Effects of acid irrigation and liming on the migration of radiocesium in a forest soil as observed by field measurements

The Science of the Total Environment, 101 (1991) 181-189 Elsevier Science Publishers B.V., Amsterdam

181

EFFECTS OF ACID IRRIGATION AND LIMING ON THE MIGRATION OF RADIOCESIUM IN A FOREST SOIL AS OBSERVED BY FIELD M E A S U R E M E N T S

W. SCHIMMACK and K. BUNZL Gesellschaft fiir Strahlen- und Umweltforschung, Institut fi~r Strahlenschutz, D-8042 Neuherberg, Federal Republic of Germany K. KREUTZER and R. SCHIERL Lehrstuhl fi~r Bodenkunde der Universit(~t, D-8000 Miinchen, Federal Republic of Germany (Received December 15th, 1989; accepted January 29th, 1990)

ABSTRACT The effect of normal irrigation, acid irrigation, and liming on the vertical migration of radiocesium from Chernobyl and global fallout was investigated by determining the activity of '~Cs and I37Cs in the upper horizons (LOft, Of2, Oh, Aeh, Alh, A1) of six experimental plots of a forest soil (spruce stand) as a function of time. For the Of2 and Oh horizons (but not for the LOft horizon) of the control plots during our period of observation (600 days from 30 April 1986), we found a significant increase in the mean residence half-time (T) of Chernobyl-derived cesium from ~ 70 to 500 days, indicating that Cs is sorbed more strongly with time. The different types of soil management showed an effect only in the Of2 horizon. Irrigation, especially when acid, prevents the increase of ~ with time. Liming reduces the rate of migration of Cs significantly, even if the plots are irrigated with normal or acid water. Cesium-137 from global fallout was present mainly in the Oh and Aeh horizons, but was less mobile than Chernobyl-derived Cs by one order of magnitude.

INTRODUCTION Long-term acid deposition on a forest ecosystem can have serious impacts on m a n y p h y s i c o - c h e m i c a l p r o c e s s e s i n t h e s o i l ( A b r a h a m s e n e t al., 1977; H o v l a n d e t al., 1980; U l r i c h e t al., 1980; U l r i c h , 1983; G r i m m e i s e n et al., 1986). I n t h i s c o n t e x t , s i n c e 1984 e x t e n s i v e i n v e s t i g a t i o n s h a v e b e e n c a r r i e d o u t i n t h e ' H S g l w a l d ' , a n o l d N o r w a y s p r u c e s t a n d n e a r M u n i c h ( K r e u t z e r , 1986; K r e u t z e r a n d B i t t e r s o h l , 1986). S i x e x p e r i m e n t a l p l o t s w e r e e s t a b l i s h e d i n o r d e r t o s t u d y the potential disturbances caused by artificial acid irrigation and compensative liming: a control area without any treatment; one plot with additional acid irrigation; one plot with additional normal irrigation; and three corresponding limed plots. I n 1986, d u r i n g t h e c o u r s e o f t h e s e i n v e s t i g a t i o n s , a v a r i e t y o f r a d i o n u c l i d e s were deposited in the canopy and on the forest floor of the HSglwald following t h e r e a c t o r a c c i d e n t a t C h e r n o b y l ( B u n z l e t al., 1989a). T h e a m o u n t o f '37Cs

182

derived from Chernobyl was about 10 times larger than that present in the soil before Chernobyl, which was derived from the fallout from above-ground nuclear weapons testing in the sixties. The migration of radiocesium from both sources, which can be determined separately, has been investigated previously in the untreated control area of the HSglwald field experiment (Bunzl et al., 1989b). The purpose of the present study was to elaborate the effects of acid irrigation and liming on the migration of radiocesium by comparing its activity distributions and mean residence times in the various soil horizons of the irrigated and limed plots with those of the control area. Because it was found that the mobility of Chernobyl-derived cesium was greater than that of global fallout cesium by a factor of three to six in the control area (Bunzl et al., 1989b), it was of interest to investigate whether or not the rates of migration of radiocesium from both sources would also be different in the irrigated and limed plots. MATERIAL AND METHODS

