Dynamics and sources of colloids in shallow groundwater in lowland wells and fracture flow in sloping farmland

Field-scale studies of natural colloid mobilization and transport in finely fractured aquifer as well as the source identification of groundwater colloids are of great importance to the safety of shallow groundwater. In this study, the daily monitoring of fracture flow from a sloping farmland plot and the biweekly monitoring of three lowland shallow wells within the same catchment were carried out simultaneously in 2013. The effects of physicochemical perturbations on groundwater colloid dynamics were explored in detail using partial redundancy analysis, structural equation modeling, Pearson correlation and multi-linear regression analyses. The characterization and source identification of groundwater colloids were addressed via multiple parameters. The daily colloid concentration in the fracture flow varied between 0.54 and 31.90 mg/L (1.64 mg/L on average). Unique periods of high colloid concentration (5.59 mg/L on average) occurred during the initially generated flow following the dry season. In comparison, a narrower colloid concentration range of 0.24–11.66 mg/L was observed in the lowland shallow wells, with a smaller temporal variation than that of the fracture flow. A low percentage (2.4–7.0%) of colloids and a high percentage (47.7–92.0%) of coarse particles (2–10 μm) were present in the lowland well water. Hydraulic perturbation by rainwater infiltration in the sloping farmland was the dominant mechanism for colloid mobilization in general; this effect retreated to secondary importance behind chemical perturbations (pH, Mg2+ and DOC) at low flow discharges (