Occupational dosimetric assessment (inhalation pathway) from the application of phosphogypsum in agriculture in South West Spain

Occupational dosimetric assessment (inhalation pathway) from the application of phosphogypsum in agriculture in South West Spain J.M. Abril a , * , R. Garcı ´a-Tenorio b , R. Peria ´n ˜ez a , S.M. Enamorado a , L. Andreu c , A. Delgado c a Departamento Fı ´sica Aplicada I, Universidad de Sevilla, EUITA, Carretera de Utrera Km 1, 41013 Seville, Spain b Departamento Fı ´sica Aplicada II, Universidad de Sevilla, ETSA, Avenida Reina Mercedes s/n, 41012 Seville, Spain c Departamento Ciencias Agroforestales, Universidad de Sevilla, EUITA, Carretera de Utrera Km 1, 41013 Seville, Spain article info Article history: Received 23 May 2008 Received in revised form 14 September 2008 Accepted 28 September 2008 Available online 18 November 2008 Keywords: Phosphogypsum Agriculture Inhalation pathway Radiological Radioactive-aerosols abstract Phosphogypsum (PG) has been traditionally applied as Ca-amendment in saline marsh soils in SW Spain, where available PG has 710 40 Bq kg 1 of 226 Ra. This work assesses the potential radiological risk for farmers through 222 Rn exhalation from PG-amended soils and by inhalation of PG-dust during its application. A three-year field experiment was conducted in a commercial farm involving two treatments: control and 25 t PG ha 1 with three replicates (each 0.5 ha plots). The 222 Rn exhalation rate was positively correlated with potential evapotranspiration, which explained 67% of the variability. Statistically significant differences between the control and PG treatments were not found for 222 Rn exhalation rates, and mean values were within the lowest quartile of the typical range for 222 Rn exhalation from soils. Airborne dust samples were collected during the application of PG and sugar-beet sludge amendments. The highest PG-attributable 226 Ra concentration in the dust samples was 3.3 10 2 m Bq m 3 , implying negligible dose increment for exposed workers. Ó2009 Elsevier Ltd. All rights reserved. 1. Introduction Phosphogypsum (PG) is the main waste of phosphoric rock processing plants, which use phosphate rock as raw material. In general, levels of fluoride, certain naturally occurring radionuclides and some heavy metals are elevated in PG. Concentrations of these hazardous elements depend on the origin of the rock phosphate sources, the type, and the efficiency of the wet process used (Rutherford et al., 1994). The worldwide annual production of PG in 2006 was estimated to be about 170 10 6 tonnes, and most of it is stock-piled. Agriculture uses of PG are being extensively studied since they could become the main worldwide use for this waste. Thus, studies have been conducted, among others, on the effect of PG in the improvement of soil structure and crop yield (May and Mortvedt, 1986; Mullins and Mitchell, 1990), reducing soil erosion (Zhang et al., 1998), and increasing levels of available S and P (Delgado et al., 2002). In SW Spain, PG has being traditionally used as a Ca-amendment for soil reclamation (Domı ´nguez et al., 2001). Initial recommended rates were 20–25 t ha 1 with repetition of every two years. Under current practices, PG is applied (after being sun-dried) over a previously tilled soil; with additional deep-tillage immediately after PG application, which provokes dilution in a soil horizon up to 40 cm. No attention is paid to PG grain size and, in practice, it is possible to distinguish from sub-mm up to several cm grain sizes immediately after application. The source of the applied PG in SW Spain is the phosphoric rock processing plant located in Huelva (SW Spain), which has produced a total PG amount of about 810 7 tonnes (Borrego et al., 2007). Most of them have been disposed in stacks lying on the right bank of the Tinto River, in the vicinity of Huelva city, covering an area of about 1200 ha. Nowadays, PG production is about 3 10 6 tonnes per year, mainly obtained after processing phosphate rocks from Morocco (with 238 U concentrations around 1000 Bq kg 1 , after Bolı ´var et al., 1996). About 85% of the uranium present in phosphate rock goes to the resulting phosphoric acid, while about 90% of the 226 Ra remains in the PG wastes (Bolı ´var et al., 1996). Recent Spanish regulations (R.D. 824/2005, from July 2005) explicitly allowed the use of PG as soil amendment with no mention to its radioactive content, while the US EPA has specific regulation for the agriculture use of PG (64 FR 5574, USEPA, 1992), allowing it if 226 Ra concentration is below 370 Bq kg 1 . Major environmental concerns related to the agriculture use of PG are the radionuclide (and other PG-related pollutants such as Cd) uptake by plants and their build-up in soil (Rutherford et al., *Corresponding author. Tel.: þ34 954486473; fax: þ34 954486436. E-mail address: [email protected] (J.M. Abril). