The distribution of fallout 137Cs and 210Pb in undisturbed and cultivated soils

Appl. Radiat. lsot. Vol. 48, No. 5, pp. 677-690, 1997

~

Pergamon

PII:S0969-8043(96)00302-8

~: 1997 ElsevierScienceLtd. All rights reserved Printed in Great Britain 0969-8043/97 $17.00+ 0.00

The Distribution of Fallout 137Cs and 21°Pb in Undisturbed and Cultivated Soils Q. H E and D. E. W A L L I N G * Department of Geography, University of Exeter. Exeter, EX4 4R J, U.K. (Received 25 July 1996; in revL~ed Jbrm 23 September 1996)

The affinity of bomb-derived fallout ~37Csand naturally-occurring fallout "°Pb for soil and sediment particles make them valuable sediment tracers, and they have been used in a wide range of environmental investigations. A knowledge of their behaviour and distribution in soils is vital for understanding their movement within the environment and therefore for interpreting the information that they provide as sediment tracers. The study reported in this paper combines both empirical evidence and theoretical reasoning to develop an improved understanding of the distribution of fallout h37Cs and 2L°Pb in undisturbed and cultivated soils. Results from field experiments suggest that the initial distribution of these radionuclides in topsoils is approximately exponential. The primary factors influencing the post-depositional redistribution of these radionuclides in stable undisturbed soils have been represented as effective diffusion and convection processes, and a one-dimensional transport model has been employed to describe temporal changes in their vertical distribution in the soil profile. Cultivation and soil erosion are the dominant processes controlling their vertical distribution in cultivated soils. The information obtained is essential for exploiting fully the potential for using these fallout radionuclides as tracers in studying soil erosion, sediment delivery and sediment deposition, and associated sediment budgets. ~; 1997 Elsevier Science Ltd

Introduction The fallout radionuclides '37Cs (half-life 30.2 yr) and unsupported 2~°Pb (half-life 22.2 yr) have a strong affinity for soil and sediment particles (cf. Tamura, 1964; Sawhney, 1972; Bolt et aL, 1978; Livens and Baxter, 1988a, 1988b; Livens and Rimmer, 1988; Van Hoof and Andren, 1989; Ritchie and McHenry, 1990; He and Walling, 1996a), and as a result they have been widely used as environmental tracers for studying sediment deposition in fluvial, lacustrine, marine and other sedimentary systems (e.g. Appleby and Oldfield, 1978; Krishnaswami and Lal, 1978; Robbins, 1978; Benninger and Krishnaswami, 1981; Smith et al., 1987; Walling and Bradley, 1989; Ritchie and McHenry, 1990; Robbins et al., 1990; Ely et al., 1992; Walling and He, 1992; Abril and Garcia-Le6n, 1994; Whicker et al., 1994; He and Walling, 1996b; Walling et al., 1996) and erosion and sediment delivery processes in drainage basins (e.g. Campbell et al., 1986; Martz and de Jong, 1991; Walling and Quine, 1990, 1991; Sutherland, 1992; Walling and Woodward, 1992; Elliott and Cole-Clark, 1993; Walling et al., 1993, 1995; Loughran and Campbell, 1995; He and Owens, 1995; Quine, 1995; Wicherek and Bernard, 1995). Although both radionuclides are *To whom all correspondence should be addressed.

used as sediment tracers in similar ways, they differ in both their origin and the temporal pattern of their fallout. 137Cs is a man-made radionuclide associated with the testing of nuclear weapons, and its deposition from the atmosphere has exhibited significant temporal variation. Global fallout commenced in the early 1950s in the Northern Hemisphere, with a maximum deposition rate in 1963, the year of most intensive nuclear weapons testing (cf. Fig. 1). Since the late 1970s, rates of 137Cs deposition have been very low, although, in some parts of Europe and adjacent regions, an additional short-term input of '37Cs was received in 1986 as a result of the Chernobyl nuclear accident (cf. Cambray et al., 1989). Figure 1 depicts the atmospheric 137Cs deposition fluxes observed at Milford Haven, U.K., from 1954 to 1984, based on data reported by Cambray et al. (1989). In contrast, the deposition of naturally-occurring fallout '~°Pb has been relatively constant through time (cf. Turekian et al., 1977; Appleby and Oldfield, 1978; Krishnaswami and Lal, 1978; Nozaki et al., 1978; Robbins, 1978; Crickmore et al., 1990). 2~°Pb is a product of the 23~Udecay series. It is derived from the decay of gaseous 222Rn, the daughter of Z~rRa. 226Ra occurs naturally in soils and rocks and diffusion of a small proportion of the t u r n from the soil introduces 2~°Pb into the atmosphere. Fallout 2~°Pb is commonly termed unsupported or excess ~"°Pb when incorpor677

678

Q. He and D. E. Walling 140

>"

105

E 0 O"

m

70

X

1954

1960

1966

1972

i978

1984

Year Fig. 1. '~TCsdeposition fluxes observed at Milford Haven, U.K. (based on data reported by Cambray et al., 1989).

ated into soils or sediments in order to distinguish it from the 2'°Pb produced in situ by the decay of 226Ra. Although their temporal patterns of fallout differ, the deposition of both '37Csand 2'°Pb occurs primarily in association with precipitation (cf. Peirson and Salmon, 1959; Mishra and Sadasivan, 1972; Nevissi, 1985; Ritchie and McHenry, 1990), and their initial distribution in the topsoil will reflect the adsorption of the radionuclides by the surface horizon of the soil as the precipitation infiltrates into the soil (cf. He, 1993; Bunzl et al., 1994; Owens et al., 1996). Post-depositional redistribution of both radionuclides within the soil will occur in response to a range of physical, physico-chemical and biological processes operating in the soil system, which will in turn result in temporal changes in the radionuclide distribution in the soil profile. A knowledge of the initial distribution of fallout a37Csand :~°Pb in surface soils and their post-depositional redistribution in soils is central to understanding their movement within the environment. For example, because of their adsorption by soil particles, the transfer of these radionuclides within the environment will be associated with the mobilisation of sediment particles from the catchment by erosion and their transport to the river network and to contemporary or permanent sinks such as lakes and reservoirs, and will therefore be closely controlled by their behaviour in catchment soils (cf. Carlsson, 1978; Wise, 1980; McCall et al., 1984; Robbins, 1985; Dominik et al., 1987; Smith and Ellis, 1982; Smith et al., 1987; Wan et al., 1987; Walling and He, 1992; Monte, 1995). Furthermore, a knowledge of the initial distribution of ~37Cs and unsupported 2'°Pb in topsoils and of temporal changes in their distribution in the soil profile will be important for interpreting the information provided by these radionuclides when they are used as tracers to study soil erosion and to establish the depositional history of sediment deposits (e.g. Smith et al., 1987;