Site and soil The spruce stand 'HSglwald', situated between Munich and Augsburg, 40km northwest of Munich, consists of 85-year-old Norway spruce (Picea abies). The mean annual precipitation is 800mm. The soil is classified as a podzolic Parabrown earth soil (FAO, Orthic acrisol; U.S. soil taxonomy, Hapludult). A typical soil exhibited the following horizons: LOft (7-4.5 cm); Of 2 (4.5-2 cm); Oh (2~} cm); Aeh (0-5 cm); Alh (5-10 cm); A1 (10-30 cm). The corresponding pH values are given in Table 1. The clay content was (percent of soil after removal of carbonate and organic matter): Aeh (19); Alh (21); and A1 (18). And organic carbon content (%): LOft (49); Of2 (49); Oh (40); Aeh (2.8); Alh (1.3); and A1 (0.9). For further details on properties such as soil texture, density, CEC, nitrogen and exchangeable cations, see the report of Kreutzer and Bittersohl (1986).

Experimental plots The experimental plots (area ~ 2500 m 2 each) were: (i) control, (ii) irrigated, (iii) acid-irrigated, (iv)limed, (v) limed and irrigated, (vi) limed and acidirrigated. Normal irrigation (pH 5.5, corresponding to the local precipitation) or acid irrigation (H2SO4, pH2.7) resulted in an additional precipitation of 170 mm per year. Irrigation was accomplished using sprinklers (15-18 events of 10-12mm for 70min each during the growing season of each year). The resulting pH values in the soil are given in Table 1. Lime was applied on one occasion in April 1984 before the first irrigation, using 4000 kg ha 1 dolomite (corresponding to 881 kg Ca ha-1 and 519 kg Mg ha-1 ). The resulting pH values are given in Table 1. Further effects of the above soil managements on various

183 TABLE 1 pH (CaCl~) in the horizons of the forest soil as a result of irrigation and liming, as observed in November 1987, i.e. 3.5 years after the beginning of treatment Soil horizon LOft

Of2

Oh

Aeh

Alh

A1

Control plots (no additional irrigation)

3.5

2.8

2.8

3.1

3.4

3.7

Plots with: Irrigation (pH 5.5) Acid Irrigation (pH 2.7)

3.5 3.2

2.9 2.8

2.9 2.8

3.0 3.0

3.4 3.4

3.6 3.6

Limed plots No additional irrigation Irrigation (pH 5.5) Acid irrigation (pH 2.7)

6.0 5.9 5.7

5.5 5.8 5.9

3.8 4.2 4.4

3.2 3.4 3.4

3.4 3.5 3.5

3.7 3.5 3.6

soil properties, such as CEC or composition of the soil solution in the various horizons, have been studied in detail and can be found in the report of Reiter et al. (1986).

Sampling Six samples were taken between May 1986 and November 1987. Horizons LOft, Of2, Oh and Aeh were sampled with a frame (10 × 10 cm); deeper horizons were sampled using an auger. Six samples were taken from the middle of each plot (~ 3 m from tree trunks) and mixed. To obtain some information on the variability, however, the six samples from each horizon of the control plot were analysed separately.

Procedures Radiocesium was determined in the soil samples by direct gamma spectrometry, using a high-purity germanium detector. In the case of 134Cs, losses by sum coincidences were taken into account. The experimental error obtained from measuring four replicates was about + 10%. All activities reported are corrected for radioactive decay with respect to 30 April 1986, i.e. the day when the Chernobyl plume first arrived. The activity of Chernobyl-derived '37Cs was obtained by multiplying the activity of 134Cs, which is present in the soil only as a result of Chernobyl fallout, by 1.75 (HStzl et al., 1987; reference date 30 April 1986). The activity of '37Cs derived from global fallout is the difference between total 137Cs and Chernobyl-derived '37Cs.