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad 0265-931X/$ – see front matter Ó2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2008.09.006 Journal of Environmental Radioactivity 100 (2009) 29–34 1994). The radiological impact related to the agriculture use of PG has been studied, among others, by Alcordo et al. (1999) in Florida, by El-Mrabet et al. (2003) in reclaimed marsh soils in SW Spain, and, more recently, by Papastefanou et al. (2006) in Greek soils. Concerning the inhalation pathway, airborne particulate matter originating from PG during its application as soil amendment is a potential source of radioactivity. With current practices in SW Spain, large clouds of dust are formed when PG is spread over the soil. Increasing 222 Rn exhalation from 226 Ra-enriched soils is another point of concern. Thus, Alcordo et al. (1999) reported significant increments in 222 Rn exhalation rates following PG application in soils under bahiagrass pasture in Florida. A significant fraction of the total radioactive dose received by the world population is associated with radioactivity inhalation via aerosol particles (UNSCEAR, 2000). In addition, 222 Rn and its shortlived decay products are responsible for an important fraction of the total internal dose received by the population. The present work is aimed to study 226 Ra concentrations in particulate matter during the application of PG under current practices in a commercial farm in SW Spain and to monitor 222 Rn exhalation rates in PG-amended plots. Measurements of 222 Rn exhalation rates typically show large spatial and temporal variability, although the factors affecting such variability are still poorly understood. Thus, attention will be paid to establish the potential effect of agricultural and environmental factors on 222 Rn exhalation. All this information will be useful to support a radioactive dose assessment through the inhalation pathway related to the agriculture use of PG in SW Spain. 2. Materials and methods 2.1. Experimental site and experimental design An experiment was conducted on a 6 ha (250 240 m) plot of a commercial farm in the ‘‘Marismas of Lebrija’’, in the marsh area of the Guadalquivir River, SW Spain (37  1.2 0 N, 6  7.4 0 W). The plot was flat (0.1% slope) and lengthwise crossed by drainage lines 250 m long and spaced 5 m which were placed at a depth of about 1 m. After reclamation, these marsh soils can be classified as Aeric Endoaquepts (Soil Survey Staff, 1998). More detailed information about this experimental site can be obtained elsewhere (Delgado et al., 2006). The farm soils, as the rest of the area, have received some previous PG amendments before 2001, being not well-documented. Nevertheless, they could correspond to some 6 typical doses of 25 t ha 1 each (Abril et al., 2008a). Two treatments (control – with no additional PG, and 25 t ha 1 PG on a sundried weight basis, applied in April 2003 and repeated in September 2004) were applied in triplicate and randomly distributed in 0.5 ha (250 20 m) plots (see Fig.1). In April 2003, sugar-beet sludge was applied to three other elemental plots at a rate of 30 t ha 1 . Airborne dust samples were collected during the application of these amendments. The 222 Rn exhalation was monitored only in control and PG- amended plots. In the first and third seasons (2003 and 2005), cotton (Gossypium hirsutum L.) was grown under furrow irrigation (sown in April and harvested in November); in the second, sugar-beet (Beta vulgaris L.) was grown under sprinkler irrigation at 2.5 mm h 1 (from January to July 2004). For both crops, fertiliser was applied to all the plots at pre-plant stage [52 kg N ha 1 ,68kgPha 1 and 43 kg K ha 1 as a mixture of (NH 4 ) 2 HPO 4 , urea and KNO 3 ]. A detailed study on 226 Ra and 238 U in these soils, including depth profiles and a comparison against nonreclaimed soils has been presented in a separate work (Abril et al., 2008a). 