Wan et al., 1987; Walling and Quine, 1990, 1991; Kachanoski, 1993; Walling and He, 1992, 1993; Walling et al., 1995, 1996). Numerous studies have been undertaken to investigate the distribution of 137Cs in soils. These have included laboratory and field experiments, field observations and mathematical modelling (e.g. Pegoyev and Fridman, 1978; Bachhuber et al., 1982; Frissel and Pennders, 1983; Walling and He, 1992, 1993; He, 1993; Garcia-Oliva et al., 1995; Bunzl et al., 1994; Schimmack et al., 1994; Shand et al., 1994; Owens et al., 1996). However, the initial distribution of fallout '37Cs in topsoils and its implications for studies employing '37Cs as a tracer have received only limited attention. Furthermore, existing studies of the mechanisms associated with the post-depositional redistribution of fallout 137Cs in undisturbed soils are primarily based on analysis of results obtained from column experiments, and many researchers have attributed the movement of '37Cs in soils to the reversible adsorption of 137Cs by soil particles (i.e. physico-chemical processes). A retardation factor Rd is commonly introduced into the one-dimensional transport equation that describes the movement of ~37Cs in the soil solution (e.g. Bachhuber et al., 1982; Reynolds et al., 1982). However, it should be recognised that the parameters derived from such column experiments may not be representative of natural soils receiving continuous fallout inputs, because both the radionuclide concentrations and the rainfall intensities employed are frequently unrepresentative of natural conditions. Since redistribution of ~37Csin natural soils involves a complex suite of processes including bioturbation and mechanical mixing, these must also be taken into consideration. Further investigation of the distribution of fallout 137Csin soils is therefore required to provide an improved understanding of its behaviour as a sediment tracer. For fallout 2r°pb, there is less published information concerning its distribution in

Fallout ~37Csand 2~°Pbin soils soils (cf. Nozaki et al., 1978; D6rr and Miinnich, 1989, 1991). However, similar information is again an important requirement for its successful application as an environmental tracer. The study reported in this paper combines field experiments and observations and theoretical reasoning to develop an improved understanding of the distribution of fallout ~37Cs and 2~°Pb in both undisturbed and cultivated soils. Experiments were undertaken to study the initial distribution of mCs and 2"'Pb fallout inputs in the surface horizons of soils. Soil cores were also collected from field sites in order to examine the distribution of the radionuclides in catchment soils receiving longer-term continuous fallout inputs. Based on an analysis of the results obtained and identification of the factors likely to affect the mobilisation of both ~'Cs and fallout -~°Pb in undisturbed and cultivated soils, models have been developed to represent the various processes involved. The results obtained have implications for the use of these radionuclides as tracers in the study of soil erosion, sediment transport and sediment deposition and of associated sediment budgets.

Data Acquisition

679

soils, soil cores ca. 15 cm in length were taken from the middle of the plots and returned to the laboratory. Additional soil cores from adjacent areas outside the plots were also collected to provide information on the pre-existing distribution of unsupported 21°pb in the soils. The soil cores were sectioned into < 5 mm slices and weighed. After air-drying, samples were re-weighed, ground and analysed for ~4Cs and 2~°Pb concentrations. Collection qf soil cores f?om undisturbed and cultivated soils

In order to study the distribution of fallout ~'TCs and 2"Pb in natural soils receiving continuous fallout inputs, soil cores ca. 40 cm in length were collected from both undisturbed permanent pasture and cultivated fields in the catchment of the Jackmoor Brook (Pasture soil 2 and Ploughed soil 2) and the River Start (Pasture soil 3 and Ploughed soil 3), Devon, U.K., for '37Cs and -~t°Pbmeasurements. The characteristics of these soils (top 5 cm) are again listed in Table 1. The soil cores were sectioned into 1-2 cm increments in the laboratory and the resultant soil samples were air-dried, ground, and analysed for ~'Cs and 2"'Pb concentrations.

Experiments and collection q[ topsoil cores

Laboratory analysis

in order to study the initial distribution of fallout ~3~Csand -'"~Pbin surface soils, a solution spiked with ~'~Cs (which has the same chemical properties as t37Cs) and :~°Pb was applied using a sprinkler to a 4 m-' plot of soil in both a pasture field (Pasture soil 1) and a bare cultivated field (Ploughed soil 1) within the catchment of the Jackmoor Brook, Devon, U.K. The basic characteristics of the soils (top 5 cm) are listed in Table I. The equivalent rainfall was about 40 mm with the intensity controlled at the level of typical rainfall events in the catchment, t34Cs was used as a surrogate for '3;Cs in these experiments because surface soils in the catchment already contained fallout ~3'Cs. The resultant "4Cs concentrations in the soils were comparable with those of ~37Csencountered in natural soils. A surrogate for 2~°pb was not available, and as a result the concentration of 2~°Pbin the applied solution was such that the resulting unsupported ~"°Pb concentrations in the surface soil were about 10 times those of fallout '~°Pb in natural soils. This permitted the newly applied 2~°Pb to be readily distinguished from the existing unsupported -~'"Pb in the soils. About 1 h after the cessation of sprinkling, when the water had infiltrated into the

