Mobility of Fertiliser-Derived Uranium in Arable Soils and Its Contribution to Uranium Concentrations in Groundwater and Tap Water
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Mobility of fertiliser-derived uranium in arable soils and its contribution to uranium concentrations in groundwater and tap water by Geerd Ahlrich Smidt A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Geosciences Approved Dissertation Committee Prof. Dr. Andrea Koschinsky Name and title of Chair Prof. Dr. Dr. Dr. h.c. Ewald Schnug Name and title of Committee Member Prof. Dr. Michael Bau Name and title of Committee Member Date of Defence: 20.12.2011 School of Engineering and Science Abstract Phosphorus (P) mineral fertilisers are found to contain high concentrations of uranium (U) (up to 206 mg U kg-1) and other trace elements (TE), such as Cd, Pb, Ni, Cu, Zn, Th, Nb, Sr, V, and rare earth elements. The content of U and other trace elements is depended on the sedimentary of igneous origin of the rock phosphate. In this study, the production of P fertilisers has been shown to contaminate top soil horizons with U and other trace elements in the close vicinity of a factory located in Southern Brazil. In contrast to this point source, agricultural P fertilisation leads to a diffuse contamination of the agro-ecosystem with U and other fertiliser-derived trace elements on a large scale. Top soil horizons of arable land accumulate fertiliser-derived U. According to the geochemical behaviour of U(VI) species under oxidising conditions, the mobilisation capacity for U in top soil horizons is considered to be high, contrary to other fertiliser-derived heavy metals (e.g. Cd). Hence, it is assumed that U can be leached to shallow groundwater and can reach fresh water resources potentially used for drinking water supply. The aims of this study were to investigate the concentration of U and other contaminants in P fertilisers, to identify geochemical processes of fertiliser-derived U mobility and mobilisation from arable top soil horizons to the groundwater, and to evaluate the origin of U in German groundwater and tap water. This study presents the broadest recent data set on regional distribution of U concentrations in German tap water to which 76 % of the German population has access. The mean U concentration was 0.68 µg L-1, the median 0.50 µg L-1. 1.3 % or 1 million of the 80.6 million inhabitants in Germany are exposed to U concentrations in tap water which are higher than the German drinking water threshold limit of 10 µg L-1. The regional distribution of U concentrations largely agrees with the geological setting reported for mineral waters, however, in addition evidence for fertiliser- derived U in tap water was found in certain areas in Northern and Southern Germany. To test the hypothesized fertiliser-derived U leaching to groundwater, samples from different depths were taken in an area intensively used for cropping production in Lower Saxony. The - correlation between fertiliser-derived nitrate (NO3 ) and U especially in shallow (< 15 m) groundwater samples indicates the anthropogenic origin of U. However, there is not yet a clear picture of quantities and processes of U leaching from fertilised soil into groundwater. The future aim is to investigate these processes of U migration via isotope fingerprinting and geochemical modelling to provide a solid foundation for future risk assessment. Table of Contents Chapter 1 - Introduction ............................................................................................................ 1 Outline .................................................................................................................................... 1 Objectives ............................................................................................................................... 3 Current state of the research ................................................................................................. 4 Uranium in fertilisers .......................................................................................................... 4 Uranium in soils .................................................................................................................. 4 Anthropogenic load of U to soils ........................................................................................ 5 Uranium speciation and U mobility in soils and the vadose zone ..................................... 7 Leaching of U from soil to groundwater ............................................................................ 9 Uranium mobilisation governed by microbial induced redox processes ........................ 11 Chapter 2 - Publication 1 .......................................................................................................... 15 Heavy metal concentrations in soils in the vicinity of a fertilizer factory in Southern Brazil .. 16 Abstract ................................................................................................................................ 16 Zusammenfassung ................................................................................................................ 17 Introduction .......................................................................................................................... 18 2 Materials and methods ..................................................................................................... 19 2.1 Study area ................................................................................................................... 19 2.2 Sampling ..................................................................................................................... 20 2.3 Sample preparation and chemical analysis ................................................................ 21 2.4 Analytical procedures ................................................................................................. 22 2.5 Statistical analysis and calculation of enrichment factors for trace elements .......... 23 3 Results and Discussion ...................................................................................................... 23 3.1 Major and trace elements in P fertilizers traded in Brazil ......................................... 23 3.2 Major and trace elements in top and sub soils .......................................................... 25 3.3 Variations of element enrichment factors ................................................................. 28 3.4 Elemental composition of groundwater .................................................................... 30 3.5 Rare earth elements in soils and fertilizers ................................................................ 31 4 Conclusion ......................................................................................................................... 32 5 Acknowledgement ............................................................................................................. 33 6 References ......................................................................................................................... 33 Chapter 3 - Publication 2 .......................................................................................................... 37 Cadmium and uranium in German and Brazilian phosphorous fertilisers ............................... 38 Abstract ................................................................................................................................ 38 Introduction .......................................................................................................................... 38 Cadmium and uranium in soils ......................................................................................... 40 Accumulation of fertiliser-derived Cd and U in soils ........................................................ 40 Material and Methods.......................................................................................................... 41 Results and Discussion ......................................................................................................... 42 Cadmium and uranium in fertilisers ................................................................................. 42 Fertiliser-derived Cd and U loads to German and Brazilian soils ..................................... 43 Conclusions ........................................................................................................................... 45 References ............................................................................................................................ 45 Chapter 4 - Publication 3 .......................................................................................................... 49 Uranium in German tap and groundwater - occurrence and origins ...................................... 50 Abstract ................................................................................................................................ 50 Introduction .......................................................................................................................... 50 Materials and Methods ........................................................................................................ 52 Tap and groundwater sampling and analyses .................................................................. 52 Geocoding of samples ...................................................................................................... 53 Results and Discussion ......................................................................................................... 54