2.2. Sampling air filters, soils and charcoal canisters for 222 Rn measurements The PG application lasts only a few hours, but large clouds of dust are formed. Thus, to collect total suspended particles (TSPs) the following suitable solution was adopted: A vacuum-pump aspired air through a Nucleopore polycarbonate filter, 0.45- m m pore size and 47-mm diameter, and a gas-volume meter. The pump was powered by a commercial portable electric generator. Mean flow rate was 3.2 m 3 h 1 and typical sampling time was 35 min per filter. Fig. 1 shows the experimental plotdesign and the situation of the collected air filters. For a number of measurements, the filter support system was placed at 1.6 m above ground level in the border of the elemental plot while the tractor was applying the amendment. This kind of measurement may roughly represent the situation of a farmer during the PG application. Other samples were collected by placing the filter support in the cabin of the tractor (in the outside mirror) in order to have a proxy to the situation of the driver in the worst and fairly improbable situation of working without (or with open) windows. Filters were weighed (by using a 0.1 mg precision electronic weighing machine) before and after collection to measure TSP (values ranged from 2 to 6 mg for most of the filters). Soils were sampled (0–30 cm horizon) at the central point of each control and PG-plot in January 2006 to study the 226 Ra and 238 U contents after the two treatments. Samples were air-dried and ground to pass a 0.5 mm screen prior to analysis. The 222 Rn exhalation was measured by the activated charcoal canister method, following the procedure described by Duen ˜as et al. (2007). Each device, provided by TECNASA-Spain, had some 75 g of activated charcoal in an open faced metal canister of 10 cm diameter. The 222 Rn collector is deployed by firmly twisting the cap into the soil surface to be measured, with exposure times of 24 h (starting early in the morning). Selected exposure time allows integration of diurnal variations in 222 Rn emissions without significant charcoal-saturation effects (after Hartley et al., 1983) and thus, it meets the USEPA method 115 for monitoring 222 Rn emissions (USEPA, 1992). Sampling points were defined at the centre of each elemental control and PG- amended plots. Three canisters were systematically placed at each sampling point in the vertices of an equilateral triangle of 1 m side (with one of the vertices orientated northwards). Canisters were weighed before and after sampling to determine the adsorbed moisture by charcoal for subsequent corrections in exhalation determination (Duen ˜as et al., 2007). The 222 Rn exhalation was measured for four sampling campaigns (September 2004, October 2004, February 2005 and February 2006). In natural soils, under sunny conditions, net radiation can lead to significant water evaporation and/or to an increase in soil temperature, affecting consequently the radon emissions. Potential evapotranspiration, ETo, is defined for a reference crop and standard soil conditions, and it is a reflection of the energy available to evaporate water, and of the wind available to transport the water vapour from the ground up into the lower atmosphere. Daily ETo, as estimated by the Penman– Monteith FAO56 method (Allen et al., 1998), is a function of the net radiation, average air temperature, wind speed and water vapour pressure. For any particular crop and crop conditions, it is possible to estimate actual evapotranspiration from ETo, corrected by an appropriate crop-coefficient. Daily ETo has been selected to study its potential effect on 222 Rn emissions. For each sampling period, ETo was obtained from the closest agro-meteorological station from the Andalusian network (107 stations operating since 2000 with online access): Lebrija I (36  58 0 40 00 N, 06  07 0 30 00 W, h¼25.0 m). ETo provided by the Andalusian network is estimated by the referred Penman–Monteith FAO56 method. 