Factors affecting uranium and thorium fractionation and profile distribution in contrasting arable and woodland soils

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Highlights

  • Vertical distribution and fractionation of U and Th in soils have been studied.

  • U was enriched in an arable topsoil due to long-term P-fertilizer application.

  • Fertilizer-derived U in the arable topsoil was much more reactive than native U.

  • Reactive U in the arable topsoil was adsorbed mainly by humus and Fe–Mn oxides.

  • The reactive proportion of native soil U was five times greater than for Th.

Abstract

This work investigated the vertical distribution and fractionation of uranium and thorium in soils under contrasting arable and woodland sites based on the same parent material. The effect of long-term application of phosphate fertilizer on U and Th concentrations in the arable soil was also assessed. The arable surface soils contained higher amounts of U compared with underlying layers and the woodland soil profile; by contrast, the Th distributions were very similar in both arable and woodland soils. The U and phosphate concentration profiles within the arable soil were broadly similar; U was strongly associated with Ca (r = 0.94) and soil P content (r = 0.86) whereas the U concentration profile in the woodland soil was virtually uniform with depth. The ‘excess’ U in arable soil, associated with long term P fertilizer application, was approximately 2.5 kg ha 1 with 80% in the top 30 cm of the soil profile. A sequential extraction technique was used to fractionate U and discriminate between ‘reactive’ (non-residual) and ‘residual’ forms of U. The reactive U was adsorbed, mainly by organic matter and Fe–Mn oxides in the arable topsoil and there was a very strong relationship between reactive U and soil phosphate content in the arable soil (r = 0.99, P < 0.001). The ‘excess’ U accumulated in the arable topsoil was also considerably more reactive than the co-existing native U. Thus the reactive fraction of the excess U ranged from 29 to 42% compared with 14 to 15% reactivity of the native U. Thorium in both soils showed a very consistent, and low, reactivity down the soil profile at about 4% of total soil Th content (96% residual), bound almost completely to humus.

Introduction

Since the 1960s, human exposure to natural radionuclides has increased by two or three orders of magnitude (Baxter, 1991). However, recently more attention has been given to the magnitude and distribution of natural radionuclides (e.g. U and Th) in soils, as radiation from natural sources is the main contribution to human exposure (Barisic et al., 1992, Kiss et al., 1988). Natural radioactivity in soils is largely a product of the mineral composition of the soil parent material and is primarily associated with uranium, thorium and their decay series. However, additional inputs of U into the environment have occurred in the past half century from sources that include mining, the nuclear industry, military use and application of fertilizers to arable soils (del CARMEN RIVAS, 2005, Tunney et al., 2009). A simple hazard ranking of potential contaminants in phosphate fertilizers was performed by Sauerbeck (1993) who examined the range of contaminant concentrations in phosphate rocks and compared it to average element concentrations in the Earth's crust; the conclusion was that As, Cd, Cr, F, Sr, Th, U and Zn are most likely to accumulate in P-fertilized soils. Research to date has confirmed this for Cd, F and U (Stacey et al., 2010 and references therein). A considerable amount is already known about the chemistry and soil-to-plant transfer of Cd, while much less information is available for fertilizer-derived F and U. In agricultural ecosystems the uranium concentration in soil has undoubtedly increased due to the long term-application of phosphate fertilizer (Nanzyo et al., 1993, Rothbaum et al., 1979, Takeda et al., 2005, Takeda et al., 2006, Yamaguchi et al., 2009). However, there is disagreement concerning the mobility and bioavailability of U in soils. Some studies suggest that U accumulation from long-term application of P fertilizer to field plots is confined only to surface soil layers with insignificant uptake by plants or leaching to surface waters (Rothbaum et al., 1979, Taylor, 2007). In contrast, Spalding and Sackett (1972) attributed the apparent increase in U concentrations measured in North American rivers to phosphate fertilizers applied to agricultural soils. The concentration ratio of U to Th has been suggested as a reliable indicator of U enrichment in soils arising specifically from the application of phosphate fertilizer (Takeda et al., 2004, Takeda et al., 2006, Yoshida et al., 1998). Measuring total elemental concentrations in soil is useful to assess the extent of contamination but provides only limited information on mobility and bioavailability. To assess the likely bioavailability and mobility of radionuclides it is often useful to have knowledge of soil chemical characteristics and the chemical forms of radionuclides present, as well as the source of radioactivity. The most reactive forms of natural radionuclides are adsorbed onto soil geocolloidal components (organic matter, clays, carbonates, Fe/Mn oxides) enabling participation in biogeochemical processes (Navas et al., 2005). Quantification of these phases can utilize experimental protocols and classification schemes widely applied to trace metals. The latter typically include fractions such as ‘exchangeable’ (mainly on alumino–silicate clays), ‘specifically adsorbed’ (to Fe and Mn oxides, humus etc), ‘co-precipitated’ (within carbonates, sulphides, phosphates and silicates) and ‘residual’ (eg primary mineral forms) (Li and Thornton, 2001). Each operational fraction may occur in a variety of structural forms. Thus, sequential extraction procedures (SEPs) are commonly used to fractionate metals in soils (Lake et al., 1984, Tessier et al., 1979, Ure et al., 1993). Of the many SEPs available, the Tessier protocol and its variants, have been the most widely applied for heavy metals and for radionuclides (Blanco et al., 2004, Bunker et al., 2000).

The aims of this study were:

  • (i)

    to investigate the vertical distribution and mobility of uranium and thorium in soils, at adjacent but contrasting arable and woodland sites based on the same parent material;

  • (ii)

    to assess the effect of long-term application of phosphate fertilizer on U and Th concentrations in the arable soil, and

  • (iii)

    to determine the distribution and chemical fractionation of U and Th within arable and woodland soil profiles.

Section snippets

Study area and soil samples

The study area is shown in Plate 1. Soil samples were collected from an arable field within the University of Nottingham farm, Leicestershire, UK (52049′48″N–1014′23″W) and from an adjacent mature woodland strip. The soil belongs to the Wick series (a sandy loam) based on fluvio-glacial sand and gravel (c. 80–100 cm) overlying Triassic Keuper Marl. Soil samples were taken as triplicate auger borings from the soil surface, in 10 cm depth intervals, to 90 cm in the arable field and to 70 cm depth in

General soil characteristics

Table 1a, Table 1b show measured soil properties in the arable and woodland soil profiles, respectively; values represent the average of three auger borings at each site. The soil pH decreased slightly with depth in the woodland soil, from pH 4.9 at the surface to 4.5 at 70 cm. In the arable soil, pH values were near neutral (6.75) at the soil surface and increased gradually through the soil profile to a maximum value (7.22) at 90 cm (Table 1a, Table 1b) suggesting prolonged movement of

Conclusions

There was clear evidence of U enrichment in the surface layers of the arable soil (0–40 cm) due to long-term application of phosphate fertilizers, as shown by the distribution of U concentrations in the soil profile, the U/Th ratio, a comparison with an adjacent woodland soil and a strong correlation between ‘reactive U’ and phosphate.

A sequential extraction technique was used to investigate the soil constituents that contribute to the retention of U in soil. Fertilizer-derived U was adsorbed,

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