Factors affecting uranium and thorium fractionation and profile distribution in contrasting arable and woodland soils
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,
References (42)
- et al.
Radium and uranium in phosphate fertilizers and their impact on the radioactivity of waters
Water Res.
(1992) Personal perspectives on radioactivity in the environment
Sci. Total Environ.
(1991)- et al.
Sequential extraction for radionuclide fractionation in soil samples: a comparative study
Appl. Radiat. Isot.
(2004) - et al.
Fractionation of natural radionuclides in soils from a uranium mineralized area in the south-west of Spain
J. Environ. Radioact.
(2005) - et al.
Solid/Liquid REE fractionation in the lateritic system of Goyoum, East Cameroon: the implication for the present dynamics of the soil covers of the humid tropical regions
Geochim. Cosmochim. Acta
(1998) - et al.
Determination of radionuclide exchangeability in freshwater systems
Sci. Total Environ.
(2000) - et al.
Influence of pH, soil humic/fulvic acid, ionic strength and foreign ions on sorption of thorium(IV) onto γ-Al2O3
Appl. Geochem.
(2007) - et al.
Effects of soil pH and organic matter on distribution of thorium fractions in soil contaminated by rare-earth industries
Talanta
(2008) - et al.
Partitioning and speciation of solid phase iron in saltmarsh sediments
Geochim. Cosmochim. Acta
(1994) - et al.
Chemical partitioning of trace and major elements in soils contaminated by mining and smelting activities
Appl. Geochem.
(2001)