2.3. Radon measurements In total, 72 charcoal canisters were collected. The 222 Rn activity was determined through the 609 keV g -emission of 214 Bi by using 3 low-level gamma-ray spectrometric systems equipped, respectively, with 5 00 5 00 NaI(Tl), HPGe Xtra (37.1% relative efficiency, FWHM of 1.76 keV at 1332 keV of 60 Co) and ReGe (31.4% relative efficiency, FWHM of 1.98 keV at 1332 keV of 60 Co) detectors. Typical counting times were 6–18 h per sample with typical counting uncertainties below 10%. These systems were calibrated with charcoal spiked with a tracer solution of 226 Ra (after homogenization), once it was allowed to reach secular equilibrium with 222 Rn and its decay products. Blank charcoal canisters were also measured for background corrections. More details on the applied method can be found in Duen ˜as et al. (2007) and Quindo ´s et al. (2001). C1 C2 C3 PG1 PG2 PG3 SBS1 SBS2 SBS3 (37o1’15.99”N, 6o7’23.09”W) N F1 F2 F3 F4 F5 F6F7 250 m 20 m Fig. 1. Situation of the experimental site with a schematic representation of the experimental design and the locations of the dust sampling filters. Filters are numbered following the temporal sequence of sampling; those associated with arrows indicate sampling in the cabin of the tractor during the application of the amendment (PG, phosphogypsum 25 t ha 1 ; SBS, sugar-beet sludge 30 t ha 1 ; C, control), and those associated with crosses indicate sampling at a fixed point (filter placed at 1.6 m above ground level). J.M. Abril et al. / Journal of Environmental Radioactivity 100 (2009) 29–3430 2.4. 226 Ra and 238 U measurements in soils Homogenised soil samples were introduced in Petri dishes, sealed to preclude radon emanation and stored during one month to allow secular equilibrium of 226 Ra with its decay products 214 Bi and 214 Pb. Gamma measurements were carried out on the ReGe and Xtra low-background high purity Ge detectors previously described. Typical counting times were 48 h per sample. The target radionuclides were 226 Ra (determined through the 352 keV emission of 214 Pb) and 238 U (through the 63 keV emission of 234 Th, following El-Daoushy and Herna ´ndez, 2002), both naturally occurring and whose concentrations may be enhanced by the PG treatment. Efficiency calibration (in Petri dish geometry) was performed using farm soils from the studied site (to ensure the same matrix conditions that the target samples) spiked with known amounts of a multig -emitters tracer solution. Measured efficiencies were cross-calibrated against Monte-Carlo simulations. 2.5. 226 Ra measurements in air filters Filters were dissolved with HCl and HNO 3 , to form aqua regia, and the resulting solution was taken to 0.5 L with distilled water. Five milligrams of Ba carrier was added to the sample and precipitation of Ba–RaSO 4 was then carried out. The precipitate was collected by filtration through a 0.45- m m pore size Millipore filter. After 20 days (to allow secular equilibrium of 226 Ra with its decay products) the alpha activity of the sample was measured in a Berthold LB 770 low-background gasflow proportional counter previously calibrated for total efficiency versus precipitate mass thickness. The measured background was 0.050 0.003 cpm (mean and one standard deviation, n¼12). The procedure is standard and may be seen in more detail, for instance, in Peria ´n ˜ez and Garcı ´a-Leo ´n (1993). 2.6. Agriculture inputs Phosphogypsum is usually obtained from a non-active PG stack (37  15 0 21.26 00 N, 6  54 0 2.07 00 W) in Huelva (SW Spain). The PG stack has been extensively sampled (n¼42) to determine its content in 226 Ra (by liquid scintillation, using a Wallac Quantulus 1220) and 238 U and 210 Po (by a spectrometry, using an alpha spectrometer with 8 independent chambers, equipped each one with a 450 mm 2 PIPS detector), resulting (on a dry weight basis) in 720 260 Bq kg 1 of 226 Ra, 170 110 Bq kg 1 of 238 U and 660 160 Bq kg 1 of 210 Po (mean and one standard deviation, Abril et al., 2008b). The PG used in our field experiment came from this stack. A representative sample was submitted for analysis, providing 630 4 and 196 6Bqkg 1 of 226 Ra and 238 U, respectively, on a dry matter basis. Borrego et al. (2007) provided a reference value of 280 