LOADS AND FATE OF FERTILIZER-DERIVED URANIUM

Cover: “Uranyum“ hand sewn textile collage from velvet and silk (38 X 37 cm)
by Bettina Franckenberg, Bodrum, Turkey (2007)
The Rainbow Serpent is a monster etched into the cliff walls of theUpper Yule River in Australia long before European people everset food in the region - and discovered uranium there. It was
originally painted by the Njamal people of northeast Australia to
guard over the elemental forces buried in the Earth. According toNjamal legend, the Serpent sleeps within the Earth. Its job is toguard over the elemental forces that lie outside the realm of humancontrol, but if the Serpent is disturbed, it will rise in vengeance ina deluge of destruction and death. The places where according to
the tales the serpent sleeps were sacred to the Aborigines and were
avoided. Frequently these places coincide with uranium deposits.
The message of the Njamal: “Leave the uranium in the earth”.
In her artwork “Uranyum” Bettina Franckenberg attempts aconnection between the tales of the Australian Aborigines and thetheme of the symposium “Fate and loads of fertilizer-derived
uranium” (June 4 - 6, 2007 at FAL in Braunschweig, Germany). In
the lower left quadrant the sleeping serpent represents the naturaluranium in the soil with unspoiled environment and life in theworld above it (upper left). But when the sleeping serpent isdisturbed by man-made uranium contaminations (lower right) it
may raise in vengeance with fatal consequences for life on earth(upper right).
About the artist: Bettina Franckenberg was born February 3, 1956
in Essen, Germany. Handcraft and artistic abilities wereencouraged early in her parent's house. After graduating fromcollege in 1975 she continued with her education in the field ofoccupational therapy, 1979 graduation and starting to specialize inpsychiatry, supplementary qualification as psychodrama assistant at the same time initiation to "patchwork". In 1982 she discovers
the Mediterranean (living the first 2 years in Greece) as well as
her "textile ambitions" - 1986 she opens her first exhibition in theCastle of St. Peter in Bodrum. She goes back to live in Berlin
for some years - between 1989 and 1992 she works as an
occupational therapist in psychiatry and organizes several "textile
art" exhibitions in Germany, Switzerland, Latvia and Turkey.
Since 1992 Bettina Franckenberg lives permanently in Bodrum,
Turkey and between 1994 and 2005 she had her own ATELlERGallery in the holiday-village Sea-Garden in Yaliçiftlik. She
continues working as a freelance textile artist living in Kizilagac
near Bodrum.

LOADS AND FATE OF FERTILIZERDERIVED URANIUM

Edited by Luit J. De Kok and Ewald Schnug

BACKHUYS PUBLISHERS, LEIDEN, 2008

Editors
Luit J. De Kok
University of Groningen
Haren, The Netherlands/
University of Melbourne
Creswick, Australia

Ewald Schnug
Julius Kühn-Institut (JKI)
Braunschweig, Germany

Cover design: Dick Visser (Groningen, The Netherlands)
Cover art: Bettina Franckenberg (Bodrum, Turkey)
ISBN/EAN 978-90-5782-193-6© 2008 Backhuys Publishers, Leiden, The Netherlands
All rights reserved. Nothing from this publication may be reproduced, stored in a computerized
system or published in any form or in any manner, including electronic, mechanical, reprographic
or photographic, without prior permission from the publishers, Backhuys Publishers, P.O. Box
321, 2300 AH Leiden, The Netherlands.
Printed in Germany

CONTENTS
Prefaceix

Radiological aspects of uranium contamination - Inge Schmitz-Feuerhake andRosalie Bertell

1

Advanced biochemical and biophysical aspects of uranium contamination Chris Busby and Ewald Schnug

11

Bioaccumulation of radionuclides and induced biological effects in situations ofchronic exposure of ecosystems – an uranium case study
Pascale Henner

23

Uranium in German soils - Jens Utermann and Michael Fuchs

33

Mobility of uranium from phosphate fertilizers in sandy soils - Beate Huhle,
Sascha Kummer, Susanne Stadler and Broder Merkel

47

Modelling the fate of uranium from inorganic phosphorus fertilizer applications
in agriculture - Diederik Jacques, Dirk Mallants, Jirka Šimůnek and Martinus
Th. van Genuchten

57

The fate of uranium in phosphate-rich soils - David Read, Stuart Black, Helen
Beddow, Elaine Trueman, Thuro Arnold and Nils Baumann

