Dynamics and source identification of colloids in surface and subsurface waters

Natural colloids may act as carriers of contaminants and enhance the transport of strongly-sorbing pollutants toward surface waters and groundwater. Field investigations on colloid transport remain rare. This is of high importance for the hilly central Sichuan in the upper reaches of the Yangtze River, which is characterized by thin (mostly <60 cm) purple soil cover and underlying finely fractured mudrock and impermeable sandstone. In Jieliu catchment of Yanting, colloid dynamics in surface and subsurface flows for two sloping (6°) farmland plots (24 m² and 1500 m²) as well as in stream flows at two weirs (3 ha and 35 ha) were monitored based on individual rain events to identify the major factors governing colloid transport. Daily monitoring of fracture flow from the 1500 m² plot and biweekly monitoring of three lowland shallow wells was also conducted simultaneously throughout a whole year to identify the source of colloids in groundwater.

Results show that colloid concentration in the surface runoff was 1-2 orders of magnitude higher than that in the subsurface flows (i.e., the interflow from the soil-mudrock interface and fracture flow from the mudrock-sandstone interface). The lowest colloid concentration was observed in the interflow, probably as a result of pore-scale colloid straining. The rainfall intensity and its temporal variation govern colloid dynamics in both surface runoff and subsurface flows. Analyses of δ¹³C and mineral composition, organic matter and carbonate content in groundwater colloids and the upper geological samples can be used combinedly to identify the sources of groundwater colloids. By using the δ¹³C tracking technique, it was found that the suspended fine sediment export via stream flow during rain events at the catchment outlet came mainly from the sloping farmland, while the deposited fine sediment on stream bed derived from multiple sources including paddy field (55.3%), woodland (29.7%) and sloping farmland (15.0%).