Environ. Sci. Technol. 2003, 37, 4592-4596
Reduction of Crop Contamination by Soil Resuspension within the 30-km Zone of the Chernobyl Nuclear Power Plant T E R E S A S A U R A S - Y E R A , * ,† J O A N A T E N T , § YURI IVANOV,| T. G. HINTON,⊥ GEMMA RAURET,§ AND R A M O N V A L L E J O †,‡ Departament Biologia Vegetal, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain, Departament Quı´mica Analı´tica, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Spain, Ukrainian Institute of Agricultural Radiology, 7, Mashinostritelej St, vil. Chabany, Kiev, Ukraine, Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29801, and CEAM, Parque Tecnolo´gico, Charles Darwin 14, E- 46980 Paterna, Spain
A field experiment was conducted within the 30-km zone of the Chernobyl Nuclear Power Plant to analyze whether the application of mulching reduced resuspension of 137Cs contaminated soil in oat (Avena sativa) crops. In 1993, we applied a mulch treatment at a dose of 200 g m-2, and soil resuspension was measured by estimating soil loadings onto plant surfaces from Ti concentrations in plants. In 1994, two mulch doses were applied, 200 and 50 g m-2, and we estimated the contribution of soil resuspension by using artificial resuspension collection devices (ARC). In the 1993 experiment between 4.6 and 34.4% of the plant’s total 137Cs contamination was attributed to external soil contamination. The mean amount of soil-derived 137Cs attached to vegetation was 124.7 Bq kg-1plant in control plots and 53.7 Bq kg-1plant in mulched plots. In the 1994 experiment, covering the soil with a mulch layer decreased the radiocesium content in ARC by about 70%. Results obtained in these experiments suggest that soil resuspension was a significant mechanism for plant contamination and that mulching was effective in reducing that contamination.
Introduction The restoration of the contaminated areas of the Chernobyl NPP accident is of great economic and ecologic importance. When Chernobyl’s radioactive aerosol was released to the atmosphere, it initially contaminated plants by direct deposition onto plant surfaces and subsequent foliar uptake (1). After the first cropping period, however, the main pathways for plant contamination became root uptake and the resuspension of contaminated soil particles onto plant surfaces. The dominant resuspension mechanisms for plant contamination by soil are rainsplash and wind (2-4). Therefore restoration techniques have been addressed to decrease these * Corresponding author phone: 34-93-402 14 62; fax: 34-93-411 28 42; e-mail:
[email protected]. † Departament Biologia Vegetal, Universitat de Barcelona. § Departament Quı ´mica Analı´tica, Universitat de Barcelona. | Ukrainian Institute of Agricultural Radiology. ⊥ University of Georgia. ‡ CEAM, Parque Tecnolo ´ gico. 4592
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 20, 2003
two mechanisms. Several studies have demonstrated the importance of resuspension by rainsplash in plant contamination (5-10). Further, contaminated soil adhering to plant surfaces can be ingested by grazing animals (11, 12). Therefore, reliable estimation of the plant’s external contamination is essential in obtaining information on radionuclide uptake from soil particles deposited on plant surfaces. Several methods that estimate the amount of soil adhered onto plant surfaces have been proposed. Plutonium, titanium, and scandium have widely been used as soil tracers due to their low solubility, poor root uptake, and low foliar absorption (9, 13). It is well-known that radiocesium, when strongly sorbed to clay minerals, has a reduced availability for plant root uptake. Three years after the Chernobyl accident, root uptake of radiocesium by crops decreased 3-4-fold and reached a steady state (14). At this time, when root uptake declined and was stabilized at low levels, the relative contribution from soil resuspension to plant contamination could become more significant because radiocesium activity concentrations in the top soil layer can be some orders of magnitude higher than plant activity concentrations. In areas with a high soil contamination it is of interest to reduce the contamination of plant surfaces by reducing the soil resuspension processes. Mulching, a common agricultural practice traditionally used to reduce soil erosion, topsoil temperature fluctuation, and soil water evaporation, was applied as a restoration technique to reduce 137Cs crop contamination in the Chernobyl area (15). We found that mulching reduced plant activity concentration by about 30-40%, and we hypothesized that reduction of soil resuspension was the main mechanism explaining the effectiveness of mulching in decreasing crop contamination. In this paper we analyze the following: (1) whether resuspension of 137Cs contaminated soil particles significantly contributed to total plant contamination, and (2) whether soil resuspension is reduced by using mulching as a restoration technique within the 30-km zone of Chernobyl 6 yr after the accident.