65

Uranium ammunition in soil - Nils Baumann, Thuro Arnold and David Read

73

Uranium in stream water of Germany - Manfred Birke and Uwe Rauch

79

Uranium in German mineral and tap waters - Ewald Schnug, Manfred Birke,
Nathalie Costa, Friedhart Knolle, Jürgen Fleckenstein, Kerstin Panten, Holger
Lilienthal and Silvia Haneklaus

91

A critical evaluation of phytoextraction on uranium contaminated agriculturalsoils - Silvia Haneklaus and Ewald Schnug

111

Combining energy and fertilizer production - vision for China’s future - Zhengyi
Hu, Hongxun Zhang, Yanfen Wang, Silvia Haneklaus and Ewald Schnug

127

Uranium in soils of German long-term fertilizer experiments - Jutta Rogasik,
Sylvia Kratz, Ute Funder, Kerstin Panten, Dietmar Barkusky, Michael
Baumecker, Reinhold Gutser, Peter Lausen, Heinrich W. Scherer, Lothar
Schmidt and Ewald Schnug

135

The fate of uranium contaminants of phosphate fertilizer - Matthew Taylor andNick Kim

147

Soil to plant interface of uranium - Judith Schick, Susanne Schroetter, Maria
Lamas, Maria Rivas, Sylvia Kratz and Ewald Schnug

157

Food chain transfer of uranium series radionuclides - Patricia A. Thomas

169

Uranium balances in agroecosystems - Sylvia Kratz, Florian Knappe, Jutta
Rogasik and Ewald Schnug

179

Significance of uranium and strontium isotope ratios for retracing the fate ofuranium during the processing of phosphate fertilizers from rock phosphates Mamdoh Sattouf, Sylvia Kratz, Katrin Diemer, Jürgen Fleckenstein, Olaf
Rienitz, Detlef Schiel and Ewald Schnug

191

Setting normative rates of natural radionuclides in phosphorus fertilizers Vladimir Drichko, Alexandra Izosimova, Elvira Lisachenko, Stanislav
Graschenko and Viktor Shamov

203

Legal aspects of uranium in environmental compartments - Felix Ekardt andEwald Schnug

209

Index of authors

219

Subject index

221

vi

This book is dedicated to Dr. Jürgen Fleckenstein,senior scientist at FAL (1974-2008), lately head chemist at theInstitute for Crop and Soil Science in Braunschweig.
He dedicated significant parts of his life to uranium chemistry and the environmental
impacts of fertilizer-derived uranium.

viiA quote from President John F. Kennedy's speech to the people of the United States on July 26, 1963:” …the number of children and grandchildren with cancer in their bones, with leukaemia in theirblood, or with poison in their lungs [due to radioactive pollution] might seem statistically small tosome, in comparison with natural health hazards. But this is not a natural health hazard -- and it is
not a statistical issue. The loss of even one human life, or the malformation of even one baby -- whomay be born long after we are gone -- should be of concern to us all. Our children and grandchildren
are not merely statistics toward which we can be indifferent.....”

viii

PREFACE“Sustainable development is development that meets the needs of the present withoutcompromising the ability of future generations to meet their own needs”. One important
aspect of sustainable development in agriculture is the responsible management of
natural resources of which soils and water bodies are the most endangered ones. A
major threat for soils are loads with toxic heavy metals applied unintentionally withfertilizers and which accumulate and may pass through soils into ground and surface
waters. Any agricultural system where the input of heavy metals exceeds the sum of
removal by crops and unavoidable losses to the environment is in fact non-sustainablebecause it leaves a burden for further generations. One of the most dangerous heavy
metals is uranium, because it is not only bio-chemically toxic but also radioactive.
Uranium is dispersed in significant amounts for instance via phosphorus fertilizers.
Although a lot of uranium research has been done under many viewpoints, theagricultural aspect and its environmental impact has not been paid much attention so far.
In this context the “Task Force Sustainable Agriculture (TFSA)” of the Agenda 21 for
the Baltic Sea Region (BALTIC21) has organized a series of symposia entitled“Protecting Water Bodies from Negative Impacts of Agriculture”. Commissioned by theFederal Minister of Food, Agriculture and Consumer Protection and encouraged by her
Scientific Advisory Board on Fertilizer Issues, the Federal Environment Agency Soil
Protection Commission (KBU) and the Federal Institute for Risk Assessment (BFR), theformer Institute of Plant Nutrition and Soil Science (PB) of the Federal Agricultural
Research Centre (FAL) in Braunschweig (from 01.01.2008 on “Institute for Crop andSoil Science, Federal Research Centre for Cultivated Plants, Julius Kühn-Institut (JKI)”)and TFSA conducted under this general headline from June 4-6, 2007 the symposium“Loads and Fate of Fertilizer-derived Uranium”. The symposium brought together theworld’s leading scientists on the subject. The papers of the symposium are compiled inthis book and have been peer-reviewed. They present the most recent andcomprehensive collection of data and references on agricultural matters related touranium. It provides a solid base of scientific information for responsible decisionmaking in agricultural politics towards sustainable development in agriculture.
Luit J. De Kok
Ewald Schnugix

