Characterization of preferential flow and transport pathways under farmland with different land uses

Abstract

Groundwater contamination in agriculturally intensive utilization areas is serious and widespread. Understanding the leaching of pollutants into groundwater is necessary, particularly the roles of preferential flow with macropores and solute transport in agricultural landscapes. However, limited studies focus on the quantitative effect of preferential flow on soil water and pollutants transport under different land uses in in-situ fields. This study quantified the process and paths of soil water and solute transport. This work also evaluated the impacts of three land uses (rice–wheat rotation (RW), vegetable (VG), and pear orchard (PO) fields) on the displacement of sodium bromide (Br⁻) and brilliant blue on a profile scale combined with field infiltration experiments. Results showed that in RW, VG, and PO fields, the stained area ratio decreased by 81.8 %, 82.6 %, and 86.7 % within 0–30 cm, respectively. Across the entire soil profile (0–100 cm), the majority of stained path width ratio concentrated at < 1 cm, accounting for 53.8 %, 62.0 %, and 56.7 % in the RW, VG, and PO fields. In 0–20 cm soil layers, the proportion of stained path width ratio ranging from 1–8 cm relative to the entire profile was higher in RW and VG fields (51.2 % and 54.0 %, respectively) compared to the PO field (38.1 %). Moreover, the high macroporosity corresponds to the low stained area ratio. Further analysis revealed that water flow in the surface layers of all three land uses was primarily matrix flow. In contrast, macropore flow, predominantly mixed with low interaction, dominated in the subsurface layers (below 30 cm). Macropores were the main influencing factor of preferential flow. The mean residual concentration of Br⁻ ion in the profile followed the pattern RW (0.11∙10^–3 mol L^–1) > PO (0^.08∙10^–3 mol L^–1) > VG (0.04∙10^–3 mol L^–1). The RW field exhibited a deeper infiltration depth and faster movement speed. Consequently, the RW field with the characteristics of drying–wetting exhibited a more pronounced preferential flow than VG and PO in the profile, especially below the root zone (> 40 cm). This study provides valuable insights into the migration pathways of nonpoint source and groundwater pollution in plain river network areas.