184

Definition of the residence half-time of radiocesium The vertical migration of radiocesium in the forest soil is the result of several processes: leaching (including convective-dispersive mass transport and sorption processes), bioturbation and root uptake. In addition, however, radionuclides present in the LOft horizon will be found, after some time, in the Of2 horizon as a result of fragmentation and microbial decomposition of organic material in the LOft horizon. To a smaller extent, material from the Of2 horizon also undergoes conversion and transportation to the Oh horizon. Because the above processes are quantitatively unknown we did not use a process-based model for data evaluation, but rather a compartmental model (Frissel and Penders, 1983; Boone et al., 1985). The various soil horizons were taken as the compartments, and the rate of cesium transport was characterised by a corresponding residence half-time within each horizon. The transfer of radiocesium activity A i (Bq m 2) in compartment 'i' in a small time interval At (days) is expressed by: AAi At

Ki ,'Ai 1 - K i ' A ~

(1)

where the dimensions of the compartment correspond, in this case, to the soil horizons LOft, Of2, Oh, Aeh, Alh and A1. K i (day 1) is the fractional rate of transfer from compartment ~i' to compartment 'i + 1'. The residence half-time, v~(days), of A~ in compartment ~i' is (1/K~). In 2. Since all A i a r e corrected with respect to 30 April 1986, radioactive decay need not be considered in Eqn (1). The rate of deposition (Bq m 2 day 1) to the first compartment, which has to be known as a function of time in order to solve Eqn (1), can be found in the report of Bunzl et al. (1989b). RESULTS AND DISCUSSION

Chernobyl-derived radiocesium To study the effect of various treatments on the migration of radiocesium, information on the variability of migration within the plot is needed. Determination of the '37Cs activity (Bq m -2) of the six samples from each horizon of the control plot separatelyproduced the following coefficients of variation, CV (%): LOft, 40; Of2, 20; and Oh, 16. The comparatively high CV for the LOft horizon is most probably the result of the rather inhomogeneous composition of this horizon. In the following, we consider the effect of a given treatment (e.g. acid precipitation) is only significant if it produced, in a given horizon, a change in the '37Cs activity which is at least as large as the CV, so that the 95% confidence limits of the respective values do not overlap.

Time dependence of the activity The Chernobyl-derived Cs activity in the organic horizons of the six plots is

185 II

Not limed

Limed

II

o.o, 10

~'~ E

>,

.>_

c~n

~Oh

o 20

LOfl •

LOfl ,~Of2

-ErOf 2

O' 'r'*~~'~ 0 h

,~ 20

Oh

LOfl

LOfl ~

l0

°f2

0

ACid-

irrigated

Of 2

o'~'~

Irrigated

'S

. -"

/.00 0 /.0O Time ( d a y s after 3 0 A p r i l

Oh

800 1986)

Fig. 1. Activity of Chernobyl-derived137Csin the organic horizons of the six experimentalplots in the spruce stand as a function of time after the arrival of the Chernobylplume on 30 April 1986. Activities given are corrected for radioactive decay to this date. shown in Fig. 1. For all plots the activity until the end of 1987 was essentially present in the layers dominated by organic material (O horizons), while only a small fraction of the activity reached the mineral soil. Within the organic layers, however, considerable l~Cs transport was observed. For the LOft horizon of all plots the time dependence of the 137Csactivity per square meter was rather similar and was characterised by an initial increase after 30 April 1986. This was the result of weathering processes (rain, wind, litterfall), which transferred the radionuclides still present in the canopy to the forest floor, with a half-life of ~ 90 days (Bunzl et al., 1989a,b). Because the deposition of radiocesium to the forest floor decreased while the vertical transport of this radionuclide from the LOft to the Of2 horizon continued, the overall 137Cs activity per square meter in the LOft horizon passed through a maximum after ~ 200-400 days. As is evident from Fig. 1, the subsequent decrease in 137Cs activity in the LOft horizon is independent of liming and the type of irrigation. This suggests t h a t the vertical transport of '37Cs from the LOft to the Of2 horizon is not the result of leaching processes, but rather due to comparatively rapid decomposition and transport of organic material from the LOft to the Of2 horizon. Transport of radiocesium in the LOft horizon by earthworms (bioturbation) was negligible in the soil investigated here. In contrast to the LOft horizon, the behaviour of radiocesium in the Of2 and Oh horizons depended on the method of irrigation (see graphs for unlimed plots in Fig. 1). While the activity increased uniformly in the Of2 horizon of the unirrigated plot, this increase was slower for the irrigated plots. As a consequence, the activity in the Oh horizon increased more rapidly in the irrigated plots than in the unirrigated plots. Thus, compared with the untreated plots, additional irrigation (,,-30%) of unlimed plots resulted in a more rapid transport of radiocesium from the Of2 to the Oh horizon. This effect is initially