Measurements of ~TCs and unsupported :"'Pb activities in the soil samples were undertaken simultaneously by gamma-ray spectrometry, using a high-resolution, low-background, low-energy, hyperpure n-type germanium coaxial y-ray detector (EG and G ORTEC LOAX HPGe) coupled to an ORTEC amplifier and multichannel analyser. The samples were contained in plastic pots (inner diameter 7.0 cm and height 8.0 cm~ and sealed for 20 days prior to assay in order for equilibrium between 226Ra and its daughter ~22Rn (half-life 3.8 days), an inert gas, to be achieved. The efficiencies of the detection system were calibrated using soil samples prepared with standard solutions containing U4Cs, "Cs, ~°Pb and ~26Ra. In order to calibrate efficiencies for samples of varying geometry, these sources contained different masses of soil. The samples were placed on top of the detector head and counted for over 50000 s, providing a precision of ca. + 10% at the 90% level of confidence for the gamma-ray spectrometry measurements. The activities of '34Cs and " C s in the samples were obtained from the counts at the 605 keV peak and those at the 662 keV peak,

Table I. Characteristics of the soils studied Total organic carbon (%) Pasture soil 1 Pasture soil 2 Pasture soil 3 Ploughed soil 1 Ploughed soil 2 Ploughed soil 3

4.25 6.30 4.52 1.52 3.45 1.44

Clay ( < 2 ~m) (%)

Silt (2 63 lam) (%)

Sand ( > 63 tam) {%)

pH

13.3 10.2 12.1 11.9 8.9 11.2

53.5 69.3 50.8 44.6 63.5 49.3

33.2 20.5 37.1 43.5 27.6 39.5

5.1 5.5 4.9 6.7 6.3 7. I

680

Q. He and D. E. Wailing

respectively, in the measured "/-ray spectrum. The total 2~°Pb concentrations of the samples were obtained using the 46.5 keV gamma ray from 2'°Pb, and the :-'6Ra concentrations were obtained using the 351.9 keV gamma ray from 2'4pb, a short-lived daughter of 226Ra. Unsupported 2~°Pb concentrations of the samples were calculated by subtracting the 226Ra-supported 2'°Pb concentrations from the total 2'°Pb concentrations (cf. Josh/, 1987). The grain size distributions of samples < 63 lain were measured using a laser-diffraction apparatus (Malvern MasterSizer) following standard procedures. The > 63 lam fractions were obtained by sieving. The pH values of the samples were measured using a pH meter at 25°C and measurement of the total organic carbon content was undertaken using a CarloErba C/N Analyser following standard analytical procedures.

Results and Discussion Initial distribution offal~out radionuclides in surface horizons of soils Figure 2 plots the initial distributions of '3"Cs and unsupported :'°Pb concentrations against cumulative dry mass depth for the topsoils obtained from the experiments. The pre-existing unsupported 2'°Pb content of the samples, as represented by the unsupported :]°Pb content of the soil cores collected from outside the experimental plots, was taken into account by subtracting it from the measured total unsupported 2]°Pb content. Cumulative mass is used as a measure of depth in order to avoid the need to consider soil compaction effects on the the radionuclide profile shapes. Concentrations of both radionuclides decrease with increasing depth from the soil surface. Over 90% of both '34Cs and 2'°pb in

Unsupp. 21°pb Activity (mBq g -1)

la4Cs Activity (mBq g-l) 0.0 0.0

16.0 A

24.0 I

32.0 I

jJ

0.8

A

8.0 I

0 0.0

0.8

1.6

2.4

2.4

350 m

525 m

I

jJ

A

1.6

175 '

700 '

B

E

¢3

3.2

v

3.2

_~

~0

Undisturbed

Undisturbed a

4.0

4.0

@

>

0.0 E

0.0

8.0 =

16.0 =

I

o

24.0 I

32.0 I

I

0 0.0

c

350 I

I

o.e

i

0.8

1.6

--

1.6

2.4

--

2.4

3.2

.

3.2

525 =

I

700 J

D

Cultivated

Cultivated 4.0

175 =

4.0

Fig. 2. The initial distributions of 134Cs and 2'°Pbobserved in a pasture topsoil (A and B) and in a ploughed topsoil (C and D) aRer radionuclide application by sprinkling.

Fallout '~TCsand 2~°pbin soils Unaupp. =l°Pb Activity (mBq g-l)

~STCs Activity (mBq g-~) 12.0 i

0.0

0.0

24.0

36.0

I

I

681

48.0 i

0

35 I

0.0

70

105

&

140

I

!

I 4.0

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j-

8.0

•, - '7

12.0

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v

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1

5-

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I

B

12.0 .

r

E

_

16.0 .

16.0

a

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River Start 2 5 0 mBq cm ~

20.0

River Start 520 mBq c r n

20.0 _

~

Q

0.0 "

E ,...i

9.0 '

0.0

18.0 I

4.0

8.0

27.0 !

I

r

36.0 i

t °

i

_

0 0.0

4.0

.

8.0

_

j12.0 .

16.0 _

16.0

_

Jackmoor Brook 2 0 0 mBq c m "-2

50

75

I

I

!

l

12.0 _

20.0

25

20.0

F-

]

100 I

D

f Jllckmoor Brook 305 mBq om ~

Fig. 3. '~TCsand fallout 21°pbprofiles associated with cores collected from undisturbed soils in the River Start catchment (A and B) and the Jackmoor Brook catchment (C and D). the soil profiles is contained in the top 1.5 cm. If erosion occurs, the sediment mobilised from the surface will contain higher levels of fallout radionuclides than sediment originating from lower in the soil profile. The decrease in 134Csand 2'°Pb concentrations with soil depth reflects the nature of the adsorption of these radionuclides by soil particles. Catchment soils can be treated as porous media. When rain water infiltrates into a soil, the radionuclides contained in the water can be transported downwards into the soil profile. Most of the radionuclides in the water will, however, be adsorbed by the surface soil during the infiltration process. For example, physical adsorption of water and associated radionuclides by soil particles will occur and these radionuclides may subsequently be adsorbed through chemical reactions with soil particles. Adsorption of those radionuclides in the infiltrating water that are in direct contact with pore walls will occur through chemical processes.

