Field-scale study of colloid transport in fracture flow from a sloping farmland of purple soil

To evaluate natural colloid transport potentials into groundwater, via fracture flow, a field-scale study was undertaken on a large sloping farmland plot (1 500 m²) in central Sichuan in the summer of 2013. Results indicated that colloid response times varied between 30—90 min; this depending mainly on the preceding soil moisture content and the intensity of rainfall. Colloid flushing occurred prior to peak fracture flow; with colloid concentration peaks appearing earlier than maximum discharge peaks. Peak colloid concentrations were 1—2 orders of magnitude larger than background colloid concentrations. Maximum rainfall intensity and patterns of rainfall dominated fracture flow and colloid concentration. Processes at the air-water interface are suggested to be the main driving factor controlling colloid release and transport during initial responses to rainfall. At these times, the flushing of colloids from soil mesopores and macropores surfaces, driven by the mixing of event rainwater and pre-event mobile soil water, is suggested to be the major mechanism of colloid mobilization and subsequent transport. It is submitted that colloid-facilitated transport could be a very important pathway for the migration of contaminants, particularly those that show large adsorption affinities to soil particles (e.g. phosphorus and hydrophobic pesticides), in the vast purple soil region of Sichuan.