186

enhanced for acid-irrigated plots. In contrast to the transport of radiocesium from the LOft to the Of2 horizon, leaching processes obviously contribute significantly to the transport of this radionuclide from the Of2 to the Oh horizon. The effect of liming can be studied by comparing limed and unlimed plots. For the limed areas, only a very small increase of Cs activity in the horizon was observed. In this case, obviously, cesium remained for a longer time in the Of2 horizon before it was transported to the Oh horizon. The comparatively slow movement of 137Cs from the Of2 to the Oh horizon can be explained by the enhanced binding of metal ions by organic matter with increasing pH (Stevenson, 1982).

Mean residence half-time of radiocesium The migration of radiocesium in the forest soil, as discussed qualitatively above, can be described in a more quantitative way by calculating the residence half-times for 137Csin each horizon with the help of a compartmental model. The results are shown as a function of time in Fig. 2, where for the sake of clarity the corresponding errors are given only for two residence half-times in the Of2 horizon. The residence half-time in the LOft horizon of the six plots varied between 140 and 280 days, while those of the Of2 and Oh horizons increased, in general, from ~ 70 to 500~00 days. Thus, Chernobyl-derived cesium was obviously sorbed more strongly in the Of2 and Oh horizons with time. The increasing sorption of Cs cannot be the result of a simultaneously increasing pH in the soil horizons, because the pH remained almost constant or even decreased slightly during the period of observation. The effect of the different soil treatments is not perceptible in the LOft horizon, in accordance with the qualitative discussion (see above), but it is clearly evident in the Of2 horizon. For the plots not treated with lime the residence half-time depended on irrigation: if no irrigation was applied, it increased slowly to ~ 500 days. For normal irrigation it increased initially, but

LOll o

0

•

~

~

~

o

C

600

Oh Controt

~

area

Irrigated

2. "- ~

nO

'""""

"~ 300 o

II

i

600 300

g.

Of= • -J-

300 . .--

II

600

...v:.>._I.~~

o ~oo o ~00 Time (days after 3 0 A p r i ( 1 9 8 6 )

~oo

Acid irrigated

8oo

Fig. 2. Residence half-times of Chernobyl-derived 13VCs in the organic horizons of the six exp e r i m e n t a l plots [( ) unlimed; (. . . . ) limed] in the spruce s t a n d as a function of the time after the arrival of the Chernobyl plume on 30 April 1986.

187 then remained almost constant at ~ 300 days. For acid irrigation the residence half-time remained constant at ~ 200 days. The additional irrigation thus mainly produced an enhanced mass flow of radiocesium. For all limed plots a rapid increase of the residence half-time to > 600 days was observed, independent of wh eth er irrigation was applied or not (Fig. 2). Liming thus caused, as a result of the pH increase, enhanced sorption of Chernobyl-derived cesium in the Of2 horizon.