The remaining radionuclides in the water will move downwards with the water. The radionuclide concentrations in the infiltrating water will therefore decrease with increasing depth. When the rainfall ceases and the water has penetrated completely into the soil, a stable distribution of water in the surface soil will be temporarily established and the t'allout radionuclides remaining in the water will be gradually adsorbed by soil particles. The resulting initial distribution of fallout radionuclides in surface soil will therefore depend upon the rainfall intensity and duration, and the physical and chemical properties of the soil and its antecedent moisture status. Fallout ~TCs and 2t°pb in undisturbed soils receiving continuous inputs Figure 3 shows the distribution of ~TCs and unsupported 2~°Pb in two soil cores collected from stable undisturbed permanent pasture fields in the catchments of the River Start and Jackmoor Brook.

682

Q. He and D. E. Walling

These profiles are representative of those encountered in most undisturbed soils (cf. Walling and He, 1992; Owens et aL, 1996; Walling et al., 1995, 1996). Both radionuclides extend down to a depth of about 15 cm in the soil profile, which is substantially deeper than the depth of the initial distribution of ~34Csand 2~°Pb in surface soils obtained from the experiments. This indicates that both '37Cs and unsupported 2'°Pb have been redistributed in the soil profiles after deposition from the atmosphere. The precise features of the vertical distribution of fallout radionuclides in soils can therefore be expected to change through time. The shapes of the two mCs profiles depicted in Fig. 3 have the following common features. Firstly, there is a peak in both ~37Csprofiles and the position of the maximum concentration is not at the soil surface, but several centimetres below the surface. Secondly, the peaks are broad, with half-height widths, defined as the distance from the position of the maximum concentration downwards to the position where the ~37Cs concentration is half the maximum concentration, generally of the order of several centimetres. Finally, there is a downward tail in the lower part of both '37Cs profiles. Differences between the two ~-~TCsprofiles also exist in terms of the magnitude of the "VCs concentration in surface soil, peak position and concentration, maximum penetration depth and the total inventory. For example, the core from the catchment of Jackmoor Brook has a '37Cs concentration of ca. 20.0 mBq g in surface soil and a total inventory of ca. 200 mBq cm 2, while the core from the River Start catchment has a " C s concentration of ca. 31.9 mBq g Lin surface soil and a total inventory of ca. 250 mBq cm 2. The mCs peak of the core from the catchment of Jackmoor Brook is at a depth of 3 . 0 g c m -2, 1 . 5 g c m : greater than the value of 1.5 g cm -' for the core from the catchment of the River Start. The difference in the total mCs inventory between the cores reflects the difference in precipitation between the two catchments. The mean annual rainfall in the catchment of Jackmoor Brook is ca. 850 ram, while that in the catchment of the River Start is ca. 1150 mm. The concentration of fallout 2~°Pb in both soil cores is at its greatest at the surface and decreases with increasing depth. Again, differences in the vertical distribution of unsupported 2~°Pb between the two cores exist. The fallout z~°Pbconcentrations in surface soil are 113.9 mBq g - ~ for the core from the River Start catchment and 70.5 mBq g - ' for the core from the Jackmoor Brook catchment, The core from the catchment of the River Start has a total inventory of ca. 520 mBq cm -2, which is ca. 70% higher than the value of 305 mBq cm-~ for the core from the Jackmoor Brook catchment. The difference in the total fallout 2'°Pb inventory between the two cores may reflect the difference in annual precipitation refered to above, as well as the difference

in soil geology between the two catchments. Assuming constant deposition fluxes, the atmospheric 2'°pb deposition fluxes are estimated to be 9.5 mBq cm 2 yr ' for the catchment of Jackmoor Brook and 16.2 mBq cm 2 yr ' for the catchment of the River Start. These values lie within the range of deposition fluxes of fallout 2~°Pb in Britain reported by Crickmore et al. (1990). Redistribution of fallout radionuclides in an undisturbed stable soil will reflect the influence of a range of physico-chemical and biological processes operating in the soil system. Physico-chemical processes, including diffusion and convection, can play an important role in the redistribution of mCs and unsupported 2J°Pb in soils. For example, when ion exchange is responsible for the adsorption of fallout mCs and 2~°Pb by soil particles and the reaction is reversible, then both mCs and unsupported 2'°Pb can be replaced by other ions and re-enter the soil solution. Radionuclides released from one site in the soil can be re-adsorbed at another site, and they may be transported downwards in the pore water. Radionuclides in solution can also be subject to molecular diffusion. These processes will be affected by the soil mineralogy and soil chemical and physical properties. For example, Squire and Middleton (1966) found that the movement of '37Cs in a soil with a high clay content was less than that in other soils. Bioturbation associated with vertical mixing by soil fauna can represent an important mechanism redistributing fallout radionuclides by redistributing soil particles containing fallout radionuclides within the soil profile. For example, Telfair and Luetzelschwab (1963) found that the high levels of fallout radionuclides in deeper soils could be attributed to transport by earthworms or other soil organisms. Fallout t37Cs a n d 2~°pb in cultivated soils receiving continuous inputs

Figure 4 presents the '37Cs profiles (A and C) and unsupported z'°Pb profiles (B and D) for two soil cores collected from cultivated fields in the catchments of the River Start and Jackmoor Brook. In contrast to their distribution in undisturbed soils, the radionuclide concentrations are almost uniform throughout the plough layer as a result of mixing associated with cultivation. The total '37Cs inventories are 220 mBq cm-2 for the core from the catchment of the River Start and 197 mBq cm -2 for the core from the Jackmoor Brook catchment, respectively. The total fallout 2~°Pb inventories for the two cores are 450 mBq c m - 2 and 295 mBq cm-2, respectively. Mean concentrations of mCs and unsupported 2'°Pb in the plough layer are ca. 13.4 mBq g - ' and 24.5 mBq g-~, respectively, for the core from the River Start catchment, and ca. 6.5 mBq g-~ and 8.2 mBq g-~, respectively, for the core from the Jackmoor Brook catchment. The concentrations of both ~37Cs and unsupported 2'°Pb in the surface

Fallout ~37Csand 2'°pb in soils laTCs Activity (mBq g -1) 0.0

A

0.0

5,0 i

10.0 i

15.0 i

Unsupp. 21°pb Activity (mBq g-1) ~.0 ,

0.0

6.0

12.0

12.0

1

24.O

~

3o.o

24.0 River Start 2 2 0 mBq cm -=

27.0 i

~.0 i

1

I

o

w IB

18.0 i

]

18.0

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9.0 t

0.0

6.0

18.0

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River Start 4 5 0 mBq cm -2

30.0

Q

E

0.0

0.0

3.0 I

6.0

9.0 t

I

0

]

8.0

24.0 .

