Physical, chemical and biological behaviour of radioactive chlorine and iodine in soils

A series of laboratory-based soil column studies has been carried out to determine the comparative behaviour of the radioactive halogens 36Cl and 125I (as a surrogate for 129I) in the soil-plant system. Physical migration of 36Cl in the soil appears to be unretarded due to its presence predominantly in the chloride form. This lead to substantial accumulation of 36Cl at the soil surface in a column experiment in which the net flux of water was upwards. Physical migration of 125I in soils appears to be relatively rapid at low oxidation-reduction potentials, but is substantially retarded under more oxic soil conditions. This lead to accumulation of 125I in the zone of transition between anoxic and oxic zones in the soil. Gel filtration chromatography confirmed that the predominant chemical form of 36Cl in the soil is the chloride anion. Conversely, 125I showed a strong tendency to associate with both high and low molecular weight fractions of soil humic substances, though its attachment to the low molecular weight fraction appears to be more specific than to the high molecular weight fraction. Soil-plant transfer was measured for both 36Cl and 125I in rye grass swards in undisturbed soil columns. Biological assimilation of 36Cl was particularly significant, with more than 50 % of the total 36Cl activity in the soil being taken up by the grass sward after three weeks' growth. Uptake of 125I by grass swards was less spectacular, though still resulted in approximately 1 % of the total soil 125I activity being assimilated even though the standing biomass of the grass sward was low