. Fig, Relationship between the total Fe concentration (i.e. 200 nm-filtered Fe) measured in field (Fe(Tot)Field) and in the laboratory (Fe(Tot) lab) in groundwaters (grey diamonds), and for soil waters (black diamonds) and the relationship between the percentage of Fe(II) (in all the different size fractions) and the total Fe concentration measured in field (Fe(Tot) field) in groundwater and soil water samples (grey and black circles symbols, respectively)

R. Bauer, M. Blodau, and C. , Arsenic distribution in the dissolved, colloidal and particulate size fraction of experimental solutions rich in dissolved organic matter and ferric iron, Geochimica et Cosmochimica Acta, vol.73, issue.3, pp.529-542, 2009.
DOI : 10.1016/j.gca.2008.10.030

M. Bravin, F. Travassac, L. Floch, M. Hinsinger, P. Garnier et al., Oxygen input controls the spatial and temporal dynamics of arsenic at the surface of a flooded paddy soil and in the rhizosphere of lowland rice (Oryza sativa L.): a microcosm study, Plant and Soil, vol.237, issue.1-2, pp.207-218, 2008.
DOI : 10.1007/s11104-007-9532-x

J. Buffle, D. Perret, and M. Newman, The use of filtration and ultrafiltration for size fractionation of aquatic particles, colloids, and macromolecules, Environmental Particles, pp.171-230, 1992.

J. Chen, B. Gu, R. A. Royer, and W. D. Burgos, The roles of natural organic matter in chemical and microbial reduction of ferric iron, The Science of The Total Environment, vol.307, issue.1-3, pp.167-178, 2003.
DOI : 10.1016/S0048-9697(02)00538-7

Z. Chen, Y. G. Zhu, W. J. Liu, and A. A. Meharg, Direct evidence showing the effect of root surface iron coating on arsenite and arsenate uptake into rice (Oryza sativa) roots. New Physiol, pp.91-97, 2005.

J. B. Christensen, J. J. Botma, and T. H. Christensen, Complexation of Cu and Pb by DOC in polluted groundwater: a comparison of experimental data and predictions by computer speciation models (WHAM and MINTEQA2), Water Research, vol.33, issue.15, pp.3231-3238, 1999.
DOI : 10.1016/S0043-1354(99)00020-2

M. C. Ciardelli, H. Xu, and N. Sahai, Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by coprecipitation in synthetic and natural groundwater, Water Research, vol.42, issue.3, pp.615-624, 2008.
DOI : 10.1016/j.watres.2007.08.011

C. C. Davis, W. R. Knocke, and M. Edwards, Implications of Aqueous Silica Sorption to Iron Hydroxide:?? Mobilization of Iron Colloids and Interference with Sorption of Arsenate and Humic Substances, Environmental Science & Technology, vol.35, issue.15, pp.3158-3162, 2001.
DOI : 10.1021/es0018421

S. Dixit and J. G. Hering, Comparison of Arsenic(V) and Arsenic(III) Sorption onto Iron Oxide Minerals:?? Implications for Arsenic Mobility, Environmental Science & Technology, vol.37, issue.18, pp.4182-4189, 2003.
DOI : 10.1021/es030309t

E. Doelsch, J. Rose, A. Masion, J. Bottero, and D. Nahon, Ligands, Langmuir, vol.18, issue.11, pp.4292-4299, 2002.
DOI : 10.1021/la011605r

E. Doelsch, A. Masion, J. Rose, W. E. Stone, J. Y. Bottero et al., Chemistry and structure of colloids obtained by hydrolysis of Fe(III) in the presence of SiO 4 ligands, Colloids Surf., A: Physicochem. Engin. Aspects, vol.217, pp.1-3, 2003.