32.0

_J

0.0

0.0

5.0 I

10.0

[

Jackmoor Brook 197 mBq crn -2

20.0 t

15.0

I

I

C

I /

16.0

40.0

12.0 t

D 8.0

16.0

1 24.0

32.0

_J 40.0

Jackmoor Brook 2 9 5 mBq cm -2

Fig. 4. The 137(_;sand fallout 21°pbprofiles associated with cores collected from cultivated soils in the River Start catchment (A and B) and the Jackmoor Brook catchment (C and D).

soil of these two cores are significantly lower than those for the two equivalent undisturbed soil cores (cf. Fig. 3). The differences in the radionuclide concentrations between the two cores primarily reflect the different plough depths and total fallout -'~°Pb deposition fluxes in the two catchments. The plough depth associated with the core from the Jackmoor Brook catchment is ca. 30.7 g cm --~ (26 cm), while that for the core from the River Start catchment is ca. 17.0 g cm -2 (18 cm). The ~37Csand unsupported 2~°pb inventories for the core from the River Start catchment are significantly lower than those measured for the stable undisturbed soil core. and, assuming uniform deposition fluxes of fallout 137Cs and 2~°Pb across the catchment, this indicates that loss of these radionuclides associated with soil erosion has occurred at the coring site (see Section 5). The ~37Cs and unsupported 2~°Pb inventories for the core from the Jackmoor Brook catchment are close

to those for the stable undisturbed soil core from the catchment, indicating that negligible soil erosion has occurred at the point where the core was collected.

Modelling t h e D i s t r i b u t i o n o f F a l l o u t 2'°Pb in Soils

|37Cs a n d

Initial distribution ~[.fallout ~TCs and :'°Pb in topsoils The results obtained from the experiments demonstrate that both ~34Cs and unsupported 2~°Pb concentrations in topsoils decline rapidly with increasing cumulative mass depth. In the case of a soil receiving continuous fallout t37Csand 2~°Pbinputs, the annual deposition flux can be approximated as a pulse input l,(t') (mBq c m - : yr -~) (n = ~37Cs or unsupported 2~°Pb), although in reality it will be associated with many rainfall events of different

684

Q. He and D. E. Walling

intensity. He (1993) has suggested that the initial distribution of a fallout radionuclide associated with precipitation inputs to a soil can be approximated as:

c,,.~(z,t') : ~l,(t') e_ :.~,

(1)

where C,j(z,t') (mBq g ~yr ~) is the radionuclide concentration at cumulative mass depth z (g cm-Z) from the soil surface. The constant /4, (g cm 2) in Eq. (1) can be termed the relaxation cumulative mass depth of the initial distribution, representing the depth of penetration into the soil. High values of H, will imply a deeper penetration of the radionuclide into the soil. For an eroding site, if the erosion rate is known, Eq. (1) can be used to estimate the proportion of the freshly deposited fallout radionuclides removed from the site by erosion. Distribution o f fallout "TCs and 2Wpb in undisturbed soils ~'Cs profiles. The characteristics of the "TCs profiles in undisturbed soils, such as those depicted in Fig. 3, provide a basis for the mathematical description of the post-depositional redistribution of radiocaesium in the soils. Firstly, considering the initial distribution of 137Cs in topsoils as observed in the experiments, the maximum concentration of ~37Cs in a soil profile will be associated with surface soil if downward migration of ~37Cs in the soil has been negligible. Secondly, since the atmospheric ~37Cs deposition fluxes have been very low since the late 1970s, a ~37Cs concentration peak below the soil surface in the soil profile will be an indication of its downward migration in the soil. The ~37Cs fallout deposited in 1963 will be closely related to the 137Cs peak in the soil profile, because the ~37Cs fallout receipt in 1963 was significantly greater than that in other years. Finally, in view of the shallow initial distribution of "7Cs in topsoils and the slow downward migration evidenced in the soils, these broad ~37Cspeaks and the prolonged tails in the lower part of the profiles suggest that dispersion of 137Cs within the soil profile has occurred. Based on these features and the discussions presented in Section 3.2, and assuming that stable undisturbed soils can be treated as semi-infinite homogeneous porous media and that the atmospheric fallout inputs are initially uniformly distributed across the soil surface, a one-dimensional transport model may be used to describe the movement of "VCs in an undisturbed soil profile. In this case, the redistribution of ~37Cs in the soil profile can be treated as being caused by effective convection (producing downward shifting) and diffusion (producing dispersion) processes characterised by a migration rate and a diffusion coefficient. It is stressed that the effective convection and diffusion processes are referred to here in a purely descriptive context to represent the results of all the physicochemical and biological processes operating in the

soil system. Taking into account the radionuclide decay effect, the variation of the ~37Cs concentration Cu,c~(Z,t) (mBq g - t) of the soil with cumulative mass depth z and time t (yr) in the soil profile can be represented by the following partial differential equation: dC.,cs(Z,t) O2Co.cs(z,t) ~,t - Dc~ ~z 2 I/c OCu'cs(z't) - 2c~C,,c,(z,t) Oz