Bq kg 1 for 230 Th in PG stacks in Huelva. Pe ´rez-Moreno (2005) found 1680 150 Bq kg 1 of 238 U and non-detectable amounts of 226 Ra in diammonium-phosphate produced in SW Spain. Under field conditions, after being sun-dried, the remaining moisture content in PG was 19%. Thus, at 25 t PG ha 1 rate (sun-dried weight), the amendment was incorporating 12.8 MBq ha 1 of 226 Ra and 4.0 MBq ha 1 of 238 U. The application of 68 kg P ha 1 as diammonium-phosphate incorporates 0.49 MBq ha 1 of 238 U, and negligible amounts of 226 Ra. 2.7. Statistical analysis Statgraphics Plus 5.1 software was applied to carry out tests for each data-set distribution. An analysis of variance (1-way ANOVA) test was applied for comparison between means. All pair wise contrasts were carried out by Tukey tests at 95% CL, and quoted groups by 95% CL LSD method. 3. Results and discussion 3.1. The effect of PG amendment in radionuclide build-up in soils and 222 Rn exhalation Table 1 summarizes 226 Ra and 238 U concentrations in soils and 222 Rn exhalation rate from control and PG-amended plots for the different sampling campaigns. The 226 Ra activity concentrations were 35.3 0.8 and 39.3 2.3 Bq kg 1 (mean and one standard deviation of mean), for control and PG-amended plots, respectively. There were no statistically significant differences (95% CL) between the two treatments. Mean 226 Ra activity concentrations were close to the average value for soils (35 Bq kg 1 ) reported by UNSCEAR (2000). Concerning 228 U concentrations, measured values were 25.3 0.4 and 23.9 1.5 Bq kg 1 for control and PG-amended plots, respectively, without statistically significant differences. The 222 Rn exhalation values from the three canisters of each sampling point were normally distributed. No significant differences (95% CL) were found between means of points corresponding to the same sampling campaign and treatment. There were significant differences (p<0.0001) between mean values from subsequent sampling campaigns (for both control and PG- amended plots). Differences between treatments were not statistically significant (95% CL) except for the sampling of September 2004. Mean exhalation rates were within the range 13–55 Bq h 1 m 2 corresponding to the lowest quartile of typical 222 Rn exhalation rates from soils (30–150 Bq h 1 m 2 , as reported by Duen ˜as et al., 1997). It is important to note that the general absence of statistically significant differences between treatments under our particular experimental setup does not necessarily imply that PG has not any effect on radionuclide enrichment in soils or in increasing 222 Rn exhalation. Furthermore, the mean 226 Ra/ 238 U activity ratio was 1.5 0.1, far from the secular equilibrium, which indicates a PG- attributable 226 Ra enrichment (PG from SW Spain has a 226 Ra/ 238 U fingerprint of 4.3 0.5) due to the cumulative effect of all the previous PG applications, as shown in Abril et al. (2008a). The 222 Rn exhalation rates were positively correlated with the daily potential evapotranspiration ETo (R 2 ¼0.689 at a CL 95%), as shown in Fig. 2. The ETo explains 67% of the variability in radon emissions. Table 1 226 Ra and 238 U concentrations a in agriculture soils and 222 Rn exhalation b measured in several sampling campaigns. Sampling point 226 Ra (Bq kg 1 ) 238 U (Bq kg 1 ) 222 Rn Exhalation (Bq h 1 m 2 ) January 2006 January 2006 September 2004 c October 2004 d February 2005 c February 2006 e Control plots C 1 36.9 0.7 25 635521.41.0 10.1 1.4 16 3 C 2 34.7 0.7 24.8 1.3 33 4 18.9 1.4 14 5185 C 3 34.3 0.9 26 440730514627.72.0 Arithmetic mean control 35.3 0.8 25.3 0.4 36a 23.4b 12.7c 20.6b PG-amended plots PG 1 34.7 1.1 26 35213 27.7 2.6 15 5 24.4 1.0 PG 2 41.0 0.9 21 4489 25.4 2.2 40 30 28 7 PG 3 42.1 1.1 24.7 2.8 66 14 36 4283 18.9 2.0 Arithmetic mean PG 39.3 2.3 23.9 1.5 55a 30b 28b 23.7b ANOVA NS NS * NS NS NS There was not significant differences in 222 Rn exhalation between sampling points within each sampling campaign but differences are significant (p<0.0001) between sampling campaign (quoted groups a,b,c, by 95% CL LSD method). NS: not significant. *Significant (95% CL). a Measurement (by gamma spectrometry) with 1 s analytical error. Soil samples from the 0–30 cm horizon. b Mean and standard deviation