Transportation regularity of colloid particle with surface runoff and fracture flow from sloping farmland in thin purple soil

Abstract: Colloids are defined as suspended fine particles with <10 ?m diameter, which are ubiquitously present in subsurface geologic media. Excessive transport of colloids from terrestrial to aquatic environments may lead to various adverse impacts. It is important to quantify the export of colloids from agricultural field via surface runoff and subsurface flows as they may act as carriers of various strongly sorbing contaminants (e.g., radionuclides, heavy metals, pesticides). A field study on the dynamics of flow processes and associated colloid transport was carried out in a 1 500 m2 sloping (6°) farmland plot with thin purple soil layer overlying fractured mudrock in the hilly area of central Sichuan Basin, Southwest China. The plot was hydrologically isolated from the surrounding geologic formations with cement walls inserted into the impermeable sandstone layer for 20 cm that underlies the mudrock layer. Rain-fed cultivation was practiced under corn-wheat rotation at the study site. There were 6 tensiometers installed at the 3 locations on the upslope (at depths of 15 and 25 cm), mid-slope (at depths of 15 and 45 cm) and downslope (at depths of 15 and 45 cm) to determine the dynamics of soil water potential. The discharge of surface runoff and fracture flow was recorded with tipping buckets connected an event data logger. Monitoring results for 3 representative rain events with distinctively different rain amounts, durations and intensities in the summer of 2015 showed that: 1) The generation mechanism of surface runoff varied over time as the rainfall proceeded, depending on antecedent soil wetness and the intensity and duration of rainfall; the highest depth (3.10 mm) of surface runoff was observed in the event on June 29 having the highest rainfall intensity (27.2 mm/h) and the greatest preceding rainfall of 64.6 mm. 2) Rain amount governed the dynamics of fracture flow. Moreover, due to the apparently better connectivity of transport pathways and hydrodynamic condition, surface runoff showed quicker response to rainfall and earlier discharge peak as compared to fracture flow. 3) Contributions of the mid-slope and downslope to surface runoff as well as fracture flow were obviously higher than that of the upslope. 4) The dynamics of colloid concentration responded faster to rainfall than both surface runoff and fracture flow despite the distinct differences in transport pathway, retaining mechanism and discharge; colloid concentration peak of fracture flow appeared later than that of surface runoff. 5) Colloid concentration in surface runoff was controlled by soil water saturation and hydrological conditions, and showed a more distinct trend of quick rising and falling as compared to fracture flow. During storm events, surface runoff was much stronger than fracture flow in transporting colloids, as indicated by markedly higher colloid concentration and cumulative colloid discharge observed in surface runoff. For example, during the rain event on June 29 in which surface runoff dominated, and the maximum concentration and cumulative discharge of colloids in surface runoff (4 827 mg/L and 7 177 g, respectively) were both more than 30 times of those in fracture flow (146 mg/L and 129 g, respectively). In the rain event on September 9, which was dominated by fracture flow (the flow depth of surface runoff and fracture flow was 1.08 and 9.81 mm, respectively), the cumulative colloid discharge in surface runoff (628.51 g) was still higher than that in fracture flow (546.12 g). Given the potential of facilitating the rapid transport of nutrients (e.g., phosphorus in particular) and pesticides over a long distance for colloid, our results can provide the reference for accurately predicting their export fluxes from farmland.