and wheat tissue type (Table 9). From these results, it is clear that soil depth had a highly significant effect on the TFw, with higher relative transfer of radionuclides into wheat from deep soil compared with wheat grown above a shallow water table. For each radionuclide a highly significant (p < 0.001) change in TFw occurred from year to year. The TFw values for 137Cs and 60Co, in both deep and shallow lysimeters, decreased significantly from 1990 to 1993, whereas in the case of 22Na a significant increase in TFw values was observed over the experimental period. Significant differences (p <= 0.05) in TFw values were found between the different wheat tissues for each radionuclide except 60Co.


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  		Table 8. Weighted soil–plant transfer factors for the uptake of gamma-emitting radionuclides by wheat growing in deep and shallow lysimeters during the period 1990–1993 (means with standard deviations).

 

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  		Table 9. Results of three-factor (fixed effects) analysis of variance (ANOVA) for data relating to weighted soil–plant transfer factors (TFw) of gamma-emitting radionuclides in deep and shallow lysimeters from 1990–1993.

 
Oxidation–Reduction (Redox) Potentials in Soil Profiles
Figure 6 shows a summary of the distribution of redox potentials throughout both deep and shallow soil profiles from summer 1992 to summer 1994. The solid lines indicate the mean redox potentials over this period, whereas the broken lines indicate maximum and minimum values obtained over the two years during which redox measurements were taken. The area within the broken lines represents the "envelope" of measured redox potentials at all soil depths within both deep and shallow lysimeters. The mean redox potentials for both deep and shallow lysimeters indicate that at a depth of 10 to 15 cm above the Geotex there was a transition from relatively high, positive Eh values to relatively low Eh values (negative in the case of deep lysimeters, approximately zero in the case of shallow lysimeters). This transitional redox zone coincides with the partially saturated capillary fringe immediately above the water table and clearly represents an intermediate zone between aerobic conditions in the upper, nonsaturated regions of the soil profiles and anaerobic conditions in the saturated zone at and beneath the water table. The variability of redox potentials around the mean Eh profile in the deep lysimeters was substantial, spanning approximately 400 mV, but was considerably less than in the shallow lysimeters. Here, the Eh "envelope" spanned nearly 800 mV at the level of the water table indicating that, despite the fact that the water table depth was controlled to within ±1 cm, large fluctuations in redox potential could occur into both positive and negative regions of the Eh scale.



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  		Fig. 6. Vertical distributions of soil redox potentials (49 measurements made fortnightly between August 1992 and July 1994) in one deep (a) and one shallow lysimeter (b). The solid line in each plot represents the means of all measurements while the dotted lines represent the maximum and minimum values recorded.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
As described in the introduction to this paper, the radionuclides chosen for this study were selected to provide a range of chemistries in order to determine the range of likely effects that physico–chemical processes, such as sorption onto soil solids, may exert on radionuclide mobility and plant uptake when an initially uncontaminated soil profile is contaminated from below the water table. Despite the differences in the known abilities of elements such as Na and Cs to sorb onto soil solids, however, the degree of vertical mobility of each radionuclide within the lysimeter soil profiles in this study was greater than expected. As observed in Fig. 2a– c, each radionuclide was transported to the soil surface during the first 6 mo of the experiment. Additionally, in the case of 137Cs and 60Co, there was clear evidence of accumulation at the soil surface at the first harvest. The dry spring and summer of 1990 are likely to have played a part in this rapid transport of radionuclides up the soil profiles—the positive water fluxes across the bases of both deep and shallow lysimeters in 1990, shown in Fig. 1 and Table 3, indicate a strong upward movement of water in both sets of lysimeters. This must have been instrumental in transporting the poorly sorbed 22Na (Kd approximately 1–10 mL g-1; Sheppard and Thibault, 1990) rapidly up the soil profiles, though for the more highly sorbed 137Cs and 60Co it is questionable whether the rate of advective water flux up the soil profile could have been sufficient to result in transport over 35 or 65 cm in 6 mo. Sheppard and Thibault (1990) quote geometric mean Kd values of 4.6 x 103 and 1.3 x 103 mL g-1, respectively, for 137Cs and 60Co in loam soil. These Kd values suggest retardation factors (R = 1 + Kd{rho}/{theta}, where {rho} is soil bulk density and {theta} is soil moisture content; Schnoor, 1996) of the order of 2.5 to 8 x 103 (dimensionless). Thus, an advective water velocity of 1 m yr-1 would only be expected to result in migration of 137Cs and 60Co over a fraction of a millimeter over one year. The question therefore remains as to the processes that might have given rise to the considerably greater observed migration of 137Cs and 60Co in the spring and summer of 1990.

