HYDROCHEMICAL MODELING OF THE RETENTION AND TRANSPORT OF METALLIC RADIONUCLIDES IN THE SOILS OF AN UPLAND CATCHMENT

The CHemistry of the Uplands Model (CHUM) describes the transport of c hemicals through upland catchments with acid, organic-rich soils, by a combination of sub-models for equilibrium soil chemistry, hydrology, weathering, and nitrogen cycling. CHUM was used to simulate the retent ion and transport of metallic radionuclides (Go, Sr, Cs, UO2, U(IV), T h, Am), in the soils of a small catchment in Cumbria, UK, for 2 years after their atmospheric deposition in a single hypothetical precipitat ion event. Export of radionuclides to streamwater is calculated to occ ur most readily following deposition of the dissolved elements at high water saturation of the catchment, when little incoming rainwater is required to make up the small moisture deficit of the organic surface horizon, and solutes can move to greater depths in the soil profile. D eposition when the catchment is drier, or of particulate radionuclides , leads to stronger retention. Radionuclide retention or transport dep ends on the strength of chemical interaction with the solid phases of the different soil horizons; this varies among the elements, and also with oxidation state, U(IV) species being more strongly retained than UO2. For purely organic soils, the least strongly retained radionuclid e is Ca, brit the presence in the mineral soil horizon of small amount s of clay mineral with high selectivity towards Cs can markedly increa se its retention. For the actinides, binding by dissolved organic matt er is important; for example, the rate of transport of Th to the strea m is increased by more than two orders of magnitude by complexation wi th dissolved fulvic acid. The model assumptions suggest that, in the l onger term, losses from the catchment of Co, Sr and Cs would take plac e on a time-scale of decades, whereas the actinides would be much more persistent. (C) 1997 Elsevier Science Ltd.