Y. Dudal and F. Gérard, Accounting for natural organic matter in aqueous chemical equilibrium models: a review of the theories and applications, Earth-Science Reviews, vol.66, issue.3-4, pp.199-216, 2004.
DOI : 10.1016/j.earscirev.2004.01.002

J. W. Gaffney, K. N. White, and S. Boult, Oxidation State and Size of Fe Controlled by Organic Matter in Natural Waters, Environmental Science & Technology, vol.42, issue.10, pp.3575-3581, 2008.
DOI : 10.1021/es702880a

J. Garnier, F. Travassac, V. Lenoble, J. Rose, Y. Zheng et al., Temporal variations in arsenic uptake by rice plants in Bangladesh: The role of iron plaque in paddy fields irrigated with groundwater, Science of The Total Environment, vol.408, issue.19, pp.4185-4193, 2010.
DOI : 10.1016/j.scitotenv.2010.05.019

Y. Ge, D. Macdonald, S. Sauve, and W. Hendershot, Modeling of Cd and Pb speciation in soil solutions by WinHumicV and NICA-Donnan model, Environmental Modelling & Software, vol.20, issue.3, pp.353-359, 2005.
DOI : 10.1016/j.envsoft.2003.12.014

W. Guo, Y. L. Hou, S. G. Wang, and Y. G. Zhu, Effect of silicate on the growth and arsenate uptake by rice (Oryza sativa L.) seedlings in solution culture, Plant and Soil, vol.17, issue.5, pp.173-181, 2005.
DOI : 10.1007/s11104-004-4732-0

J. P. Gustafsson, WinHumicV For Win95/98/NT, 1999.

M. Hassellöv, K. O. Buesseler, S. M. Pike, and M. Dai, Application of cross-flow ultrafiltration for the determination of colloidal abundances in suboxic ferrous-rich ground waters???, Science of The Total Environment, vol.372, issue.2-3, pp.636-644, 2007.
DOI : 10.1016/j.scitotenv.2006.10.001

B. Jansen, J. Mulder, and J. M. Verstraten, Organic complexation of Al and Fe in acidic soil solutions, Analytica Chimica Acta, vol.498, issue.1-2, pp.105-117, 2003.
DOI : 10.1016/j.aca.2003.08.054

D. G. Kinniburgh, W. H. Van-riemsdijk, L. K. Koopal, M. Borkovec, M. Benedetti et al., Ion binding to natural organic matter: competition, heterogeneity, stoichiometry and thermodynamic consistency, Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol.151, issue.1-2, pp.147-166, 1999.
DOI : 10.1016/S0927-7757(98)00637-2

V. Lenoble, C. Garnier, A. Masion, F. Ziarelli, and J. M. Garnier, Combination of 13C/113Cd NMR, potentiometry, and voltammetry in characterizing the interactions between Cd and two models of the main components of soil organic matter, Analytical and Bioanalytical Chemistry, vol.71, issue.2, pp.749-757, 2008.
DOI : 10.1007/s00216-007-1678-0

URL : https://hal.archives-ouvertes.fr/hal-01096823

F. Liu, A. De-cristofaro, and A. Violante, EFFECT OF pH, PHOSPHATE AND OXALATE ON THE ADSORPTION/DESORPTION OF ARSENATE ON/FROM GOETHITE, Soil Science, vol.166, issue.3, pp.197-208, 2001.
DOI : 10.1097/00010694-200103000-00005

W. J. Liu, Y. G. Zhu, F. A. Smith, and S. E. Smith, Do phosphorus nutrition and iron plaque alter arsenate (As) uptake by rice in hydroponic culture? New Phytolog, pp.481-488, 2004.

J. T. Mayo, C. Yavuz, S. Yean, L. Cong, H. Shipley et al., The effect of nanocrystalline magnetite size on arsenic removal, Science and Technology of Advanced Materials, vol.204, issue.1-2, pp.71-75, 2007.
DOI : 10.1039/b409601k

A. A. Meharg and M. Rahman, Arsenic Contamination of Bangladesh Paddy Field Soils:?? Implications for Rice Contribution to Arsenic Consumption, Environmental Science & Technology, vol.37, issue.2, pp.229-234, 2003.
DOI : 10.1021/es0259842

N. Mladenov, Y. Zheng, M. P. Miller, D. Nemergut, T. Legg et al., Dissolved Organic Matter Sources and Consequences for Iron and Arsenic Mobilization in Bangladesh Aquifers, Environmental Science & Technology, vol.44, issue.1, pp.15-23, 2010.
DOI : 10.1021/es901472g

S. Norra, Z. A. Berner, P. Agarwala, F. Wagner, D. Chandrasekharam et al., Impact of irrigation with As rich groundwater on soil and crops: A geochemical case study in West Bengal Delta Plain, India, Applied Geochemistry, vol.20, issue.10, pp.1890-1906, 2005.
DOI : 10.1016/j.apgeochem.2005.04.019