xMODELLING THE FATE OF URANIUM FROM
INORGANIC PHOSPHORUS FERTILIZER APPLICATIONSIN AGRICULTURE6
Diederik Jacques1, Dirk Mallants1, Jirka Šimůnek 2 and Martinus Th. van
Genuchten31Institute for Environment, Health, and Safety, Belgian Nuclear Research Centre(SCK•CEN), Boeretang 200, B-2400 Mol, Belgium; 2Department of Environmental
Sciences, University of California, Riverside, CA 92521, USA; 3U.S. Salinity laboratory,
USDA, ARS, 450 W. Big Spring Rd, Riverside, CA 92507, USA

Introduction
When inorganic P-fertilizers (e.g. monocalcium phosphate or triple superphosphate) areapplied to agricultural fields, U is also added due to its natural occurrence in the raw
material apatite. Compared to phosphate rock, mineral P-fertilizers are enriched in U
since this element remains in the liquid phase during acidification of the phosphate rock(Barišić et al. 1992; Saueia and Mazzilli 2006). P-fertilizers hence often contain naturalalpha-activity due to the presence of 238U, among other alpha emitters. Uranium
concentrations (naturally occurring U contains 99% by weight 238U) in P-bearing
fertilizers have been reported to be in the range of 300 to 3000 Bq kg-1 of fertilizer (EC1999), or 1700 to 9200 Bq kg-1 of fertilizer for both 238U and 234U (Cogné 1993).
The environmental impact of P-fertilizers is in general relatively modest. Different
studies have found different relative contributions of U from inorganic P-fertilizers to U
concentrations in drainage and surface waters. For example, Rothbaum et al. (1979) andZielinski et al. (1997) found that most U was retained in the soil profile, and that only
very little U from P-fertilizers ended up in the drainage water. On the other hand,
Spalding and Sackett (1972) attributed the increase in U in North American rivers toapplication of P-fertilizers on agricultural land. According to a study of Barišić et al.
(1992), more than 20 percent of the annually deposited U by fertilizers is transported todrainage channels in the Kanovci area of Croatia. The mobility of agriculture-applied U(and P) depends strongly on soil conditions such as the pH, soil moisture, mineralogy
and texture (Zielinski et al. 1997). Transient flow conditions in the soil (infiltration,
evapotranspiration, moisture redistribution) furthermore can influence specificgeochemical conditions in the soil (e.g., pH and solid phase reactivity), and thus also
affect the mobility of aqueous elements (Jacques et al. 2007a).
Saueia and Mazzilli (2006) calculated internal doses due to the long-term applicationof phosphate fertilizers. Pathways for the internal doses were through the ingestion ofmilk, meat and terrestrial food via crops grown on fertilized fields. Radionuclide
activities in these pathways were related to radionuclide activity concentrations in the
Loads and Fate of Fertilizer-derived Uranium, pp. 57-64
Edited by L.J. De Kok and E. Schnug 2008 Backhuys Publishers, Leiden, The Netherlands

57soil through transfer factors quantifying U uptake by food crops and livestock. However,lower activities in the root zone or soil due to leaching to deeper soil layers or
groundwater were not considered. Still, the doses calculated by Saueia and Mazzilli(2006) were well below 1 mSv yr-1 in most cases. Another exposure pathway for U is
via soil leaching and groundwater flow to surface water or drinking wells. Estimates of
U-fluxes to groundwater are required as input for these types of pathway analyses. In
this paper, we estimate such U-fluxes using the HP1 code of Jacques and Šimůnek
(2007), which is a coupled reactive transport model that integrates a broad range ofwater flow, solute transport and geochemical processes in the vadose zone. The modelallows one to account for specific U-soil-water interactions such as sorption on solid
surfaces and aqueous complexation (Guzman et al. 2002). Groundwater concentrations
for U will be calculated on the basis of a flux-concentration relationship determined
separately (Gedeon and Mallants 2004). We further calculate the radiological impact
associated with U intake via groundwater used by a small self-sustaining farming
community. Concentrations and doses thus obtained will be compared with natural
background values typical for Northern Belgium.