of mean (n¼3, 1 m spaced). Measurements by charcoal canister method. c Un-vegetated and tilled (w20 cm) soil. d Un-vegetated and deep-tilled (w40 cm) soil. e Sugar-beet crop at nascence stage. J.M. Abril et al. / Journal of Environmental Radioactivity 100 (2009) 29–34 31 Basic processes governing radon transport from soils to air (as reviewed by Nazaroff, 1992) are relatively well understood. They provide a basis for understanding the effect of evapotranspiration in the enhancing of radon emissions. A reduction of soil moisture enhances 222 Rn emanation and the transfer of 222 Rn from liquid to gaseous phases. Radon transportation through the connected pore space towards the earth’s surface involves diffusion (enhanced by temperature, and the increase of connected empty-pore spaces) and advection processes. Advection is governed by intrinsic permeability (affected by moisture) and pressure gradients. Other meteorological parameters like temperature difference between soil and surface air and wind velocity also affect the advection processes. Three canisters were placed for each sampling point. A normalized variable can be defined as the ratio of the exhalation rate of each canister and the mean value of each sampling point. This way a set of 36 data is generated (3 canisters 3 sampling points 4 sampling campaigns) for control and PG-amended plots. Data were normally distributed with standard deviations of 0.32 and 0.40 for control and PG-amended plots, respectively. Despite the factors affecting the mean values of 222 Rn exhalation rates, there persisted an intrinsic variability of roughly 30% (control) to 40% (PG-plots), which can be attributed to the variability in the w1 m scale of the micro and meso-pore structure and to irregularities in the PG-grains (sizes and distribution) within PG- amended soils. 3.2. 226 Ra in airborne dust samples Table 2 summarizes results for the measurements of 226 Ra activities in air filters (the numbering corresponds to the temporal sequence of sampling). TSP and 226 Ra activity were below the corresponding decision threshold (L C ) in laboratory and field blanks (the latter having been sampled before the treatment started). The lowest measured 226 Ra activity value was found in filter F2, sampled in the tractor cabin during the first run of the PG application, not being exposed to the dust cloud formed behind the tractor and with still negligible resuspension resulting after the PG spreading. From reference 226 Ra concentrations in soils (Table 1) and PG from Huelva (Abril et al., 2008b), and the measured TSP and 226 Ra activity, it is possible to estimate the corresponding PG- attributable percentages, as shown in Table 2. Thus, the PG contribution in filter F2 cannot be calculated, but it explains 77% of the measured 226 Ra activity in filter F3 (tractor cabin, second run of PG application) with 13% of TSP. The PG-attributable contribution to TSP decreases during the application of the sugar-beet sludge (Filters F4 and F5). The origin of PG in these filters was the resuspension from plots PG2 and PG3 (see Fig. 1) due to the moderate wind blowing laterally. The 226 Ra was not detectable in filter F6; sampled in a fixed point without any PG-amended plot upwind (Fig. 1). The highest 226 Ra activity value corresponds to filter F7, measured 1.6 m above ground during the third run of PG application. In this case the PG-attributable contribution to TSP was 22%, explaining 86% of the measured 226 Ra activity. There are not many reports on 226 Ra concentrations in air. The reference level defined by UNSCEAR (2000) is 1 m Bq m 3 for a typical dust concentration in air of 30 m gm 3 at ground level. Measurements reported in current literature are of the order of such reference level. For instance, in a semi-rural environment in Germany, the mean 226 Ra concentration detected during years 1983–1985 was 1.2 m Bq m 3 , with a mean particle concentration of 59 m gm 3 (Hotzl and Winker, 1987). A lower value, 0.6 m Bq m 3 , has been reported for the USA (UNSCEAR, 2000). Slightly higher concentrations, from 2.9 to 9.31 m Bq m 3 , were measured during 10-year observations in a rural area of Japan (Yunoki et al., 1995). In Poland, 226 Ra in air at ground level was 3.28 m Bq m 3 (Kownacka et al., 