It is likely that, in the low redox potentials associated with the region of the soil around the water table and capillary fringe (Fig. 6), the predominant form of nitrogen is the ammonium (NH+4) ion. This formation of ammonium is known, from column studies in our own laboratory (Hu, 1998), to be one of the important chemical effects associated with the establishment of reducing conditions above water tables in soils. The NH+4 ion has a strong ability to displace other cations from exchange sites on soil surfaces and is often used as a standard ion in analytical extractions of metals and nutrient ions from soils. The net effect of increasing the NH+4 ion concentration in a soil is to reduce the Kd (and therefore decrease the retardation factor) of other ions: in the case of 137Cs the effect of the NH+4 ion is known to be particularly important and enhanced ammonium concentrations due to low redox potentials in lake sediments are known to reduce 137Cs fixation (Pardue et al., 1991). Column experiments by Hu (1998) have shown that conversion of iron to the mobile and bioavailable FeII form occurs strongly in the region of a fixed water table. Whereas the effects of reduced redox potentials on radiocesium are likely to be indirect, in the case of elements, such as iron, with chemistries that are directly controlled by redox potential, it is likely that changes in mobility and bioavailability will occur in the reduced region of the soil. In the case of 60Co, however, the cobaltous form (60CoII) predominates under normal environmental conditions, so is unlikely to be enhanced due to low redox potentials near the lysimeter water tables.

If low soil redox potentials favored the mobility of 137Cs (and possibly other ions) in the deeper regions of the lysimeter soils, it is also likely that crop root uptake of this radionuclide was also enhanced, as a larger fraction of the inventory would have been associated with the soil solution. This raises the possibility that at least a portion of the radionuclide activities that were transported to the soil surfaces during spring and summer of 1990 were transported within root systems following absorption in the highly contaminated regions nearer the water table. Suvornmongkhul (1996) has demonstrated that soil-to-root transfers of 137Cs are enhanced when this radionuclide is supplied at the base of soil columns with fixed water tables, compared with columns with bands of 137Cs contamination at the surface. Bishop and Beetham (1989) have reviewed the literature indicating that deep-rooted plant species can enhance radionuclide transport up soil profiles, but it is clear from the present study that even species such as wheat, which is not normally considered to be deep rooting, can augment physical transport of radionuclides in soils.

Statistical examination of radionuclide activity concentration profiles in deep and shallow lysimeter soil profiles in this study has indicated that, over the four-year experimental period, significant variation in the characteristics of the radionuclide profiles occurred (see results of ANOVA in Table 4). Just as the degree of vertical transport of 137Cs and 60Co is somewhat surprising, so is the observation that the year-to-year variability in distributions of these radionuclides was significant. For the less highly sorbed 22Na, not only did the absolute activity concentrations within lysimeters vary significantly from year to year, but the vertical distributions of 22Na within the soil profiles also changed in a significant manner, as indicated by the highly significant interactions shown in Table 4. This is as expected since radionuclides with Kd values as low as those of Na should exhibit sufficient mobility within the soil–plant system to result in pronounced seasonal fluctuations in their profiles due to capillary rise in the summer months and downward flushing in the winter months.

Crop root density is a parameter that exhibits considerable variability from year-to-year and from season-to-season, though in the summary of data presented here only year-to-year variability is discussed. Climatic controls on root growth in the lysimeter experiment are not as pronounced as might have been expected. The major apparent effect of the dry spring and summer of 1990 on root distribution in the lysimeters was to reduce root density at the surface of the deep lysimeters and to increase root density in deeper parts of the deep lysimeters. This effect was also visible, though less pronounced, in the shallow lysimeters in 1990. This climatically induced skew in root distributions in 1990 may well have been at least partially responsible for the high degree of radionuclide uptake by the wheat crop and the rapid movement of radionuclides up both deep and shallow soil profiles in that year.

In terms of the two overall measures of soil-to-crop transfer used in this study, the simpler was the so-called inventory ratio (IR). This is useful as it gives some indication of the total removal of radionuclides from a soil by a crop, an issue of major relevance to the use of plants to phytoremediate contaminated soils. The highest IR values in this study were recorded for 137Cs in both deep and shallow lysimeters in 1990 although, as already noted above, less than 1% of the total soil inventory of 137Cs was transferred to the crop tissues at harvest, clearly insufficient transfer to enable efficient phytoremediation. The general decline in IR values calculated for each radionuclide over the four-year experiment was most likely due to increasing interaction with the soil, especially in the case of 60Co and 137Cs: the latter is known to undergo strongly time-dependent fixation in soils.