Materials and Methods Area of Study. This study was carried out in Kopachy, Ukraine. The experimental area was 5 km south of the Chernobyl Nuclear Power Plant and was contaminated with fuel and condensed particles. The 137Cs activity concentration of soil in 1986 was 1100-2500 kBq m-2 (14). The soil type was soddypodzolic, the climate continental, with a mean annual temperature of 6.5 °C and mean annual precipitation of 589 mm. Experimental Design. Mulching experiments were conducted in 1993 and 1994. In experiment 1 (1993), a straw mulch treatment was applied at a dose rate of 200 g m-2. The 137Cs activity concentration of the mulch was less than 0.04 kBq kg-1 and was considered uncontaminated. A similar mulch treatment was applied in experiment 2 (1994), but two mulch doses of uncontaminated straw were tested: 200 and 50 g m-2. Control plots were left unmulched. For all experiments, the crop was oat (Avena sativa), and the different treatments were replicated four times in 35 m2 plots following a Latin square design. Seed density was 15 g m-2. More details on experimental design (cropping schedule, fertilization, air temperature, and precipitation) were described by SaurasYera et al. (15). Measuring Soil Resuspension. Contribution of soil resuspension to crop contamination was studied by two different methods: 10.1021/es026377h CCC: $25.00
2003 American Chemical Society Published on Web 09/11/2003
Method 1 evaluated soil loading onto plant surfaces using Ti concentrations. Titanium content in plants, collected at flowering in control plots and at harvesting in control and mulched plots of experiment 1 (1993), was determined using AAS. Additionally, Ti content was determined in the bulk soil and in its different particle-size fractions. From the Ti concentration in soil and in vegetation samples we estimated the amount of soil mass loaded onto plant surfaces as well as the fraction of 137Cs in plant samples attributed to soil particles retained on leaf surfaces, by applying the following equations (13):
soil loading (gsoil kg-1plant) )
Ti (mg kg-1plant)/Ti (mg kg-1soil) × 1000 (1)
137
Csplant from attached soil (Bq kg-1plant) ) soil loading
(gsoil kg-1plant) × 137Cs (Bq kg-1soil)/1000 (2)
In parallel, we modified the above equation by using the fine fraction (silt + clay, < 20 µm) to calculate the fraction of 137Cs in plant from attached soil since the fine fraction is considered the main contributor to adhesion processes (5).
fine fraction (silt + clay) loading (gfine fraction kg-1plant) )
Ti (mg kg-1plant)/Ti (mg kg-1fine fraction) × 1000 (3)
137
Csplant from attached fine fraction (Bq kg-1plant) )
fine fraction loading (gfine fraction kg-1plant) × 137
Cs (Bq kg-1fine fraction)/1000 (4)
Method 2 evaluated the contribution of soil resuspension on total plant contamination by using artificial resuspension collection devices (ARC). The ARC devices consisted of 80 cm2 glass wool filter rectangles (double sided 20 cm × 2 cm rectangle). Glass wool filter material was used because of its high adsorption capacity. The filters were attached between two metallic rods in order to give a rigid structure and to facilitate installation in the field. The ARC devices were inserted perpendicular to the ground and with the bottom edge of the filter 2 cm above the topsoil. Four ARC per plot were installed in the control and mulched plots of experiment 2 (1994) just after sowing and were maintained in the field during the whole cropping period. Vegetation Sampling. Vegetation sampling at flowering was as follows: One sample per plot containing about eight plants was collected in control plots of experiment 1 (1993) and used to determine total plant137Cs activity concentration and Ti content. Vegetation sampling at harvest was similar in experiments 1 (1993) and 2 (1994). Plants were collected from four 0.5 × 1 m2 subplots within each plot, air-dried, separated manually into straw, grain, and chaff components, and ground before analyses. Chaff samples were not analyzed. In 1993, the four subsamples per plot were analyzed separately. In 1994, the four subsamples of straw and grain were pooled into one composite sample per plot. The artificial resuspension collector devices (ARC) were collected at harvest in 1994, and the wool glass filters were ashed at 450 °C overnight in a muffle furnace. Sample Analyses. The 137Cs activity concentrations in soil, vegetation samples, and on the ARC devices were determined by high-resolution gamma spectrometry. Ti content in soil samples, straw samples collected at harvest, and leaf samples collected at flowering was determined by AAS after wet digestion with fluoridric acid (16).