1999) and, in Poland as well, Bem et al. (2004) have found an average concentration of 1.56 m Bq m 3 for a mean dust concentration of 53.1 m gm 3 . Similar values have also been found by Braziewicz et al. (2004). In Greece, Papastefanou et al. (1999) have estimated a 226 Ra concentration of 0.87 m Bq m 3 in ground level air assuming a dust loading of 50 m gm 3 . It may be seen that the results in Table 2 are two orders of magnitude higher than values reported above. This is not surprising since dust concentrations are two orders of magnitude higher too, and 226 Ra levels in air are correlated with the dust concentration (Hotzl and Winker, 1987). Obviously, the increase in dust concentration is caused by the application of the PG amendment with the tractor. y = 9,3 1 x - 1,87 R2 = 0,689* 0 10 20 30 40 50 60 70 123456 ETo ( mm d-1 ) 222Rn Exhalation (Bq h-1m-2) Fig. 2. 222 Rn exhalation rates (mean with error-bars corresponding to one standard deviation of mean) as a function of the daily potential evapotranspiration. Table 2 Analysis of air filters sampled during the application of PG and sugar-beet sludge (SBS) as soil amendments a . Sample Volume (m 3 ) Time (min) TSP (mg m 3 ) 226 Ra Maximum PG- attributable (Bq m 3 ) Error (%) % (w/w) % Activity F1 Farm blank 1.237 22 ND ND F2 PG amendment. Tractor cabin. 2.207 43 2.4 6.5E-05 70 ND F3 PG amendment. Tractor cabin. 1.282 25 1.6 1.9E-04 40 13 77 F4 SBS amendment. Tractor cabin. 2.083 41 2.5 1.9E-04 25 7 62 F5 SBS amendment. Tractor cabin. 2.269 42 2.6 2.5E-04 16 10 71 F6 SBS amendment. Farm 1.596 29 2.5 ND F7 PG amendment. Farm 1.931 39 2.1 3.8E-04 13 22 86 F8 Laboratory blank 3.301 60 ND ND Sampling on April 8th 2003 (F1–F7). a TSP was measured by gravimetric method (ND not determined). 226 Ra concentration measurements by alpha counting. Decision threshold L C ¼1.1 10 4 cps (95% CL, from 12 independent background measurements of 600 min. each) or 6.2 10 5 Bq m 3 (for a volume of 2.0 m 3 and average yield and efficiency). J.M. Abril et al. / Journal of Environmental Radioactivity 100 (2009) 29–3432 3.3. Dose assessment Table 3 summarizes an estimation of the dosimetric contribution of PG-attributable dust, generated under current practices in SW Spain during its application as soil amendment. Estimations follow the methodology presented by Borrego et al. (2007), based on the Spanish Regulation RD 783/2001 (in agreement with EURATOM’96 directive). Dose assessment due to inhalation was carried out for radionuclides belonging to the 238 U decay chain, excluding the 222 Rn decay products. Volumetric activity increments for PG-attributable 226 Ra corresponding to sample F7 were selected as the worst case (Table 2). For the other radionuclides, volumetric activity increments were derived assuming the same activity ratio as in PG from SW Spain (see Section 2). Radioactive equilibrium between 238 U, 234m Pa, 234 Th and 234 U, and between 210 Pb, 210 Bi and 210 Po was assumed. These activity increments are converted into doses by using the corresponding inhalation factors provided by the Spanish regulations for exposed workers and an AMAD (activity median aerodynamic diameter) of 5 m m. The AMAD value associated to an aerosol indicates that 50% of the activity in the aerosol is associated with particles of aerodynamic diameter greater than the AMAD. Conversion factors corresponding to class M (moderate residence time in lung) were selected for all the involved radionuclides except for 210 Pb, for which default class was F (short residence time in lung). The breathed volume was estimated at a light worker rate of 1.2 m 3 h 1 (ICRP-66, 1994) with a working occupancy of 40 h per week during 48 weeks per year (after Borrego et al., 2007). The cumulative dose increment estimated in this manner for exposed workers (in the worst and fairly improbable scenario of a farmer assisting PG application outside the tractor or a driver without a protected cabin) resulted in a dose of 12 m Sv y 1 . This value is far below the limits for dose rate increment established by the European Directive (1 mSv y 1 for the general population is the most restrictive limit). We note that dust could be eventually ingested, but dose conversion factors for this pathway are smaller than those for