Predicted U-fluxes to groundwater
The HP1 simulator (Šimůnek et al. 2006; Jacques and Šimůnek 2007) was used to
predict the leaching of Ca, P and U applied to an agricultural soil in the form of amineral P-fertilizer (Jacques et al. 2007b). HP1 resulted from coupling the HYDRUS1D one-dimensional water flow and solute transport model (Šimůnek et al. 2005) with
the PHREEQC-2.13 geochemical speciation code (Parkhurst and Appelo 1999). As
such, HP1 can account for a large number of geochemical processes, including aqueous
complexation, cation exchange and surface complexation, during transient fieldsituations (i.e. for time-variable precipitation, evapotranspiration, drainage, and soilwater contents). Jacques et al. (2006) previously showed that the invoked non-iterative
operator splitting approach in HP1 is quite versatile for simulations involving transient
flow if some care is taken in selecting the numerical time-stepping scheme.
Here we use the code to predict long-term U leaching from a soil profile in Northern
Belgium that is assumed to be fertilized annually with a phosphate fertilizer naturally
enriched in uranium. Complete details of the example are given by Jacques et al.
(2007b). The soil at the site was a dry Spodosol consisting of seven soil horizons in the
top 1 m. The moisture retention characteristics, the unsaturated hydraulic conductivity,
the bulk density, soil organic matter and the Fe-oxide content of each horizon were
measured by Seuntjes (2000). We assumed that cation exchange reactions took place
only on organic matter, and surface complexation reactions only on the Fe-oxides.
The cation exchange capacity was related to the amount of organic matter byassuming that the amount of exchangeable protons on organic matter (OM) was 6 meq
g-1 (Tipping 2002). To account for different functional groups on organic matter (andthus different dissocation constants for the protons), the total capacity was divided in six
groups involving different OM functional groups, similar to the model of Appelo et al.
(1998). The capacity of the surface sites was derived for the amount of Fe2O3 measured
in the soil profile assuming 0.875 reactive sites per mole Fe (Waite et al. 1994).

58

Equilibrium constants were taken from Dzombak and Morel (1990). Elements
exchanged on the exchange complex were H+, Na+, K+, Ca2+, Mg2+ and UO2. Elements
taking part in the surface complexation reactions were taken as H+, Ca2+, Mg2+ ,UO22+,
PO43-, SO42- and F-. Meterological data from the Brogel station (Belgium) for a 30-year
period (1969-1999) were used to define daily precipitation and potential evaporation.
The effects of vegetation (transpiration and U uptake) were neglected in the example. To
obtain a 200-year time series for the modeling purposes, the 30-year period was simply
repeated seven times. In addition to the transient flow simulation, a simulation with asteady-state water flux was done. The cumulative steady-state water flux after 200 years
is equal to the long term net infiltration calculated during the transient flow simulation.
Solute concentrations of the rain water were obtained from Stolk (2001) for station 231
located in Gilze-Rijen (The Netherlands), close to the site being modeled.
The fertilizer scenario assumed an annual application of Ca, P, and U. In Flanders(Northern Belgium), the amount of applied P that originates from mineral fertilizers
decreased over the last decade from 13.9x106 kg P in 1990 to 2.38x106 kg P in 2003
(MIRA 2004). To simulate the long-term transport of P and U through the soil, thefollowing assumptions were made: (1) P-fertilizers are applied each year on May 1 at a
rate of 1 g P m-2 (or 6.35 x 106 kg P in all of Flanders) in the form of Ca(H2PO4)2, and(2) 1 kg of Ca(H2PO4)2 contains 10-3 mol U. Based on a specific activity of 2.96x106 Bq
mol-1 U, 2960 Bq of alpha activity exclusively due to 238U is present in 1 kg of
Ca(H2PO4)2. Thus, a fertilizer application of 1 g P m-2 corresponds to 1.61x10-2 mol ofCa(H2PO4)2 containing 3.77x10-6 mol of U. The calculations assumed that the fertilizer
was applied each year in the form of Ca(H2PO4)2 in 1 cm of water and that it wascompletely dissolved in the irrigation water.
Preliminary calculations with HP1 showed a significant effect of pH and solution
composition on U sorption. Consequently, an approach in which sorption is describedwith a linear distribution coefficient, Kd (the traditional linear equilibrium approach)may not be applicable. In fact, for relevant soil pH-conditions (between pH 3 and 6), theKd would increase more than three orders of magnitude (in the absence of competing Ca
and P elements from the fertilizer application) with increasing pH. We note that U ispreferentially sorbed onto the cation exchange complex at the lower pH values, and on
Fe-oxide surfaces at the higher pH’s. The composition of the aqueous phase also
influences Kd. The value of Kd decreased by almost three orders of magnitude when theconcentrations of Ca, P, and U were at the levels of the dissolved fertilizer. Sincetransient flow conditions influence aqueous concentrations and the pH of the soil
solution (e.g., Jacques et al. 2007a), Kd values will be very variable in both time andspace. This is illustrated by the time series in Fig. 1 of the Kd of U at 5 and 25 cm soil
depths for both transient and steady-state flow conditions. The transient flow simulation
showed significant short-time variations in the Kd, which are clearly absent in thesteady-state simulation results. We also found a long-time trend in Kd caused by longtime changes in the amounts of Ca, P, and U due to the annual fertilizer applications.
Because U moves faster than P through the soil, there is still less competition forsorption on the Fe-oxide surface at a depth of 25 cm (P has not yet reached this depth),leading to an increase in U sorption over time. The amplitude of the short-time
variations of Kd at the same time decreased with time. The adopted sorption isotherm istypically nonlinear, having a slope (the first derivative) that gradually decreases with59

