A laboratory investigation of the release of a conservative tracer and herbicide from topsoil aggregates under varying rainfall intensities

This paper describes the results of laboratory-based research which investi gated the losses of the herbicide, isoproturon, and a non-adsorbing solute, bromide, from topsoil aggregates under high (7 mm h(-1) and low 2.7 mm h(- 1)) intensity simulated rainfall. The structures of the micro- and macro-ag gregates from a clay soil of the Denchworth Series (from Wytham, Oxfordshir e, UM) were observed using a scanning electron microscope, and the slaking of the topsoil aggregates was also investigated in order to provide a conte xt for interpreting the experimental results. The topsoil at Wytham was fou nd to have a bimodal structure with small microaggregates of 0.71 to 1 mm d iameter clustered together to form larger macroaggregates of up to several centimetres diameter. Selected aggregates were air dried and then repacked in Buchner funnels to several centimetres depth in order to remove variabil ity in the experimental results due to the effects of surface microrelief. Under saturated surface conditions high intensity simulated rainfall was le ss efficient at removing both non-adsorbing and adsorbing solutes from the repacked topsoil aggregates than low intensity rainfall. The observed decre ase in the concentration of isoproturon and bromide in the leachate with ti me could be explained by a transport non-equilibrium effect.