Global fallout-derived cesium While Chernobyl-derived Cs is at present mainly observed in the organic soil horizons, 137Cs from the global fallout of weapons-testing in the sixties is also found in the mineral horizons (see Fig. 3). No systematic differences in the vertical profile of Cs in the soil was detected during our sampling period. Also, no effect of the different types of soil management (irrigation, liming) was detectable. Figure 3 shows, for each horizon, the median Cs activity and its 95% confidence limits, as obtained from all plots (30 values each). It is not surprising t h a t no soil t r e a t m e n t effect was observed for global fallout cesium. Firstly, the 3-4-years period during which the soil was irrigated or limed is short compared with the period of > 20 years available for the migration of global fallout cesium. Besides, as shown above, the effects of the different types of soil m a na ge m ent were observed mainly in the Of2 r a t h e r t han in the Oh horizon. At present, however, only a very small fraction of global fallout Cs is found in the Of2 horizon. For this reason the mean residence half-time in the LOft and Of2 horizons was estimated to be ~<2 years. For the Oh horizon the residence half-time was found to vary between 9 and 12 years, and for the Aeh horizon between 30 and 300 years. For several plots, however, values of ~>300 years were observed for the Aeh horizon. A comparison of the residence half-times of Cs from Chernobyl fallout and Activity

(kBq/m2/cm) 0.5 "

10

1.0

~

LOfl Of 2

0

....

Oh

u

O 10

-

20 Fig. 3. Median activity concentration and 95% confidencelimits of 1~7Csfrom global fallout as a function of depth in the soil of the six experimental plots in the spruce stand, as observed in 1986.

188 f r o m g l o b a l f a l l o u t r e v e a l s t h a t , i n d e p e n d e n t of t h e soil t r e a t m e n t a p p l i e d , g l o b a l f a l l o u t Cs r e m a i n s i n t h e O h h o r i z o n a b o u t o n e o r d e r of m a g n i t u d e l o n g e r t h a n C h e r n o b y l - d e r i v e d c e s i u m . ( B e c a u s e t h e r e s i d e n c e h a l f - t i m e of g l o b a l f a l l o u t Cs i n t h e L O f h o r i z o n s c o u l d o n l y be e s t i m a t e d to be ~ 2 y e a r s , t h e a b o v e c o m p a r i s o n c a n n o t be m a d e for t h i s h o r i z o n . ) A d i f f e r e n c e i n t h e s o r p t i o n o f Cs f r o m t h e s e t w o s o u r c e s w a s a l s o d e m o n s t r a t e d i n a r e c e n t s p e c i a t i o n s t u d y b y L i v e n s a n d B a x t e r (1988). U n l e s s we a s s u m e t h a t t h e p h y s i c o - c h e m i c a l e n v i r o n m e n t i n t h e f o r e s t soil h a s c h a n g e d s i g n i f i c a n t l y d u r i n g t h e l a s t 20 y e a r s , t h e r e is n o r e a s o n w h y , i n t h e l o n g - t e r m , r a d i o c e s i u m f r o m t h e a b o v e t w o s o u r c e s s h o u l d b e h a v e d i f f e r e n t l y . F o r t h a t r e a s o n , we c a n expect the p r e s e n t l y observed r a t e of m i g r a t i o n of C h e r n o b y l - d e r i v e d c e s i u m to c o n t i n u e to decrease f u r t h e r over the c o m i n g years.

ACKNOWLEDGEMENT The authors wouldliketothankW.

Schultz ~rtechnicalassistance.

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189 Ulrich, B., 1983. A concept of forest ecosystem stability and of acid deposition as driving force for destabilisation. In: B. Ulrich and J. Pankrath (Eds), Effects of Accumulation of Air Pollutants in Forest Ecosystems. D. Reidel, Dordrecht, pp. 1-29. Ulrich, B., R. Mayer and P.K. Khanna, 1980. Chemical changes due to acid precipitation in a loess derived soil in Central Europe. Soil Sci., 130: 193-199.