As described in the Results section, incorporation of direct measurements of rooting depth and density with measurements of vertical distributions of radionuclides within the soil profile allows variations in both distributions to be combined simultaneously to provide a summary statistic, the weighted soil–plant transfer factor (TFw), which can be used to evaluate the soil-to-crop transfer of a radionuclide. In the context of the experiment reported here, this allows a more realistic interpretation of the relative degree to which radionuclides are transferred into crops than more straightforwardly defined transfer factors such as that of the International Union of Radioecologists (1989). This issue is of considerable importance to the quantification of plant uptake of contaminants from buried sources for which the main area of contamination lies below the plant rooting depth.

One of the key observations made consistently throughout this study is that TFw values for each radionuclide are significantly higher for deep lysimeters than for shallow lysimeters. This is unexpected, since the density of crop roots in the vicinity of the water tables in the deep lysimeters was lower than in shallow lysimeters. This result suggests, however, that the smaller fraction of the total crop root system residing in the lowermost regions of deep soil profiles is more efficient in absorbing radionuclides than in shallow lysimeters. As seen from Fig. 6, the prevailing redox potentials at the bases of deep lysimeters are, on average, lower than those at the bases of shallow lysimeters. This is a major reason why crop roots are less prolific at the bases of deep lysimeters, although survival of a small number of wheat roots is possible over a short time period even under very reducing conditions, due to the ability of wheat plants to form oxygen-transporting aerenchyma within nodal root tissues (Thomson et al., 1990). Thus, when sampling soil cores, red sheaths of oxidized iron are sometimes seen surrounding individual roots. However, for the small number of roots growing in such reducing conditions the bioavailability of radionuclides may be significantly enhanced due to direct or indirect changes in radionuclide solubility or sorption to soil surfaces. It is, therefore, perhaps the physico–chemical conditions occurring at the interface between saturated and unsaturated zones of the soil that are instrumental in controlling the eventual crop uptake of radionuclides emerging from the subsurface.

Significant time-dependent effects occurred with respect to crop uptake of radionuclides during the course of the experiment. From 1990 to 1991 there was a significant reduction in the TFw for 137Cs and 60Co, and an associated reduction in the WMAC for these two radionuclides with time. From 1991 onward, the TFw remained low for these two radionuclides. The consistent decline in TFw for 137Cs and 60Co is likely to be associated with fixation processes within the soil, which have the effect of reducing both the physical mobility and bioavailability of radionuclides with time following their introduction into the soil system. This is particularly well documented for radiocesium (Cremers et al., 1988). In contrast, the TFw for 22Na increased over the four years of the experiment and from 1991 22Na exhibited the highest degree of soil–plant transfer of all the gammaemitting radionuclides present.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
In this study, a little-investigated ground water contaminant pathway has been examined in the context of the underground disposal of intermediate-level and low-level radioactive wastes. Vertical migration of radionuclides upward from a subsurface source has been studied in a specially designed lysimeter system operated over four consecutive years (1990 to 1993, inclusive). Migration from deep (65 cm) and shallow (35 cm) water tables was more rapid than expected, with breakthrough of 137Cs, 60Co, and 22Na occurring at the soil surface after only 6 mo. This is thought to be partly due to favorable water fluxes within the lysimeter system during the spring and summer of 1990, which were drier than the subsequent three years. However, it is also considered that transport of radionuclides to the soil surface was partly attributable to root uptake and translocation. Crop uptake efficiency, as measured by the weighted soil–plant transfer factor (TFw), was consistently and significantly greater for deep lysimeters in which root densities in the region of peak soil contamination, just above the water table, were lowest. It is thought that physico–chemical conditions in this region of the soil profile enhance bioavailability of radionuclides. It thus appears that crops such as wheat have a considerable ability to scavenge radionuclides from the subsurface in which such physico–chemical conditions prevail.


   ACKNOWLEDGMENTS
 
The authors would like to thank United Kingdom Nirex Limited for financial support for the Silwood Park lysimeter study (under the Nirex Safety Assessment Research Programme) and, specifically, Dr. M.C. Thorne, Dr. I. Crossland, and Dr. P. Degnan for their thorough reviews of the manuscript. This study would not have been possible without the excellent technical work of Steve Burne, John Ions, and Hu Qing, to whom we are indebted. We dedicate this paper to the memory of John Ions.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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G. Shaw, P. Wadey, and J. N. B. Bell
Radionuclide Transport Above a Near-Surface Water Table: IV. Soil Migration and Crop Uptake of Chlorine-36 and Technetium-99, 1990 to 1993
J. Environ. Qual., November 1, 2004; 33(6): 2272 - 2280.
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