(2)

where Dc~ (g2 cm 4 yr ~) is the effective diffusion coefficient, Vcs (g cm -2 yr -~) is the effective downward migration rate, and 2c~ (yr ~) is the decay constant for "7Cs. The initial condition for equation (2) for an instantaneous input Ic~(t') can be represented by equation (1). Both Dc~ and Vc~ have been assumed to be constant. The values of the diffusion coefficient and migration rate will be influenced by the soil physical, chemical and biological properties as well as by interaction between "TCs and soil particles and the soil as a whole. If the post-depositional movement of ~37Cs in the soil is restricted to the soil profile itself, mass conservation requires the following boundary conditions for Eq. (2) for an initial input Ics(t'): -- Dc~

OCu,c~(z,t) c3z 1:=0 + ~ C,,cs(z,t)l:-o = 0

(3) lira ~C~,cs(Z,t) _ 0

Formal solution C,.cs(z,t,t') ( m B q g ~yr -l) of Eq. (2) for cumulative mass z at time t under the boundary conditions specified in Eq. (3) and initial condition represented by Eq. (1) after t-t' can be obtained for an initial input (cf. Lindstrom and Boersma, 1971; Walling and He, 1993). For continuous fallout inputs, Cu,cs(z,t,t') serves as a Green-function, and the ~37Cs concentration distribution Cu,cs(z,t) (mBq g-~) in the soil profile at time t can be obtained by integrating Cu.c~(z,t,t') over time t' (cf. Walling and He, 1993): Co,cs(z,t) =

Cu,cs(Z,t,t') dt'

(4)

where t~ (yr) is the year when 137Cs fallout commenced, Equation (4) can be used to simulate the ~37Csprofiles developed in stable undisturbed soils. In the case of a soil profile experiencing erosion, the temporal variation of the vertical distribution of t37Cs may be approximated by Eq. (4) when sheet erosion predominates and the erosion rate is low. In this situation, Eq. (4) can be used to estimate the erosion rate and to establish the temporal variation of the "7Cs content of sediment derived from undisturbed surface soils, which is important for

Fallout ~3~Csand -~'°Pbin soils interpreting the ~37Csprofiles in lake sediments where allochthonous materials represent a substantial proportion of the sediment deposits (cf. Walling and He, 1992, 1993). Fallout 2~°Pbprofiles. Assuming that the mechanisms that govern the redistribution of fallout 2~°Pb are similar to those for ~37Cs, Eq. (2) and the associated boundary and initial conditions used to describe the movement of ~37Cs in undisturbed soils may also be used to describe the movement of fallout 2k°Pb in the soils. The values of the parameters involved may, nevertheless, be different from those for ~37Cs in the same soil profile. In the case of unsupported -~t°Pb, however, the situation can be simplified if, as seems reasonable, the deposition flux Iph(t') (mBq cm 2 yr ~) o f " ' P b is treated as constant over time. In this situation, a stable state of unsupported 2~°Pb in an undisturbed soil profile will be achieved due to its radioactive decay (i.e. 3Copu(z,t)/(?t = 0), and Eq. (2) reduces to: Dpb d2Cu'm'(2) dz:

[feb dCu'pb(z) d_-

/~pbCu.Pb(Z)= 0

(5)

where Dpb (g2 cm 4 yr ') is the effective diffusion coefficient, Vp, (g c m 2 yr ') is the effective migration rate and 2p~ (yr ') is the decay constant of :')Pb. The solution C,.pb(z) (mBq g-~) to Eq. (5) is:

C,.~b(z) = C~,pb(O)e-t,:

(6)

where C,.pb(0) (mBq g ~) is the fallout -'°Pb concentration in surface soil, and the constant fl (g ' c m 2) is defined as:

fi = 5

O~b -~- Dpb

D;,b

(7)

Equation (6) can be used to simulate the fallout 2'°Pb profiles in stable undisturbed soils. The concentration of unsupported 2~°pb in the surface soil remains constant.

Distribution q[~fallout ~7Cs and 2~°pb in cultivated soils In addition to the processes controlling the movement of fallout radionuclides in undisturbed soils, cultivated soils are affected by agricultural practices. Both erosional and depositional areas may exist in a cultivated field. Here, only the situation involving erosion will be considered. In the case of a soil profile experiencing erosion, cultivation and erosion will represent the dominant processes controlling the redistribution of both 137Cs and unsupported z~°Pb in the soil profile. In a cultivated soil profile, the soil may conveniently be divided into two layers. The soil properties (including the radionuclide content) in the layer above the plough depth are normally relatively uniform and differ from those in the layer below the plough depth (cf. Fig. 4). Assuming that diffusion and migration of

685

the accumulated fallout radionuclides in the soil can be neglected owing to the continual mixing by cultivation, the radionuclides will be restricted to l~he plough layer. Changes in the activity of accumulated 137Cs and unsupported 21°pb in a soil profile will thus be associated with further deposition of the radionuclides from the atmosphere and loss as a result of decay and soil erosion. ~"~Csprofiles. When the accumulated ~37Csin a soil profile is restricted to the plough layer, the changei of its activity A~.(~(t) (mBq cm 2) per unit area with time will reflect two components: dA~.cs(t)dt - (1 - r,OlcM) -

(

).c~+ ~

A~(.~(t) (8)

where Rc (g cm-2 yr ~) is the erosion rate, and D o (g cm 2) is the cumulative mass depth representing the average plough depth. The first term on the right-hand side of Eq. (8) represents deposition of the atmospheric "TCs fallout, and the second term the loss associated with radioactive decay and soil erosion. Fc~ in Eq. (8) is the percentage of the recently deposited fallout '37Cs which is removed by erosion before being incorporated into the plough layer by cultivation. When the erosion rate R,.<
(9)

Fc~ ~ Hc~

where At = 1 (yr). It is clear from Eqns (8) and (9) that the relaxation mass depth Hc~ is an important element influencing the removal of the recently deposited fallout from the site. For a constant erosion rate, the smaller the value of Hcs, the greater the proportion of the recently deposited fallout that Will be removed by erosion. Hc~ is therefore an important parameter when using fallout '3:Cs measurements to study soil erosion. Solution of Eq. (8) under continuous fallout input I(s(t') yields: Ac.c,(t) = Ac.c,(to) e ,R~o~+~,,)(, ,,,,