Statistical Analyses. Statistical analyses were performed using the SPSS program (17). Significant differences in Ti and 137Cs content in plants and 137Cs content in ARC were tested by analysis of variance and Duncan’s multiple range test. Differences between treatments for ARC filters were tested by nested analysis of variance (factor plot is nested to factor treatment), since four ARC per plot were measured. All tests were conducted at a 5% significance level.
Results Estimation of Contaminated Soil Adhered to Plants and Contribution to Total Plant Contamination by 137Cs. Table 1 shows the mean values and standard deviation of 137Cs and Ti concentration both in the bulk soil samples and in the different soil fractions collected in the experimental area. A significant positive linear relationship was found between 137Cs and Ti concentration in the different soil fractions (r2 ) 0.796, p < 0.01, n ) 5). 137Cs content in the fine fraction (silt + clay, < 20 µm) was 10 times higher than for the bulk soil, and the fine fraction had the highest 137Cs/Ti ratio. Table 2 shows the Ti concentration, the estimated soil loading, and the 137Cs activity concentration in vegetation samples attributed to soil particles retained on the plant surface according to eqs 1 and 3 both by using the bulk soil and the fine fraction. The vegetation samples were collected at flowering in control plots of experiment 1 (1993). At this sampling time, average soil loading onto plant surfaces was 7.45 ( 2.36 gsoil kg-1plant (1.2 ( 0.4 gfine fraction kg-1plant when considering the fine fraction). A positive linear relationship was found between Ti concentrations and total 137Cs activity concentrations in plant samples (r2 ) 0.913, p < 0.05, n ) 4), suggesting that soil particles adhered to plant surfaces can be relevant when calculating soil-to-plant transfer of radiocesium. 137Cs activity concentration in oat leaves at flowering derived from fine fraction (silt + clay) adhesion by applying eq 4 ranged from 46 to 98 Bq kg-1plant (Table 2). According to these results, between 4.6 and 14.5% of total plant contamination was attributed to external soil contamination at that sampling time. The Effect of Mulching on Soil Resuspension. (a) Experiment 1 (1993): Estimation of Soil Resuspension by Ti Analysis. Ti concentrations were measured in straw samples collected at harvest in control and mulched plots, and eqs 1 and 2 were applied to calculate the quantity of soil deposited on plant surfaces and the 137Cs activity concentration attributed to soil adhesion (Table 3). Soil loading in control plots was 14.7 ( 4.9 gsoil kg-1plant. In contrast, the amount of soil deposited on plant surfaces in mulched plots was 6.3 ( 0.7 gsoil kg-1plant. Analysis of variance showed significant differences (p < 0.05) between soil loading on plants within control plots and those in the mulched plots. As a result, covering the soil by a mulching layer reduced the soil loading on plant samples by about 55%. As at flowering time, a significant positive correlation was found between Ti and total 137Cs content in plants (r2 ) 0.79, p < 0.05, n ) 7) considering the two treatments. These results indicated that
TABLE 1. 137Cs and Ti Concentration in the Bulk Soil and in the Different Granulometric Soil Fractions Collected in the Experimental Areaa soil sample bulk >125 µm 63-125 µm 38-63 µm 20-38 µm
