Long-term phosphate (P) desorption from soil is described using two discrete P "pools" in the soil: one available and one strongly fixed pool. The P release kinetics for each pool are described with a firstorder rate equation. A new desorption method is used with hydrous iron oxide inside dialysis tubing acting as a P sink. The widely used iron-impregnated filter paper desorption method overestimates initial P desorption by a factor of up to 4 and underestimates the quantitative progression of desorption as a function of time. P desorption continued with substantial rates for periods longer than 1600 h. A wide range in P desorbability was observed: 15-70% of oxalateextractable P (Pox) desorbed after 1600 h. P desorbability decreased with increasing Feox + Alox content of the sample. The relative size of the quickly desorbing pool increased with increasing initial degree of P saturation oto = P0X/[Fe0X + Alox] of the soils. This fact is of direct importance for the estimation of P losses from phosphate-rich soils. This study furthermore provides evidence that all oxalate-extractable P potentially is desorbable: no irreversibly fixed Pox exists.

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This study explored the potential of eutrophic river sediment to attenuate the infiltration of chlorinated aliphatic hydrocarbon (CAH)-polluted groundwater discharging into the Zenne River near Brussels, Belgium. Active CAH biodegradation by reductive dechlorination in the sediment was suggested by a high dechlorination activity in microcosms containing sediment samples and the detection of dechlorination products in sediment pore water. A unique hydrogeochemical evaluation, including a delta2H and delta18O stable isotope approach, allowed to determine the contribution of different abiotic and biotic CAH attenuation processes and to delineate their spatial distribution inthe riverbed. Reductive dechlorination of the CAHs seemed to be the most widespread attenuation process, followed by dilution by unpolluted groundwater discharge and by surface water mixing. Although CAHs were never detected in the surface water, 26-28% of the investigated locations in the riverbed did not show CAH attenuation. We conclude that the riverbed sediments can attenuate infiltrating CAHs to a certain extent, but will probably not completely prevent CAHs to discharge from the contaminated groundwater into the Zenne River.