+

- r c ~ lcM')e ~Roo+,,.,,,-,,dt, (I0) ,J

where to (yr) is the year when cultivation started, and A~,c~(to) (mBq cm -~) is the ~37Cs inventory at t,,. Assuming that diffusion and downward migratio n of t37Cs into the soil below the plough depth a r e negligible, the concentration C%.c~(z,t) (mBq g t) of the accumulated "7Cs in the ploughed soil at time t is therefore:

Ccc~(Z,t)={A~,c~)/Dp,

z<_DOz>Dp

(11)

If the '37Cs deposition flux IcM'), the total inveniory Ac.cs(t) for the soil profile and the value of Hc i are known, Eq. (10) can be used to estimate the erosion rate. The temporal variation of the !37Cs

Q. He and D. E. Walling

686

concentration of eroded sediment can be estimated using Eqns (10) and (11) (cf. Walling and He, 1992, 1993). Fallout '~°Pb profiles. The above discussions also apply to the redistribution of fallout 2~°pb in cultivated soils. Assuming that both the erosion rate R~ and the deposition fluxes Ipb(t' ) a r e constant over time, the activity A~,pb(t) (mBq cm-2) of fallout 2t°Pb per unit area for a soil profile can be represented as: A~.pb(t) = A~,pb(to) e 'R~°~+;~'-'oJ

-+-

f(, )

-Fpb Ipb(t')

--

e - le~oo +

,.,,i, dt'

0

= A~.pb(to) e iR~.o,+~.,~l~, ,0,+ Ipb(l -- Fpb) R~/Dp + )~pb

where F~,b ~ Rc/Hpb, and A~.ph(to) (mBq cm -2) is the unsupported :~°Pb inventory at time to. The '-~°Pb deposition flux I,b can be estimated from the fallout 2t°pb inventories in stable undisturbed soils. The erosion rate R~ can be estimated from Eq. (12) when the deposition flux Ipb, the plough depth Dp and Hpb are known. |n the case that cultivation has existed for over 100years (i.e. t - to > 100), Eq. (12) reduces to: Ipb - 2pbAc,pb R c --

DpHpb

(13)

A c . p b H p t ' q- I p b D p

The concentration C'~.pb(z,t) (mBq g - ' ) of the accumulated unsupported 2~°Pb in the ploughed soil can be expressed as: C,:.e~(z,t) =

1 (1-- Fpb)Ip~ D o R~/Dp + 2p~ 0

z < Dp z>Dp

(14)

As in the case of undisturbed stable soils, the concentrations of unsupported 2~°Pb in cultivated surface soils and the eroded sediment will be constant. Model

Applications

The models outlined above have been used to replicate the results obtained from the experiments, to interpret the "TCs and unsupported 2L°pb profiles of the two cores from stable undisturbed soils depicted in Fig. 3 and to obtain estimates of erosion rates for the two cores from cultivated soils. Initial distribution

Equation (1) has been used to simulate the initial distribution of tS4Cs and unsupported 21°Pb in topsoils demonstrated by the experiments presented in Fig. 2. The estimated values of Hcs (for 134Cs)

and Hpb (for 2~°Pb) are 0.52 g cm-2 and 0.50 g cm : for undisturbed topsoil and 0.38 g cm-2 and 0.36 g c m 2 for cultivated topsoil, respectively. In the pasture topsoil, both ~34Cs and :~°Pb penetrated deeper than in the cultivated topsoil. This probably results from differences in physical condition, such as soil compaction and permeability, between these soils. The fitted results are plotted in Fig. 5. Undisturbed soils

If the temporal pattern of ~'Cs deposition fluxes to the catchments of the River Start and Jackmoor Brook are assumed to be the same as that shown in Fig. 1 (these two catchments were not affected by the Chernobyl-derived caesium fallout), the record of fallout input can be synthesised by adjusting the magnitude of the annual input according to the total inventories associated with the two cores from undisturbed soils. Equation (4), together with a value of 0.52 g cm ~- for Hc~ estimated for the initial distribution of ~4Cs in undisturbed surface soil from the experiments, can then be used to simulate the "TCs profiles shown in Fig. 3 by changing the values of Vc~ and Dc~ to minimise the sum of the squares of the deviations between the modelled and observed results. The estimated values of Vc~ and D c s a r e 0 . 0 4 g c m -'yr ~ and 0.38g2cm 4yr-~ for the core from the River Start catchment, and 0.07 g cm -~ yr- ~and 0.37 g: cm 4 yr- ~for the core from the Jackmoor Brook catchment. The simulated results shown in Fig. 6 are in good agreement with the observed results. Equation (6) has similarly been used to simulate the two fallout 2'°pb profiles shown in Fig. 3. The estimated values of fl are 0.25 g ~cm 2 for the core from the River Start catchment and 0.28 g -R cm 2 for the core from the Jackmoor Brook catchment, respectively. If the downward movement of fallout 2t°Pb in the soil profile is assumed to occur in the same manner as for mCs, then values of the diffusion coefficient Op b for n°Pb are estimated to be 0.32 g2 cm 4 yr- ' for the core from the River Start catchment and 0.15 g-' cm 4 yr ~ for the core from the Jackmoor Brook catchment [from Eq. (7)], respectively. It can be seen that the values for the diffusion coefficient for 2'°pb estimated using Eq. (7) are significantly different from those for ~'Cs estimated using Eq. (5). If the mechanisms for dispersion of both ~37Cs and unsupported 2~°Pb in a soil are the same, the diffusion coefficients for both 2~°Pb and "TCs could be expected to be similar. One possible explanation for this discrepancy is that the direct downward migration of unsupported 2~°Pb in soils may be less than for ~37Cs (cf. D6rr and Mfinnich, 1991). Unsupported 2~°Pb has also been found to be less mobile than mCs in lake sediments (cf. Davis et al., 1984). If it is assumed that the direct downward migration of 2~°Pb is negligible [with Vpb = 0 in Eq. (7)], Dpb is estimated to be 0.49 g2 cm-4 yr-~ for the core from the River Start

Fallout '37Cs and 2'°Pb in soils l~Cs

0.0

0.0 -

Activity ( m B q O-1)

8.0 i

16.0 ,

24.0 I

U n s u p p . = l ° P b Activity ( m B q O-1) 32.0 I

0

0.8

1.6

1.6

2.4

2.4

E

-4

175 i

0.0

0.8

°=

687

350 ,

525 ,

700 ,

525 I

700 I

3.2 .

v

Undisturbed ~;

4.0

Undisturbed

_

4.0

_

a 0

>

0.0

8.0 ,

0.0

E = °

0.8

_

1.6

_

/

16.0 ,

~

24.0 I

32.0 ,

0

C

3.2

175 *

0.0

350 I

~ 0.8

.