L. C. Roberts, S. J. Hug, J. Dittmar, A. Voegelin, R. Kretzschmar et al., Arsenic release from paddy soils during monsoon??flooding, Nature Geoscience, vol.298, issue.1, pp.53-59, 2010.
DOI : 10.1038/ngeo723

J. Rose, A. Manceau, J. Bottero, A. Masion, and F. Garcia, Ions. 1. Fe K-Edge EXAFS Study, Langmuir, vol.12, issue.26, pp.6701-6707, 1996.
DOI : 10.1021/la9606299

URL : https://hal.archives-ouvertes.fr/hal-01426201

J. Rose, A. Vilgé, G. Olivie-lauquet, A. Masion, C. Frechou et al., Iron speciation in natural organic matter colloids, Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol.136, issue.1-2, pp.11-19, 1998.
DOI : 10.1016/S0927-7757(97)00150-7

M. Sadiq, Arsenic chemistry in soils: an overview of thermo dynamic predictions and field investigations, Water Air Soil Pollut, vol.93, pp.117-136, 1997.

N. Sahai, Y. J. Lee, H. Xu, M. Ciardelli, and J. Gaillard, Role of Fe(II) and phosphate in arsenic uptake by coprecipitation, Geochimica et Cosmochimica Acta, vol.71, issue.13, pp.3193-3210, 2007.
DOI : 10.1016/j.gca.2007.04.008

P. L. Smedley and D. G. Kinniburgh, A review of the source, behaviour and distribution of arsenic in natural waters, Applied Geochemistry, vol.17, issue.5, pp.517-568, 2002.
DOI : 10.1016/S0883-2927(02)00018-5

Y. Takahashi, R. Minamikawa, K. H. Hattori, K. Kurishima, N. Kihou et al., Arsenic Behavior in Paddy Fields during the Cycle of Flooded and Non-flooded Periods, Environmental Science & Technology, vol.38, issue.4, pp.1038-1044, 2004.
DOI : 10.1021/es034383n

S. Thoral, J. Rose, J. Garnier, A. Van-geen, P. Refait et al., XAS Study of Iron and Arsenic Speciation during Fe(II) Oxidation in the Presence of As(III), Environmental Science & Technology, vol.39, issue.24, pp.9478-9485, 2005.
DOI : 10.1021/es047970x

URL : https://hal.archives-ouvertes.fr/hal-00125447

E. Tipping, Humic Ion Binding Model VI: an improved description of the interactions of protons and metal ions with humic substances, Aquatic Geochemistry, vol.4, issue.1, pp.3-48, 1998.
DOI : 10.1023/A:1009627214459

E. R. Unsworth, K. W. Warnken, H. Zhang, W. Davison, F. Black et al., Model Predictions of Metal Speciation in Freshwaters Compared to Measurements by In Situ Techniques, Environmental Science & Technology, vol.40, issue.6, pp.1942-1949, 2006.
DOI : 10.1021/es051246c

A. Van-geen, J. Rose, S. Thoral, J. Garnier, Y. Zheng et al., Decoupling of As and Fe release to Bangladesh groundwater under reducing conditions. Part II: Evidence from sediment incubations, Geochimica et Cosmochimica Acta, vol.68, issue.17, pp.3475-3486, 2004.
DOI : 10.1016/j.gca.2004.02.014

A. Van-geen, Y. Zheng, Z. Cheng, H. Yi, R. K. Dhar et al., Impact of irrigating rice paddies with groundwater containing arsenic in Bangladesh, Science of The Total Environment, vol.367, issue.2-3, pp.769-777, 2006.
DOI : 10.1016/j.scitotenv.2006.01.030

E. Viollier, P. W. Inglett, A. N. Hunter, P. Roychoudhury, and P. Van-capellen, The ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters, Applied Geochemistry, vol.15, issue.6, pp.785-790, 2000.
DOI : 10.1016/S0883-2927(99)00097-9

A. Wolthoorn, E. J. Temminghoff, L. Weng, and W. H. Van-riemsdijk, Colloid formation in groundwater: effect of phosphate, manganese, silicate and dissolved organic matter on the dynamic heterogeneous oxidation of ferrous iron, Applied Geochemistry, vol.19, issue.4, pp.611-622, 2004.
DOI : 10.1016/j.apgeochem.2003.08.003