inhalation; thus, the previous estimate can still be considered as an upper limit. Dose assessment due to 222 Rnwill not be accomplished within the present work because PG-attributable increments in 222 Rn exhalation could not be concluded from our present data. Additionally, in field conditions, the 222 Rn concentration in air (which effectively contributes to the radioactive dose by inhalation) depends not only on local sources, but on the general circulation of atmospheric air masses at much larger spatial scales aswell. The contribution to doses due to the 222 Rn decay products in PG-dust is negligible. Thus, assuming secular equilibrium with 226 Ra in PG-dust, the resulting volumetric activity concentration of about 0.3 mBq m 3 (see Table 3) is five orders of magnitude lower than typical indoor 222 Rn concentrations (40 and 30 Bq m 3 for the arithmetic and geometric means of the distribution of worldwide indoor 222 Rn concentrations, with a geometric standard deviation of 2.3, after UNSCEAR, 2000). 4. Conclusions Under our experimental conditions (two PG amendments applied to an agriculture soil with some previous 5–6 PG applications, and mixed in the 0–40 cm soil horizon), no statistically significant differences were found between control and PG-amen- ded plots for 222 Rn exhalation rates (except for the first campaign). Measured values were within the lowest quartile of the typical range for 222 Rn exhalation from soils (Duen ˜as et al., 1997). The 222 Rn exhalation was positively correlated with daily potential evapotranspiration, which explained 67% of the variability. Normalized distributions revealed an intrinsic variability in the w1 m spatial scale from 30% (control) to 40% (PG-plots), which can be attributed to the variability, at this scale, of the soil structure and PG-grains (sizes and distribution in the PG-amended soils). The 226 Ra/ 238 U activity ratios in soils revealed a PG-attributable 226 Ra enrichment due to the accumulation effect of recent (within the experiment) plus previous PG applications. The highest PG- attributable 226 Ra concentration in the airborne dust samples was 3.3 10 2 m Bq m 3 , implying negligible radioactive dose increment (12 m Sv y 1 ) for exposed workers (the total average worldwide exposure to natural radiation sources is 2.4 mSv y 1 , being the contribution from inhalation exposure 1.26 mSv y 1 , of which 1.15 mSv y 1 is due to 222 Rn, after UNSCEAR, 2000). Under current practices (spreading PG and mixing it within the 0–40 cm soil horizon), application of PG as soil amendment in SW Spain does not lead to a significant increment in radioactive doses for farmers through the inhalation pathway and, after three decades of practices, current 222 Rn exhalation rates from soils remain well within the normal range. Acknowledgements This work was funded by ENRESA (Spanish Public Corporation of Radioactive Residues) and by the IFAPA-C039 project from the regional Andalusia government. The authors wish to thank to the Agriculture Cooperatives ‘‘Las Marismas’’ and ‘‘La Amistad’’ for making some facilities and the experimental site available. Table 3 Estimation of the PG-attributable dose increment due to inhalation of dust during the PG application as soil amendment (current practices in SW Spain). Radionuclide Concentration in PG (Bq kg 1 ) Mean activity increment b ( m Bq m 3 ) Dose conversion factor a (Sv Bq 1 ) Volumetric dose increment (Sv m 3 ) 238 U 165 7.7 10 1 1.6 10 6 1.2 10 10 234m Pa 5.5 10 10 4.2 10 14 234 Th 5.3 10 9 4.1 10 13 234 U 2.1 10 6 1.6 10 10 230 Th 280 1.3 10 2 2.8 10 5 3.6 10 9 226 Ra 710 3.3 10 2 2.2 10 6 5.8 10 10 210 Pb 1.1 10 6 2.6 10 10 210 Bi 6.0 10 8 1.7 10 11 210 Po 640 3.0 10 2 2.2 10 6 5.9 10 10 Total volumetric dose increment 5.4 10 9 Sv m 3 Dose increment for exposed workers c 12 m Sv y 1 a Factors for exposed workers with AMAD 5 m m (from Spanish regulation RD 783/2001). Absorption rate type ‘‘M’’ has been assumed for all the radionuclides, expect for 210 Pb, with type F. b Radioactive equilibrium is assumed between 238 U, 234m Pa, 234 Th and 234 U and between 210 Pb, 210 Bi and 210 Po. Activity increments for PG-attributable 226 Ra correspond to sample F7 as the worst case (Table 2). 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