increasing aqueous concentration. The Kd, being the slope of the isotherm at a particularconcentration, then also decreases with increasing aqueous concentration. In addition,
variations in Kd with changing concentration (the second derivative of the isotherm) go
to zero with increasing aqueous U concentration. In the long term, U concentrations
increase due to the annual addition of the P-fertilizer, which may explain the decreasing
fluctuations in calculated Kd values.
1x104

4

1x10

Kd of U (l kg-1)

Kd of U (l kg-1)

3

1x10

2

1x10

1

3

1x10

1x102

1x10

5 cm

1x100
0

25

50
Time (years)

75

100

25 cm

1

1x10

0

50

100
Time (years)

150

Fig. 1. Time series of the calculated Kd of U for transient (solid line) and steady-state (dashedline) flow simulations at depths of 5 cm (left) for the first 100 years) and 25 cm (right) for the full
200 years of simulation (after Jacques et al. 2007b).

Fig. 2 shows the fluxes at three depths: the bottom of the A (7 cm), Bh2 (28 cm) andC2 (100 cm) horizons for both the transient and steady-state flow conditions. Steadystate fluxes were obtained after 40 and 100 years for the A and Bh2 horizon,
respectively. While U reached the bottom of the soil profile after 80 years, a steady-state
flux was not yet obtained within the 200-year simulation period. U fluxes increased
earlier for the transient flow simulation, which reflects faster leaching due tointeractions caused by short-time variations in water contents, water fluxes andgeochemical conditions (Jacques et al. 2007b).

Dose calculations for groundwater use
The environmental impact of uranium leaching was assessed first by calculating theconcentration of groundwater at a hypothetical well approximately 100 m downstream
from a 5 ha field site which had received the long-term P-fertilizer applications. A fluxconcentration relationship as determined by Gedeon and Mallants (2004) was used toderive groundwater concentrations for a sandy aquifer assuming no sorption onto thesandy sediments. For a unit radionuclide flux reaching groundwater over an area of 5 ha,
the groundwater transport calculations by Gedeon and Mallants (2004) predicted adepth-averaged concentration of 4.7×10-5 Bq m-3 at the well (averaged between depths

60

200

of 3.8 and 5.9 m). Our steady-state leaching calculations produced a long-term
maximum U-flux of 5.7×105 Bq yr-1 over the 5 ha area, equivalent to a maximum
concentration of 27 Bq m-3. By comparison, the transient leaching calculations suggestthat the maximum flux could be approximately one order of magnitude higher if shortterm variations in climate are accounted for. While these results are based on
calculations for the shallower soil depths where steady-state had already been reached,
we assumed that they would also hold for the fluxes at the bottom of the profile once theflux has reached its steady-state value. The latter should also be equal to the steady-state
flux at shallower depth (i.e. 5.7×105 Bq yr-1 over the 5 ha area) since the soil model
assumed a fixed sorption capacity and uptake was neglected. In other words, the longterm maximum U concentration in groundwater under realistic climate conditions wasestimated to be 270 Bq m-3. By comparison, local background U concentrations in thearea were found to be in the order of 1000 Bq m-3 or less (Mallants et al. 2004).
3