1.6

_

D

3.2 Cultivated

4.0

Cultivsted 4.0

Fig. 5. Model fitted results (smooth curves) for the initial distribution of ~Cs and 2~°Pbrecorded in the field experiments. catchment and 0.40 g2 cm-4 yr ' for the core from the Jackmoor Brook catchment, respectively. These values are closer to the values estimated from the '37Cs profiles. The simulated results are also given in Fig. 6.

Cultivated soils It has been assumed that the deposition fluxes of both fallout '37Cs and 2'°Pb are uniform across the land surface in both catchments and that they are represented by the values estimated for the two cores from stable undisturbed soils. For the cultivated soil core from the Jackmoor Brook catchment, which was collected from a flat area at the top of a ploughed field, erosion has been negligible (both nTCs and unsupported 2'°Pb inventories are close to those in stable undisturbed soil). For the cultivated soil core from the River Start catchment, assuming that cultivation has existed for over 100 years, the erosion

rates were estimated to be 0.04 g cm 2 yr ~usingjthe '37Cs data [Eq. (10)] and 0.03 g cm 2 yr ' usingjthe fallout 2'°pb data [Eq. (13)]. When using Eqns (10) and (13) to estimate the erosion rates, the values of Hcs and Hp~ were set to 0.38 g cm - -' and 0.36 g crn 2, respectively, which represented the least-squ~tres estimated values for the initial distribution of ~!4Cs and 2~°Pb in cultivated surface soils derived from Ithe experiments. The erosion rate estimated using ithe ~37Cs measurements for the core from the River Start catchment is slightly higher than that estimated u~ing the 2'°Pb method, suggesting that erostion rates may have increased in recent years.

Conclusions

Results from the experimental studies reported here indicate that the initial distribution of L37Cs[and 2L°Pb in topsoils when deposited as fallout with ]rain

688

Q. He and D. E. Walling laTCs Activity (mBq g - l )

o.o

0.0

12.0

24.0

I

I

36.0

Unsupp. 21°pb Activity (mBq g - l ) 48.0 !

I

0

4.0

4.0

8.0

8.0

-",

12.0

12.0

OD

16.0

16.0

~W

35 i

0.0

70 i

105 i

140 J

75

100 J

/

m I0

River Start

River Start

~ 20.0 Q

20.0

@

_m

E

o.o

0.0

9.0 I

18.0 I

27.0 I

36.0 J

0 0.0

25 I

50 I

L.~

o 4.0

8.0

12.0

:/

16.0

4.0 _ f

D

8.0 _

12.0 _

16.0 _ Jackmoor Brook

Jackmoor Brook

20.0

20.0 _

Fig. 6. Model simulations (smooth curves) of the fallout mCs and 2'°Pb profiles shown in Fig. 3.

is approximately exponential. This initial distribution can be represented by a cumulative relaxation mass depth, the magnitude of which will be influenced by the interaction between the radionuclides and soil particles, the rainfall regime, and the soil physical and chemical properties. The profiles of fallout '37Cs and 2~°Pb in undisturbed soils and cultivated soils receiving continuous atmospheric fallout inputs are significantly different from those of the initial distribution, reflecting their post-depositional redistribution in the soil. Redistribution of these radionuclides in soils results from a range of physical, physico-chemical and biological processes operating in the soil system. In undisturbed soils, physicochemical processes and bioturbation are the primary factors responsible for the redistribution of ]37Cs and fallout 21°pb in the soil profile, and their vertical distribution in the soil can be described using a one-dimensional transport model characterised by an

effective diffusion coefficient and migration rate. The '37Cs profiles for undisturbed soils generally exhibited a broad peak near the soil surface and a downward tail extending into the lower part of the profile. The distribution of fallout 2]°Pb in undisturbed soils is in an approximate steady state and its concentration decreases approximately exponentially with cumulative mass depth. Both 137Cs and unsupported 2a°Pb extend to roughly the same depth in the undisturbed soils investigated. In cultivated soils, the redistribution of both fallout 137Cs and 2~°Pb in the soil profile is primarily the result of mechanical mixing associated with cultivation, and the radionuclides are almost uniformly distributed in the plough layer at stable and erosional sites. Application of the models presented here could provide a basis for exploring more fully the potential for using fallout 137Cs and 21°pb in investigating erosion on soils with different land use practices and for deriving information on the

Fallout '~'Cs and 2"'Pb in soils temporal v a r i a t i o n o f their c o n c e n t r a t i o n s in m o b i l i s e d s e d i m e n t . T h e latter i n f o r m a t i o n c o u l d be o f p a r t i c u l a r v a l u e w h e n i n t e r p r e t i n g ~ C s a n d 2'"Pb profiles in c o r e s f r o m r e c e n t s e d i m e n t a r y e n v i r o n m e n t s s u c h as lakes, r e s e r v o i r s a n d river f l o o d p l a i n s , w h i c h act as s i n k s for s u c h m o b i l i s e d s e d i m e n t . ,tckmnvledgements The assistance of Dr. P. N. Owens ~lth sample collection and of Mr. J. Grapes with gamma-ray spectrometry, and the cooperation of landowners in permitting access to their land for field experiments and for collection of soil cores are gratefully acknowledged.

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