U (Bq/y/m²))

U (Bq/y/m²)

3

1x10
1x102
1
1x10
0
1x10
1x10-1
-2
1x10
-3
1x10
-4
1x10
-5
1x10
-6
1x10
-7
1x10
-8
1x10
-9
1x10
-10
1x10

7 cm
0

U (Bq/y/m²)

-3

1x10
-4
1x10
-5
1x10
1x10-6
-7
1x10
-8
1x10
-9
1x10
-10
1x10
1x10-11
-12
1x10
-13
1x10
-14
1x10

25

50
Time (year)

75

100

80
120
Time (year)

160

200

1x10
1x102
1
1x10
0
1x10
1x10-1
-2
1x10
-3
1x10
-4
1x10
-5
1x10
-6
1x10
-7
1x10
-8
1x10
-9
1x10
-10
1x10

28 cm
0

40

80
120
Time (year)

160

200

100 cm040

Fig. 2. Downward U fluxes (Bq/y/m²) at the bottom of the A (7 cm, 100 years), Bh2 (28 cm 200
years), and C2 (100 cm 200 years) horizons for the transient (dots) and steady-state (dashed line)
flow simulations (after Jacques et al. 2007b).

The final step in our assessment was the estimation of the radiological impact owing
to consumption of 238U-containing groundwater by a self-sustaining farming community.
The biosphere model developed by Zeevaert and Sweeck (2000) was used for thispurpose. We considered the effects of 238U on the dose calculations as well those of its
short-lived daughter radionuclides that are in equilibrium with 238U, i.e. 234Th, 234Pa,

61

234m

Pa, by adding their dose factors to the dose factor of 238U. When groundwater is used
for irrigation of food crops and for drinking water by both residents and cattle, the total
annual dose was 5.2×10-7 Sv yr-1, assuming a maximum groundwater concentration of
27 Bq m-3. Approximately 90% of this total dose was due to ingestion of drinking water,while ingestion of food crops, milk and meat contributed far less to the total annual
dose. Neglecting the contribution of drinking water, the upper bound to the annual total
dose was only 5.2×10-6 Sv yr-1. The latter value is about three orders of magnitude
below the natural background dose of about 2.5 mSv yr-1 in Belgium (FANC 2006). The
long-term radiological impact for the conditions described in this paper seems therefore
negligible. We note that our calculations neglected the effect of U-uptake by plants
directly from the fertilizer application. However, based on results by Saueia andMazzilli (2006), such doses are less than 1 mSv yr-1.

Conclusions
We used the recently developed HP1 coupled geochemical transport code to calculate
U-fluxes to groundwater originating from application of inorganic P-fertilizers to an
agricultural field. The HP1 code was found to be a very convenient tool for studying
interactions between water content and flow conditions, changes in pH, and retention of
U. Results show that U will migrate considerable faster during transient flow conditions
as compared to steady-state flow. A steady-state flow approach hence can only partiallyaccount for the various coupled physical and geochemical interactions in the vadosezone, whereas short-term variations involving considerable temporal variability requirea transient model. Calculations of the radiological impact of U-containing groundwater
demonstrate that the annual total dose owing to consumption of groundwater would notlead to values exceeding the natural background dose.
Our study was a first analysis that does not yet fully account for all physical andgeochemical processes potentially relevant for predicting U leaching from fertilized
agricultural soil profiles. Other important soil processes such as the carbon and nitrogen
cycles (e.g. the effects of complexation with nitrate and carbonate) and vegetation-soil
processes such as water uptake (and possibly U uptake), may well need to beincorporated in any future, more comprehensive modeling approach.
Acknowledgements: This work was partly supported through bilateral agreement
projects between SCK•CEN and USDA-ARS (Agreement No. 58-5310-0-F105) and the
University of California, Riverside (Agreement No. C0-900001412.01F), by SCK•CEN
RandD project C091002, by the National Science Foundation (Agreement No. EAR9876800), and by the Terrestrial Sciences Program of the Army Research Office.

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